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SCIENCEWEEK

ScienceWeek
December 20, 2002
Vol. 6 Number 51

An Online Research Digest Published Weekly Since 1997

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One of the symptoms of approaching nervous breakdown
is the belief that one's work is terribly important.
-- Bertrand Russell (1872-1970)

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Thematic Issue: Supramolecules

1. Introduction
2. On Supramolecular Chemistry
3. Supramolecular vs. Traditional Chemistry
4. On Non-Covalent Synthesis
5. Towards Molecular Information-Processing and Self-
Organization.
6. Self-Assembly: Biological and Chemical Aspects
7. Supramolecular Polymers
8. From Molecules to Materials: Current Trends and Future
Directions
9. On Supramolecular Chirality
10. Solid-State NMR and Supramolecular Systems
11. On the Engineering of Supramolecular Crystals
12. On the History of Crystal Engineering
13. On Self-Organizing Supramolecular Porphyrin Arrays
14. On Hydrated Amphiphiles and Supramolecular Materials
15. Molecular and Supramolecular Photoactive Switches
16. Metal-Coordination in Supramolecules
17. Selective Assembly of Supramolecular Aggregates
18. Supramolecules and Biological Movements

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1. INTRODUCTION

From the Editor: As the following excerpt illustrates,
supramolecular chemistry is a new chemistry, a chemistry of
macromolecular architectures and dynamics, and an exciting new
interface between the physical sciences and biology. As with any
attempt to sample the work of an entire discipline, many aspects
and many researchers are unfortunately not explicitly included.
Nobel Laureate Jean-Marie Lehn (Louis Pasteur University, FR) is
one of the founders of the field.

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"Today's chemists are able to understand and to make practical
use of the mutual operations of molecules both natural and
synthetic. To do so, they have had to expand their horizons, to
see chemistry not just as the science of individual molecules but
also as an investigation of how molecules come together and
interact in groups -- in pairs, in small aggregates or in vast
throngs. This is the business of supramolecular chemistry -- the
chemistry beyond the molecule, the study of ensembles of
molecules working together. Only by taking a perspective this
broad can chemists hope to understand life's molecular
complexity. Yet that will be but a by-product of the
supramolecular chemist's craft. For this chemistry 'beyond the
molecule' is demonstrating that chemistry itself has a vaster
potential than any scientist of Erwin Schroedinger's generation
would have guessed, a potential whose realization will demand not
just technical aptitude but also creative imagination. It is as
if the brick-makers have suddenly realized that their products
need not be an end in themselves but provide a means for them to
become architects...

"Civilization combats entropy through a network of information
exchange. (Information was made formally the opposite of entropy
in Claude Shannon's information theory in the 1940s.) We talk to
each other, we send letters, faxes and electronic mail, we write
things down and store them in libraries where others can look
them up. We pass on this information from generation to
generation -- and, because it comes mixed with a dash of
inevitable disorder, it changes slowly in the process. When
molecules need to get organized, they adopt analogous strategies.
This is why the key concepts of supramolecular chemistry embrace
not just those of traditional molecular chemistry -- structure
and energy -- but also a third, information. We can regard
supramolecular chemistry as a kind of molecular sociology,
wherein the behavior of the collective results from the nature of
the individuals and the relations among them. The components of
supramolecular chemistry communicate, they form associations,
they have preferences and aversions, they follow instructions and
pass on information. Central to these exchanges is the idea of
molecular recognition, whereby one molecule is able to
distinguish another by its shape or properties."

J-M. Lehn and P. Ball, in: Nina Hall (ed.): The New Chemistry.
Cambridge University Press. 2000. p.300,301.

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2. ON SUPRAMOLECULAR CHEMISTRY

Gautam R. Desiraju (University of Hyderabad, IN) discusses
supramolecular chemistry:

For a long time, chemists have tried to understand nature at a
level purely molecular, considering only structures and functions
involving strong covalent bonds. But some of the most important
biological phenomena do not involve the making and breaking of
covalent bonds, the linkages that connect atoms to form
molecules, Instead, biological structures are usually made from
loose aggregates held together by weak non-covalent interactions.
Because of their dynamic nature, these interactions are
responsible for most of the processes occurring in living
systems. Chemists have been slow to recognize the enormous
variety -- in terms of structure, properties, and functions --
offered by this more relaxed approach to making chemical
compounds. The slow shift toward this new approach began in 1894,
when Emil Fischer (1852-1919) proposed that an enzyme interacts
with its substrate as a key does with its lock. This elegant
mechanism contains the two main tenets of what would become a new
subject, supramolecular chemistry. These two principles are
molecular recognition and supramolecular function. The term
"supramolecular chemistry" was coined in 1969 by Jean-Marie Lehn
in his study of inclusion compounds and cryptands. The award of
the 1987 Nobel Prize in Chemistry to Charles Pedersen, Donald
Cram, and Lehn signified the formal arrival of the subject on the
chemical scene. Lehn defined supramolecular chemistry as "the
chemistry of the intermolecular bond". Just as molecules are
built by connecting atoms with covalent bonds, supramolecular
compounds are built by linking molecules with intermolecular
interactions.

Nature 2001 412:397

Related Background:

SUPRAMOLECULAR ASSEMBLIES: CURRENT AND FUTURE RESEARCH

One has the sense that a renaissance in materials science is
underway, a significant refocusing with a potential impact at
least as great as that following the introduction of plastics
more than a century ago. At a recent materials science symposium
on "Materials for the 21st Century and Beyond" (April 29, Hunter
College New York, US), seven leading figures in the field
presented perspectives on the near future. Nobel Laureate Jean-
Marie Lehn (Louis Pasteur University Strasbourg, FR) reviewed the
work of his group in designing and creating molecules programmed
by virtue of their structure and functional groups to
spontaneously organize themselves into larger supramolecular
assemblies held together by hydrogen bonds, metal coordination,
and so on. The interest is not so much in the mere self-assembly
into large structures, but in the fact that such self-assembled
structures exhibit a new spectrum of physical and chemical
properties with important potential practical applications.
Lehn's research involves the use of metal ions to organize and
stabilize supramolecular structures with reversible
architectures, and such structures have special redox, optical,
magnetic and other properties. Michael D. Ward (University of
Minnesota Minneapolis, US) reported on the use of molecular
building blocks to construct crystalline frameworks with
preordained architectures and new functions. Ward's structures
involve sheets of organic cations and organic anions hydrogen-
bonded to each other in a hexagonal arrays. Work by other groups
has involved supramolecular multilayers. In 1988, researchers
discovered that when certain films consisting of alternating
layers of a magnetic and a non-magnetic metal are placed in a
magnetic field, the resistance of the film changes markedly, a
phenomenon known as "giant magnetoresistance". This discovery
apparently reenergized the magnetic materials science field
because of important possible applications to information storage
technology, and Stuart P. Parkin (IBM San Jose, US) is now
leading a productive research group in this field. Ron Dagani
(Chemical and Engineering News), who authors a review of the
symposium, concludes: "Parkin's lecture made it clear that, at
least in the case of magnetic multilayers, some materials
envisioned for the 21st century are already here."

Chem. & Eng. News 1998 8 June.

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3. SUPRAMOLECULAR VS. TRADITIONAL CHEMISTRY

S.T. Nguyen et al (Northwestern University, US) discuss
supramolecular chemistry:

For over 100 years, chemistry has focused primarily on
understanding the behavior of molecules and their construction
from constituent atoms, and our current level of understanding of
molecules and chemical construction techniques has given us the
confidence to tackle the construction of virtually any molecule,
be it biological or designed, organic or inorganic, monomeric or
macromolecular in origin. During the last few decades, chemists
have extended their investigations beyond atomic and molecular
chemistry into the realm of "supramolecular chemistry". Terms
such as "molecular self-assembly", "hierarchical order", and
"nanoscience" are often associated with this area of research. In
general, supramolecular chemistry is the study of interactions
between, rather than within, molecules -- in other words,
chemistry using molecules rather than atoms as building blocks.
Whereas traditional chemistry deals with the construction of
individual molecules (1 to 100 angstroms length scale) from
atoms, supramolecular chemistry deals with the construction of
organized molecular "arrays" with much larger length scales (1 to
100 nanometers). In classical molecular chemistry, strong
association forces such as covalent and ionic bonds are used to
assemble atoms into discrete molecules and hold them together. In
contrast, the forces used to organize and hold together
supramolecular assemblies are weaker non-covalent interactions,
such as hydrogen bonding, polar attractions, van der Waals
forces, and hydrophilic-hydrophobic interactions.

Proc. Nat. Acad. Sci. 2001 98:11849

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4. ON NONCOVALENT SYNTHESIS

D.N. Reinhoudt and M. Crego-Calama (University of Twente, NL)
discuss noncovalent synthesis, the authors making the following
points:

1) With increasing understanding of the individual interactions
that govern the molecular recognition process, the focus is now
shifting to supramolecular chemistry as a tool for noncovalent
synthesis. Cooperative, weak interactions are used for the
spontaneous formation of large aggregates that have well-defined
structures (helicates, grids, molecular containers, capsules,
cyclic arrays, and the like), in which the individual components
are not connected through covalent but through noncovalent bonds.

2) In this emerging field of noncovalent synthesis, one might
expand the definition of a molecule to "a collection of atoms
held together by covalent and noncovalent bonds." Contrary to the
classical definition of a molecule, these supramolecules may be
highly dynamic on the human time scale. On the other hand,
noncovalent and covalent synthesis are not fundamentally
different; both have as the objective to introduce specific
connectivities between atoms. The advantage of noncovalent
synthesis is that noncovalent bonds are formed spontaneously and
reversibly under conditions of thermodynamic equilibrium, with
the possibility of error correction and without undesired side
products. Furthermore, it does not require chemical reagents or
harsh conditions.

3) In biosynthesis, chemical transformations are highly
stereoselective with only one of the many possible stereoisomers
(compounds with the same molecular formula that differ in the way
their atoms are arranged in space) being formed. With the current
state of chemical synthesis, a comparable stereocontrol over
covalent bond formation is possible for many types of reactions
as well. In the synthesis of noncovalent systems, this control
over stereochemistry is much more difficult, because bonds
between individual components are kinetically labile and are
continuously broken and formed. However, in noncovalent
synthesis, the stereochemistry of reaction products
(regioselectivity, diastereoselectivity, and enantioselectivity)
must also be controlled.

4) One of the areas where noncovalent synthesis has a great
advantage over covalent synthesis is the bottom-up (chemical)
assembly of nanostructures. Large-scale nanometer fabrication
will be a requirement for future molecular electronic devices,
high-density data storage, or drug delivery. Covalent synthesis
has been proven to be extremely fruitful for the synthesis of
compounds with molecular weights in the range of 100 to 3000
daltons such as palytoxin, norbrevetoxin, and taxol.
Nevertheless, with the exception of the sequential methodologies
for the synthesis of biopolymers (or oligomers), there are no
simple covalent strategies for the synthesis of pure molecules
that have molecular weights between 10^(4) and 10^(6)
kilodaltons. Such molecules have dimensions between 3 and 20
nanometers and fill the gap between small molecules and larger
nano-objects that are now accessible by top-down (physical)
fabrication methods, mainly based on lithography. This is also
the size range where quantum confinement influences the
electronic and optical properties of matter.

5) In summary: In chemistry, noncovalent interactions are now
exploited for the synthesis in solution of large supramolecular
aggregates. The aim of these syntheses is not only the creation
of a particular structure, but also the introduction of specific
chemical functions in these supramolecules.

References (abridged):

1. C. J. Pedersen, Angew. Chem. Int. Ed. 27, 1021 (1988)

2. J.-M. Lehn, Angew. Chem. Int. Ed. 27, 89 (1988)

3. D. J. Cram, Angew. Chem. Int. Ed. 27, 1009 (1988)

4. R. Ungaro, A. Arduini, A. Casnati, A. Pochini, F. Ugozzoli,
Pure Appl. Chem. 68, 1213 (1996)

5. P. Wallimann, T. Marti, A. F�rer, F. Diederich, Chem. Rev. 97,
1567 (1997)

Science 2002 295:2403

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5. FROM MOLECULAR RECOGNITION TOWARDS MOLECULAR INFORMATION-
PROCESSING AND SELF-ORGANIZATION.

J-M. Lehn (Louis Pasteur University, FR) discusses perspectives
in supramolecular chemistry, the author making the following
points:

1) The selective binding of a substrate by a molecular receptor
to form a supramolecular species involves molecular recognition
which rests on the molecular information stored in the
interacting species. The functions of supramolecules cover
recognition, as well as catalysis and transport. In combination
with polymolecular organization, they open ways towards molecular
and supramolecular devices for information processing and signal
generation. The development of such devices requires the design
of molecular components performing a given function (e.g.,
photoactive, electroactive, ionoactive, thermoactive, or
chemoactive) and suitable for assembly into an organized array.

2) Light-conversion devices and charge-separation centers have
been realized with photoactive cryptates formed by receptors
containing photosensitive groups. Electroactive and ionoactive
devices are required for carrying information via electronic and
ionic signals. Redox-active polyolefinic chains, like the
"caroviologens", represent molecular wires for electron transfer
through membranes. Push-pull polyolefins possess marked nonlinear
optical properties. Tubular mesophases, formed by organized
stacking of suitable macrocyclic components, as well as
"chundle"-type structures, based on bundles of chains grafted
onto a macrocyclic support, represent approaches to ion channels.
Lipophilic macrocyclic units form Langmuir-Blodgett films that
may display molecular recognition at the air-water interface.

3) Supramolecular chemistry has relied on more or less
preorganized molecular receptors for effecting molecular
recognition, catalysis, and transport processes. A step beyond
preorganization consists in the design of systems undergoing
self-organization, that is, systems capable of spontaneously
generating a well-defined supramolecular architecture by self-
assembling from their components under a given set of conditions.
Several approaches to self-assembling systems have been pursued:
the formation of helical metal complexes, the double-stranded
helicates, which result from the spontaneous organization of two
linear polybipyridine ligands into a double helix by binding of
specific metal ions; the generation of mesophases and liquid
crystalline polymers of supramolecular nature from complementary
components, amounting to macroscopic expression of molecular
recognition; the molecular-recognition-directed formation of
ordered solid-state structures.

4) Endowing photo-, electro-, and ionoactive components with
recognition elements opens perspectives towards the design of
programmed molecular and supramolecular systems capable of self-
assembly into organized and functional supramolecular devices.
Such systems may be able to perform highly selective operations
of recognition, reaction, transfer, and structure generation for
signal and information processing at the molecular and
supramolecular levels.

Ang. Chem. International. Ed. (Engl.) 1990 29:1304

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6. SELF-ASSEMBLY: BIOLOGICAL AND CHEMICAL ASPECTS.

J.S. Lindsey (Carnegie Mellon University, US) discusses self-
assembly, the author making the following points:

1) Molecular electronics places a premium on organized 3-
dimensional architectures. Self-assembly has been touted as a
solution to the synthesis problems of molecular electronics.
Biological self-assembly provides striking illustrations of
thermodynamically-stable architectures, including tobacco mosaic
virus, DNA, and numerous multimeric proteins. But in many other
instances biological self-assembly is regulated in a number of
characteristic ways.

2) The author introduces seven classifications of self-assembly
processes, including strict (equilibrium) self-assembly,
irreversible self-assembly, assembly following precursor
modification, assembly with post-modification, assisted assembly,
directed assembly, and assembly with intermittent processing.
Strict self-assembly is governed by equilibrium thermodynamics.
The virtues of self-assembly include minimization of information
through use of modular subunits, control of assembly and
disassembly, built-in error-checking and recovery, and overall
high efficiency.

3) In many but not all instances self-assembly is a cooperative
process involving nucleation and growth phases. A fundamental
theme of cooperative assembly processes is that one series of
interactions establishes the initial structure (nucleation),
thereby setting the stage for a subsequent and more extensive
series of interactions (growth). Cooperative phenomena are well-
known in biochemistry, but cooperative assembly is not as well-
developed conceptually in synthetic chemistry.

4) A striking feature of self-assembly is that forming several
bonds can be easier than forming only one bond. Self-assembly can
involve non-covalent and covalent bond formation. Self-assembly
lies at the heart of myriad examples in chemistry, ranging from
metal chelation to model systems for self-replication. Multi-
bridged cage molecules provide one domain for comparing modern
methods of one-flask syntheses with biological self-assembly, and
the syntheses of over 100 such cage molecules are reviewed by the
author. The rich precedents of biological self-assembly may yield
new paradigms for synthetic chemistry. Molecular electronics is
not alone in its requirement for controlled 3-dimensional
architectures, and a deeper understanding of self-assembly in all
its manifestations is expected to benefit many fields of
chemistry.

New J. Chemistry 1991 15:153

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7. SUPRAMOLECULAR POLYMERS

"With the introduction of supramolecular polymers, which are
polymers based on monomeric units held together with directional
and reversible secondary interactions, the playground for polymer
scientists has broadened and is not restricted to macromolecular
species, in which the repetition of monomeric units is mainly
governed by covalent bonding. The importance of supramolecular
interactions within polymer science is beyond discussion and
dates back to the first synthesis of synthetic polymers; the
materials properties of, e.g., nylons, are mainly the result of
cooperative hydrogen bonding. More recently, many exciting
examples of programmed structure formation of polymeric
architectures based on the combination of a variety of secondary
supramolecular interactions have been disclosed. When the
covalent bonds that hold together the monomeric units in a
macromolecule are replaced by highly directional noncovalent
interactions, supramolecular polymers are obtained. In recent
years, a large number of concepts have been disclosed that make
use of these noncovalent interactions. Although most of the
structures disclosed keep their polymeric properties in solution,
it was only after the careful design of multiple-hydrogen-bonded
supramolecular polymers that systems were obtained that show true
polymer materials properties, both in solution and in the solid
state. Polymers based on this concept hold promise as a unique
class of novel materials because they combine many of the
attractive features of conventional polymers with properties that
result from the reversibility of the bonds between monomeric
units. Architectural and dynamic parameters that determine
polymer properties, such as degree of polymerization, lifetime of
the chain, and its conformation, are a function of the strength
of the noncovalent interaction, which can reversibly be adjusted.
This results in materials that are able to respond to external
stimuli in a way that is not possible for traditional
macromolecules."

L. Brunsveld et al: Chem. Rev. 2001 101:4071

Related Background Brief:

SELF-COMPLEMENTARY QUADRUPLE HYDROGEN-BONDING MOTIFS AS A
FUNCTIONAL PRINCIPLE: FROM DIMERIC SUPRAMOLECULES TO
SUPRAMOLECULAR POLYMERS. The self-association of individual
molecules can lead to the formation of highly complex and
fascinating supramolecular aggregates. However, for binding
motifs which rely only on hydrogen bonds, a combination of
several such weak interactions is necessary to observe self-
association in solution. Systems based on four hydrogen bonds in
a linear array can be obtained which efficiently aggregate at
least in chloroform. Besides the physical-organic
characterization of these aggregates and the factors influencing
their stability, such quadruple hydrogen-bonding motifs can also
be used in the field of materials science to synthesize, for the
first time, supramolecular polymers through the self-association
of self-complementary monomers. As the formation of noncovalent
interactions is reversible and their strength depends
significantly on the chemical environment (for example, solvent,
temperature), the macroscopic properties of such polymers can be
controlled by variation of these parameters; hence a first step
towards intelligent materials with tailor-made properties is
made. C. Schmuck and W. Wienand: Angew Chem Int Ed Engl 2001
40:4363.

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8. FROM MOLECULES TO MATERIALS: CURRENT TRENDS AND FUTURE
DIRECTIONS.

A.P. Alivisatos et al (University of California Berkeley, US)
discuss supramolecular materials science, the authors making the
following points:

1) The development, characterization, and exploitation of novel
materials based on the assembly of molecular components is an
exceptionally active and rapidly expanding field. For this
reason, the topic of molecule-based materials (MBMs) was chosen
as the subject of a workshop sponsored by the Chemical Sciences
Division of the United States Department of Energy. The purpose
of the workshop was to review and discuss the diverse research
trajectories in the field from a chemical perspective, and to
focus on the critical elements that are likely to be essential
for rapid progress.

2) The MBMs discussed encompass a diverse set of compositions and
structures, including clusters, supramolecular assemblies, and
assemblies incorporating biomolecule-based components. A full
range of potentially interesting materials properties, including
electronic, magnetic, optical, structural, mechanical, and
chemical characteristics were considered. Key themes of the
workshop included synthesis of novel components, structural
control, characterization of structure and properties, and the
development of underlying principles and models.

3) MBMs, defined as "useful substances prepared from molecules or
molecular ions that maintain aspects of the parent molecular
framework" are of special significance because of the capacity
for diversity in composition, structure, and properties, both
chemical and physical. Key attributes are the ability in MBMs to
access the additional dimension of multiple length scales and
available structural complexity via organic chemistry synthetic
methodologies and the innovative assembly of such diverse
components. The interaction among the assembled components can
thus lead to unique behavior.

4) A consequence of the complexity is the need for a multiplicity
of both existing and new tools for materials synthesis, assembly,
characterization, and theoretical analysis. For some
technologically useful properties, e.g., ferro- or ferrimagnetism
and superconductivity, the property is not a property of a
molecule or ion; it is a cooperative solid-state (bulk) property
-- a property of the entire solid. Hence, the desired properties
are a consequence of the interactions between the molecules or
ions, and understanding the solid-state structure as well as
methods to predict, control, and modulate the structure are
essential to understanding and manipulating such behaviors. As
challenging as this is, molecules enable a substantially greater
ability of control than atoms as building blocks for new
materials and thus are well positioned to contribute
significantly to new materials.

5) The diversity of components and processes leads to the
recognition of the critical role of cross-disciplinary research,
including not only that between traditionally different areas
within chemistry, but also between chemistry and biochemistry,
physics, and a number of engineering disciplines. Enhancing
communication and active collaboration between these groups was
seen as a critical goal for the research area.

Advanced Materials 1998 10:1297.

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9. ON SUPRAMOLECULAR CHIRALITY

M-J. Kim et al (Kwang-Ju Institute of Science and Technology, KR)
discuss supramolecular chirality, the authors making the
following points:

1) Fundamental questions concerning chiroptical polymers arise
from the characteristics of natural polymers, which have a one-
handed helical conformation and show characteristic functionality
in living systems.(1) Conformational chirality can be optically
induced by the irradiation of photochromic molecules and
polymers.(2-5) This phenomenon has been investigated for cases of
many kinds of photochromophores, for example, azobenzenes,(2-4)
overcrowded alkenes,(5) diarylethens, binaphthalenes, and
spiropyranes.

2) Since the pioneering studies by Goodman (1967), the optical
induction of supramolecular chirality has been widely studied
using azobenzene-containing polymers. Azobenzenes are well-known
chromophores for their photoinduced linear orientation via trans 
cis   trans photoisomerization. Photoinduced chirality changes in
azopolymers have been reported for polymethacrylates,(2)
polypeptides,(3) and polyisocyanates.(4) These azobenzene
polymers contain chiral centers, and the chiral properties were
investigated in solution using two different wavelengths as the
light source.

3) The use of circularly polarized light has been demonstrated as
a method for partially resolving a racemic mixture. Recently,
Nikolova et al (1997) reported on the photoinduced chirality of
amorphous and liquid crystalline azobenzene polymers by
irradiation with circularly polarized light. The induced
chirality of the azobenzene polymers was investigated as a
function of the ellipticity of incident light. However, Iftime et
al (2000) reported that circular dichroism is not induced in an
amorphous azopolymer film by irradiation with circularly
polarized light and proposed that liquid crystalline alignment
represents one of the key factors in the creation of a chiral
superstructure. Therefore, the issue of the origin of the
photoinduced chirality of azobenzene polymer films irradiated by
light with handedness is not clear.

4) The authors report an investigation of chirality
photoinduction from amorphous and achiral azobenzene polymer
films. The suggest their results demonstrate that liquid
crystallinity is not a necessary condition for a material to
exhibit photoinduced chiral properties.

References (abridged):

1. Circular Dichroism Principles and Applications, 2nd ed.;
Berova, N., Nakanishi, K., Woody, R. W., Eds.; Wiley-VCH, Inc.:
New York, 2000.

2. Angiolini, L.; Caretti, D.; Giorgini, L.; Salatelli, E.
Macromol. Chem. Phys. 2000, 207, 533.

3. Pieroni, 0.; Fissi, A.; Ciardelli, F. React. Fund. Polym.
1995, 6, 185.

4. Muller, M.; Zentel, R. Macromolecules 1996, 29, 1609.

5. Feringa, B. L.; Jager, W. F.; Lange B. d. J. Am. Chem. Soc.
1991, 113, 5468.

J. Am. Chem. Soc. 2002 124:3504

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10. SOLID-STATE NMR AND SUPRAMOLECULAR SYSTEMS

S.P. Brown and H.W. Spiess (Max Planck Institute for Polymer
Science Mainz, DE) discuss NMR methods for supramolecular
systems, the authors making the following points:

1) In current polymer science, there is considerable interest in
the design of well-ordered superstructures based on self-assembly
of carefully chosen blocks. Of particular importance in this
context are noncovalent interactions, e.g., hydrogen-bonding and
aromatic pi-pi interactions. It has been demonstrated, for
example, that linear polymers and reversible networks are formed
from the self-assembly of monomers incorporating two and three 2-
ureido-4-pyrimidone units, respectively, because of the
propensity of these units to dimerize strongly in a self-
complementary array of four cooperative hydrogen bonds. But such
specific interactions are not a prerequisite for a well-
controlled self-assembly: e.g., the self-assembly in bulk of
dendritic building blocks into spherical, cylindrical, and other
supramolecular architectures occurs as a consequence of both
shape and complementarity and the demixing of aliphatic and
aromatic segments.

2) Despite the presence of considerable order on different length
scales, single crystals suitable for diffraction studies, and
thus full crystal structures, are not available for such self-
assembled supramolecular entities. If the mechanisms governing
self-assembly are to be better understood, analytical methods
capable of probing the structure and dynamics of these partially
ordered systems are essential. In recent years, the field of
solid-state nuclear magnetic resonance (NMR) has enjoyed rapid
technological and methodological development, and advanced solid-
state NMR methods are currently well placed to meet the challenge
of modern polymer chemistry. In particular, with such methods
much insight can be achieved with small amounts (10 to 20
milligrams) of as-synthesized samples.

Chem. Revs. 2001 101:4125

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11. ON THE ENGINEERING OF SUPRAMOLECULAR CRYSTALS

T.L. Nguyen et al (State University of New York Stony Brook, US)
discuss supramolecular crystal engineering, the authors making
the following points:

1) Crystal engineering (in this context, supramolecular
synthesis) is an important problem that requires a detailed
knowledge of intermolecular interactions. One would like to be
able to choose appropriate molecules or sets of molecules and
predict with confidence the manner in which they will
crystallize. This is a difficult problem of great complexity, and
indeed in many cases there may be no simple thermodynamic basis
for a successful prediction. Crystallization is a kinetic
process, and polymorphism often appears when it is most
inconvenient. The authors suggest that chemists persevere with a
certain confidence that by a clever design they will achieve the
structural result they seek. Success is achieved either by wisely
setting limited structural goals in the first place, or by making
judicious use of ex post facto crystal design.

2) Despite these difficulties, one can still imagine a scenario
where one could reliably predict the total structure of a crystal
purely on the basis of knowledge of molecular properties. Total
structure prediction would require specification of molecular
geometry and orientation, unit cell dimensions, and the space
group. The authors report that their simplified approach to this
problem has been to identify molecular functionalities that will
predictably and persistently lead to crystals containing defined
network structures. Each chosen functionality has a size and
shape that leads to characteristic repeat distances within its
networks, and these molecular networks are substructures of the
final crystal. The networks have repeat distances commensurate
with the unit cell of the crystal, and their group symmetries are
a subgroup of the space group of the final crystals. The distance
parameters can be predicted, and a consideration of molecular
symmetry combined with the symmetry of each anticipated
intermolecular bond can lead one to the correct network symmetry.
The authors state: "By combining good chemical insight with solid
crystallographic principles, one can design or engineer
crystalline solids that contain networks with desired structural
features."

J. Am. Chem. Soc. 2001 123:11057

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12. ON THE HISTORY OF CRYSTAL ENGINEERING

In general, in this context, "goniometry" involves the
measurement of interfacial angles for the comparison of crystals
of different development. William Wollaston (1766-1828) developed
in 1809 a reflecting (optical) goniometer for use with small
crystals: a fixed mirror is illuminated from a collimator so that
part of the parallel beam falls on the crystal, which is fixed on
an axis parallel to the mirror and a short distance above it, and
is so adjusted that the edge of the facial angle to be measured
is parallel to the axis. All the interfacial angles in a given
zone can be found by rotation of the crystal.

Mark D. Hollingsworth (Kansas State University, US) discusses the
history of crystal engineering, the author making the following
points:

1) Legend has it that modern crystallography owes its roots to an
accidental discovery reported in 1781 by the French physicist
Rene Just Hauey (1743-1822) (1). While admiring a friend's
mineral collection, Hauey dropped a particularly large crystal of
Iceland spar (calcite), which cleaved into equivalent fragments.
With keen insight, Hauey recognized that internal structure was
related to external form, and after spending the next years
smashing his mineral collection and those of his friends, he
reckoned that all crystals were composed of a limited number of
building blocks that were stacked together in simple ways. With
the subsequent development of optical goniometry, polarized light
microscopy, and other physical techniques, 19th-century chemists
and crystallographers focused on macroscopic properties of
crystals such as birefringence, optical activity, pyroelectricity
(electric polarization caused by temperature change), and, later,
piezoelectricity (electric polarization under external stress),
which was discovered by Pierre and Jacques Curie in 1880. These
efforts culminated in Paul Groth's Chemische Krystallographie
(2), which documents in five volumes what was known about the
external form and physical properties of more than 7000 organic
and inorganic crystals that had been characterized by the
beginning of the 20th century.

2) Groth's treatise and Hauey's deconstruction of macroscopic
crystalline objects provide instructive contrasts with the modus
operandi of modern-day solid state organic chemists and "crystal
engineers", who have embraced the notion of the supramolecular
"synthon" (3) as the critical design element for generating new
materials. In its renaissance, as inaugurated by G.M.J. Schmidt
and coworkers in the 1960s (4), solid state organic chemistry has
focused on the molecular building blocks and their connections
with the anticipation that reliable functional group interactions
can be used to assemble a variety of useful molecular materials.

3) The synthesis of organic molecules relies on the strength of
covalent bonds and on the relative rates of bond-forming
processes to lead in a rational and step-wise process to the
final product. It is therefore no surprise that many organic
chemists have until recently shied away from crystal synthesis.
For the supramolecular synthetic chemist, the specific goal is a
macroscopic property, and the final product is often a moving
target that changes each time the crystal synthesis yields
something different from that predicted by the imperfect models
we use. The fundamental difficulty for this field is that
molecular crystals are held together by a multitude of weak
interactions, and a huge number of free energy minima
(polymorphs) exist within a few kilojoules/mol of the global
minimum. The process of crystal engineering is therefore an
iterative one that involves synthesis, crystallography, crystal
structure analysis, and computational methods.(5)

References (abridged):

1. J. G. Burke, Origins of the Science of Crystals (Univ. of
California Press, Berkeley, CA, 1966), pp. 83-85.

2. P. A. Groth, Chemische Krystallographie (Verlag von Wilhelm
Engelmann, Leipzig, 1906-1919), vol. I-V.

3. G. R. Desiraju, Angew. Chem. Int. Ed. 34, 2311 (1995)

4. G. M. J. Schmidt, Pure Appl. Chem. 27, 647 (1971)

5. K. D. M. Harris, M. Tremayne, P. Lightfoot, P. G. Bruce, J.
Am. Chem. Soc. 116, 3543 (1994)

Science 2002 295:2410

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13. ON SELF-ORGANIZING SUPRAMOLECULAR PORPHYRIN ARRAYS

C.M. Drain et al (City University of New York, US) discuss
porphyrin arrays, the authors making the following points:

1) With the increasing demand for the ability to sculpt matter
into precise functioning devices of nanoscale dimensions, the
molecular level design of functional materials is an overarching
theme in much of the synthetic materials literature (1-5).
Inspired by biological systems, the introduction of specific
interactions is a route toward using the facile and energetically
favorable production capabilities to self-assemble materials.
Exploitation of nonspecific intermolecular interactions has
resulted also in the formation of molecular electronic devices.

2) The authors report they have used self-assembly to form a
square planar array of nine porphyrins mediated by coordination
of exocyclic pyridyl groups on three different porphyrins to 12
trans-palladium dichlorides. In addition to modulating the size
and distribution on surfaces, metalation of the porphyrin
macrocycle enables one to design nanoscale systems with a host of
photonic, magnetic, redox catalytic, and sensor capabilities.
These functions have been well studied on metalloporphyrin
monomers. Substitution of the peripheral R groups with long-chain
hydrocarbons enables the design of nanoscale aggregates that,
using nonspecific interactions, organize into two-dimensional
arrays. The authors present an overview of the design
capabilities for materials and devices by using porphyrin
supramolecular arrays.

3) In summary: The authors report that tessellation of nine free-
base porphyrins into a 3 ž 3 array is accomplished by the self-
assembly of 21 molecular entities of four different kinds, one
central, four corner, and four side porphyrins with 12 trans
Pd(II) complexes, by specifically designed and targeted
intermolecular interactions. Strikingly, the self-assembly of 30
components into a metalloporphyrin nonamer results from the
addition of nine equivalents of a first-row transition metal to
the above milieu. In this case each porphyrin in the nonameric
array coordinates the same metal such as Mn(II), Ni(II), Co(II),
or Zn(II). This feat is accomplished by taking advantage of the
highly selective porphyrin complexation kinetics and
thermodynamics for different metals. In a second, hierarchical
self-assembly process, nonspecific intermolecular interactions
can be exploited to form nanoscaled three-dimensional aggregates
of the supramolecular porphyrin arrays. In solution, the size of
the nanoscaled aggregate can be directed by fine-tuning the
properties of the component macrocycles, by choice of
metalloporphyrin, and the kinetics of the secondary self-assembly
process. As precursors to device formation, nanoscale structures
of the porphyrin arrays and aggregates of controlled size may be
deposited on surfaces. Atomic force microscopy and scanning
tunneling microscopy of these materials show that the choice of
surface (gold, mica, glass, etc.) may be used to modulate the
aggregate size and thus its photophysical properties. Once on the
surface the materials are extremely robust.

References (abridged):

1. Alivisatos, A. P. , Barbara, P. F. , Castleman, A. W. , Chang,
J. , Dixon, D. A. , Klein, M. L. , McLendon, G. L. , Miller, J.
S. , Ratner, M. A. , Rossky, P. J. , Stupp, S. I. & Thompson, M.
E. (1998) Adv. Mater. 10, 1297-1336.

2. Aviram, A. & Ratner, M. (1998) Ann. N.Y. Acad. Sci. 852, 1-
349.

3. Lehn, J.-M. (1990) Angew. Chem. Int. Ed. Engl. 29, 1304-1319.

4. Stang, P. J. & Olenyuk, B. (1997) Acc. Chem. Res. 30, 502-518.

5.  Lindsey, J. S. (1991) New J. Chem. 15, 153-180.

Proc. Nat. Acad. Sci. 2002 99:6498

Related Background Brief:

OPTIMIZATION AND CHEMICAL CONTROL OF PORPHYRIN-BASED MOLECULAR
WIRES AND SWITCHES. Porphyrin molecular wires consist of
porphyrin units fused to acene-type bridges and have been
synthesized by the authors in a range of topologies including
linear porphyrin octamers of length ca. 120 �. The authors
demonstrate, for some linear oligoporphyrins, how the electronic
coupling between the end porphyrin units can be modulated by
simple (possibly in situ) chemical modulation of the bridging
units. Specifically, the chemical systems considered involve
either pH-controlled protonation of bridge azines or conversion
of bridge quinone or quinone dioxime rings to or from benzenoid
or hydroquinone rings. In the most general terms, the electronic
coupling through oligoporphyrin molecular wires is discussed in
terms of a simple model in which complete end-to-end electronic
delocalization is required in order to provide strong long-range
interactions. Computationally, the authors monitored interorbital
coupling using an appropriate mixture of density functional and
ab initio SCF computational schemes. Finally, the authors
examined bridge modulation of the intermetallic coupling in three
homovalent bis-metallic oligoporphyrin systems. Results were
obtained both using an effective two-level model, appropriate for
spectroscopic properties, and using a more general scheme,
appropriate for molecular conduction. N.S. Hush et al: Ann New
York Acad Sci 1998 852:1.

Related Background Brief:

SELF-ORGANIZATION OF SELF-ASSEMBLED PHOTONIC MATERIALS INTO
FUNCTIONAL DEVICES: PHOTO-SWITCHED CONDUCTORS. Linear porphyrin
arrays self-assembled by either hydrogen bonding or metal ion
coordination self-organize into lipid bilayer membranes. The
length of the transmembrane assemblies is determined both by the
thermodynamics of the intermolecular interactions in the
supramolecule and by the dimension and physical chemical
properties of the bilayer. Thus, the size of the porphyrin
assembly can self-adjust to the thickness of the bilayer. An
aqueous electron acceptor is placed on one side of the membrane
and an electron donor is placed on the opposite side. When
illuminated with white light, substantial photocurrents are
observed. Only the assembled structures give rise to the
photocurrent, as no current is observed from any of the component
molecules. The fabrication of this photogated molecular
electronic conductor from simple molecular components exploits
several levels of self-assembly and self-organization. Charles M.
Drain: pnas 2002 99:5178.

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14. ON HYDRATED AMPHIPHILES AND SUPRAMOLECULAR MATERIALS

In general, "amphiphiles" are molecules with parts (groups)
having diverse affinities for different solvents. For example,
polar groups have an affinity for water, while hydrocarbon groups
have an affinity for oils. Most detergents are amphiphiles,
molecules with a polar head and a long hydrocarbon tail. In this
context, however, possible solvent interactions are only one
aspect of amphiphilic character. The important consideration is
that amphiphiles tend to self-organize: groups of amphiphilic
molecules will form stable domains of polar interactions and
nonpolar interactions. For example, amphiphiles may form
"micelles", spherical or cylindrical arrangements with an
interior forming one interaction domain while the surface forms
another interaction domain. Larger aggregates may form vesicles
with diameters in the micron range.

A simple linear polymer is a chain molecule composed of monomers
with two reactive sites (bifunctional monomers), with
monofunctional terminal units. If more than one bifunctional
monomer is present, the chain is known as a "copolymer". A
copolymer in which a number of units of the same monomer are
located adjacent to one another (in "blocks" of monomers) is
called a "block copolymer". A "diblock copolymer" is composed of
two types of monomers (e.g., A and B), and may be depicted thus:
AAAAAABBBBBAAAAAABBBBBAAAAAAA.

In general, a "homopolymer" is any polymer made up of only one
kind of constitutional repeating unit, e.g., cellulose, which
contains only glucose as the monomeric unit.

A "Langmuir-Blodgett film" (Langmuir-Blodgett multilayer) is a
film of molecules on a solid surface, the film with multiple
layers made by dipping a plate into a liquid so that it is
covered by a monolayer and then repeating the process. The
technique enables a multilayer to be built up one monolayer at a
time, and such layers have many practical applications.

A. Mueller and D.F. O'Brien (University of Arizona, US) discuss
hydrated amphiphiles, the authors making the following points:

1) Hydrated amphiphiles form various phases as a function of
molecular structure, temperature, concentration, and pressure.(1-
4), and there appears to be a one-to-one correspondence between
the structures observed for hydrated amphiphiles and that for
block copolymer.(5) Amphiphiles are characterized by having a
hydrophilic headgroup attached to at least one hydrophobic tail.
The unfavorable interfacial enthalpic interaction between the
hydrophobic tail(s) of the amphiphile with the polar water
molecules induces the former to aggregate with the hydrophobic
tail(s) of other amphiphiles.(4) The hydrophilic headgroup
therefore separates the water from the tail(s), in much the same
way that the A-B junction of a diblock AB copolymer separates the
two homopolymer blocks A and B. Self-organized arrays of
noncovalently associated amphiphiles may exist as self-supported
lamellar/vesicular, various bicontinuous cubic, or
hexagonal/cylindrical phases. Amphiphiles are also frequently
studied as supported assemblies, e.g., monolayers at the air-
water interface, Langmuir-Blodgett, or self-assembled monolayers.
During the past two decades or so, the understanding of each of
these supramolecular assemblies has advanced significantly. This
progress is a consequence of fundamental and applied research in
many laboratories.

2) The advent of methods to polymerize supramolecular assemblies,
first in monolayers in the 1970s, followed by bilayer vesicles in
the early 1980s, and more recently in nonlamellar phases, i.e.,
cubic and hexagonal phases, has led to the creation of new
materials, the development of new methods, and a widening
perspective on the potential applications of these novel
polymeric materials. These uses include the controlled delivery
of reagents and drugs, the preparation of biological membrane
mimics, the separation and purification of biomolecules, the
modification of surfaces, the stabilization of organic zeolites,
and the preparation of nanometer colloids, among others.

3) The concept of an area-minimizing surface has been used
extensively to describe the morphologies of amphiphile/water
systems.(2) The free energy of the system is described by the
topology of the surfaces. In such an analysis, a spontaneous
curvature term arises purely as a result of the fact that the
dimensions of the microdomain are only a few orders of magnitude
greater than that of the constituent molecules. This means that
the shape of the interface is influenced by the interactions on a
molecular level. In order for a system to achieve equilibrium,
the various terms in the free energy expression, chief of which
is the mean curvature, must be minimized. This theory has been
extended to describe the effects of surface charge and branched
alkyl chains on the formation of nonlamellar assemblies. The
distribution of a mixture of lipids in nonlamellar phases has
also been investigated.

References (abridged):

1. Lindblom, G.; Rilfors, L. Biochim. Biophys. Acta 1989, 988,
221-256

2. Gruner, S. M. J. Phys. Chem. 1989, 93, 7562-7570

3. Seddon, J. M. Biochim. Biophys. Acta 1990, 1031, 1-69

4. Evans, D. F.; Wennerstrom, H. The Colloidal Domain: Where
Physics, Chemistry, Biology, and Technology Meet; VCH Publishers:
New York, 1994.

5. Benedicto, A. D.; O'Brien, D. F. Macromolecules 1997, 30,
3395-3402

Chem. Rev. 2002 102:727

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15. MOLECULAR AND SUPRAMOLECULAR PHOTOACTIVE SWITCHES

S. Zeena and K.G. Thomas (Regional Research Laboratory, IN)
discuss photoactive chemical systems. The design and study of
molecular and supramolecular photoactive systems have been
actively pursued in recent years due to their potential
applications in optoelectronic devices (e.g., molecular switches,
sensors, transducers, and information processing and storage
devices). Of particular interest is the design of molecular
systems which undergo conformational changes analogous to the
folding of proteins. Synthetic molecular systems and polymers
which can fold into well-defined conformation in solution
("foldamers") through non-covalent interactions have been
reported. These include 1) solvophobically driven conformational
folding of phenyacetylene-based oligomers into ordered helical
structures, and 2) the donor-acceptor interaction or aromatic
groups leading to pleated structures. Conformational changes and
molecular motions in photoactive molecular and supramolecular
systems can be modulated by chemical, photochemical, or
electrochemical methods, and such changes, when translated into
optical or electronic properties, can form the basis of switching
devices. The authors report they have designed two nonconjugated
bichromophores which can fold and unfold by varying the solvent
polarity or by the application of external stimuli such as heat
or light.

J. Am. Chem. Soc. 2001 123:7859

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16. METAL-COORDINATION IN SUPRAMOLECULES

C.J. Kuehl et al (University of Utah, US) discuss metals in
supramolecules. Over the past decade, the use of metal
coordination as a means to drive and preserve the formation of
discrete molecular ensembles has become an established
methodology in supramolecular chemistry. However, the level of
complexity to which metal-mediated self assembly can develop as a
general synthetic strategy has yet to be realized. Nevertheless,
the design and construction of new supramolecular entities refine
our understanding of the fundamental principles of molecular
self-organization. So far, highly symmetric ring systems (e.g.,
molecular triangles, squares, pentagons, hexagons, etc.) have
generally been the most successfully characterized species,
because of their inherent simplicity over three-dimensional
constructs. Typically comprising aromatic bridging ligands
connected via transition metals, these "metallocyclophanes" have
shown promise as a new class of functional receptor molecules
that can act as hosts in host-guest complexes. Considering that
metal-containing macrocycles often possess magnetic,
photophysical, and/or redox properties not accessible from purely
organic systems, studies in basic host-guest chemistry have broad
implications for technologies such molecular sensing,
separations, and catalysis. However, precise size and highly
specific electrostatic and dispersion forces are required for
selectivity, and for the most part this remains an important
challenge for research.

J. Am. Chem. Soc. 123:9634

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17. SELECTIVE ASSEMBLY OF SUPRAMOLECULAR AGGREGATES

Y. Yokoyama et al (National Institute for Materials Science, JP)
discuss selective assembly of supramolecular aggregates on a
surface. The realization of molecule-based miniature devices with
advanced functions requires the development of new and efficient
approaches for combining molecular building blocks into desired
functional structures, ideally with these structures supported on
suitable substrates. Supramolecular aggregation occurs
spontaneously and can lead to controlled structures if selective
and directional non-covalent interactions are exploited. But such
selective supramolecular assembly has yielded almost exclusively
crystals or dissolved structures. In contrast, the self-assembly
of adsorbed molecules into larger structures has not yet been
directed by controlling selective intermolecular interactions.
The authors report the formation of surface-supported
supramolecular structures whose size and aggregation pattern are
rationally controlled by tuning the non-covalent interactions
between individual adsorbed molecules. Using low-temperature
scanning tunneling microscopy, the authors demonstrate that
substituted porphyrin molecules adsorbed on a gold surface form
monomers, trimers, tetramers, or extended wire-like structures.
The authors report that each structure corresponds in a
predictable fashion to the geometric and chemical nature of the
porphyrin substituents that mediate the interactions between
individual adsorbed molecules. The authors suggest their findings
indicate that careful placement of functional groups that are
able to participate in directed non-covalent interactions will
allow the rational design and construction of a wide range of
supramolecular architectures adsorbed to surfaces.

Nature 2001 413:619

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18. SUPRAMOLECULES AND BIOLOGICAL MOVEMENTS

Directed motion is one of the more dramatic characteristics of
many living systems, and the movements of simple organisms,
particularly of single-celled organisms, have fascinated
biologists ever since the invention of the microscope. Under a
microscope, a motile protozoan may appear as large as a rabbit,
but there are no muscles or nerves in the single cell that
constitutes such an organism, and the riddle is clear: How is
chemical energy transduced to directed mechanical and kinetic
energy in primitive biological systems? Until the era of the
electron microscope and molecular biology, little progress was
made in answering this question. That has changed: in recent
decades molecular biology has provoked a renaissance in studies
of cell movements. But if much has been learned and the questions
refined, our fascination with motion in primitive organisms has
grown rather than diminished. For it has become apparent that
directed motions in primitive biological systems are examples of
molecular-scale engineering that is often astonishing.
Vorticella, discussed below, is a ciliated protozoan common in
ponds, a single-celled organism that can be envisioned as
follows: Imagine a bell-shaped body 50 microns at its widest
part. The rim of the open end of the bell is covered with cilia
that beat synchronously to sweep water and nutrients into the
open end of the body. The closed dome end of the bell is attached
to a long thin stalk that may be 500 or more microns in length,
and the far base of the stalk is attached to a leaf or to pond
debris. When the organism is feeding, the stalk is extended. When
the organism is physically or chemically disturbed, the stalk
contracts like a spiral-shaped spring, quickly drawing the bell-
shaped body of the organism to the protection of the debris where
it is attached.     First described by Anton van Leeuwenhoek
(1632-1723), Vorticella is a legendary organism in biology. Many
children receive inexpensive microscopes as gifts when they are
ten or eleven years old, and these children often use their new
microscopes to examine everything available, including local pond
water. At the first sight of Vorticella -- the lovely bell-shaped
body with its synchronously beating cilia, the body at intervals
suddenly pulled back by the contracting spring of the stalk, the
stalk and body then slowly extending again with the cilia
resuming their synchronized beating -- such children are often
spellbound by the dynamic world of the small. If the fascination
endures, and if they are fortunate in life, they often become
biologists.

L. Mahadevan and P. Matsudaira (Massachusetts Institute of
Technology, US) present a review of recent research on cell
motility, the authors making the following points:

1) The retraction of the stalk of Vorticella (and of other
ciliates of this type: peritrich ciliates) is caused not by the
sliding action of a motor protein but by a spring that operates
according to a simple mechanism: the entropic collapse of
polymeric filaments. Although they are considered unusual engines
for motility, springs and ratchets composed of filaments and
tubules power many of the largest, fastest, and strongest
cellular and molecular movements. Just as muscles magnify forces
and movements by a geometrical hierarchy, these unusual
mechanochemical engines use a similar principle: small changes in
a protein subunit are amplified by the linear arrangement of
proteins in filaments and bundles. The authors suggest that,
considering the biochemical and physical characteristics of
several known molecular springs and ratchets, they apparently
represent ancient and biologically commonplace molecular engines.

2) In general, biological springs are active mechanochemical
devices that store the energy of conformation of proteins in
certain chemical bonds that act as latches. In the absence of an
external force, the potential energy is released and converted
into mechanical movement when the chemical bonds are broken.

3) The contractile avoidance reaction of Vorticella, first
described by Leeuwenhoek in 1676, is a dramatic example of an
active mechanochemical spring. The body of Vorticella is attached
to a leaf or to debris by a long slender stalk. Within the stalk
lies a rod-like helical cytoplasmic organelle, the "spasmoneme".
In its extended state, the spasmoneme is 2 to 3 millimeters long,
depending on the species of ciliate. When exposed to calcium
ions, but to no external energy source, the spasmoneme contracts
in a few milliseconds to 40 percent of its length at velocities
approaching 8 centimeters per second. Based on the hydrodynamics,
the force of contraction is of the order of a millidyne, whereas
the power generated is a few milliergs per second. In terms of
specific power per unit mass, the spasmoneme is among the most
powerful biological engines.

4) The authors state: "The dynamics and energetics of biological
springs and ratchets are dominated by factors that are
inconsequential on the large length scales associated with our
everyday world. In a [biological] cell, viscous forces, Brownian
motion, short-range hydrophobic interactions, screened
electrostatics, and steric effects influence the kinetics of
filament and subunit diffusion and growth. In this soft, wet, and
dynamic world, structural features are dominated by filamentous
and membranous objects, a constant reminder that all events at
this level are mediated by interfacial interactions."

Science 7 Apr 00 288:95

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