ScienceWeek

CHEMISTRY: INTERACTIONS AT DECORATED LIQUID-CRYSTAL SURFACES

The following points are made by J.M. Brake et al (Science 2003 302:2094):

1) Some of the most important biomolecular interactions occur at biological membranes, including the binding events that permit entry of protein toxins into cells (1), the binding and enzymatic events that trigger cell signaling pathways (2), the assembly of proteins, lipids, and cholesterol into rafts (3), the crystallization of proteins (4), and the binding events that are the first stage of viral infection (5).

2) Past attempts to provide facile methods of reporting these biomolecular interactions (e.g., for biological sensing) have exploited the self-assembly of the constituents of biological membranes such as phospholipids and proteins at interfaces (1 5). These systems, however, have been difficult to analyze because they generally require the use of either labeled molecules (e.g., fluorescent labels) or complex instrumentation.

3) The work of the authors was inspired by the observation that most biomolecular interactions at biological membranes are accompanied by a reorganization of the proteins, lipids, and other species that constitute the membranes (1 5). The authors report that fluid, phospholipid assemblies formed at interfaces between thermotropic liquid crystals (LCs) and aqueous phases are coupled to the orientations of the thermotropic LCs. The specific binding of proteins to these interfaces and their subsequent formation of organized lateral assemblies, as well as the activities of enzymes, are demonstrated to trigger spatially patterned orientational transitions in the LCs that are readily imaged with polarized light. This coupling permits label-free imaging of a range of dynamic molecular phenomena that occur at these interfaces. Because aqueous streams can also flow past the lipid-laden interface of the LC, these principles may also provide the basis of low-cost, passive (zero-power) indicators of the presence of targeted biological species.

4) In summary: The spontaneous assembly of phospholipids at planar interfaces between thermotropic liquid crystals and aqueous phases gives rise to patterned orientations of the liquid crystals that reflect the spatial and temporal organization of the phospholipids. Strong and weak specific-binding events involving proteins at these interfaces drive the reorganization of the phospholipids and trigger orientational transitions in the liquid crystals. Because these interfaces are fluid, processes involving the lateral organization of proteins (such as the formation of protein- and phospholipid-rich domains) are also readily imaged by the orientational response of the liquid crystal, as are stereospecific enzymatic events. The authors suggest these results provide principles for label-free monitoring of aqueous streams for molecular and biomolecular species without the need for complex instrumentation.

References (abridged):

1. X. Song, B. I. Swanson, Anal. Chem. 71, 2097 (1999)

2. R. V. Stahelin, W. Cho, Biochem. J. 359, 679 (2001)

3. C. Dietrich et al., Biophys. J. 80, 1417 (2001)

4. A. P. Gast, C. R. Robertson, S.-W. Wang, M. T. Yatcilla, Biomol. Eng. 16, 21 (1999)

5. D. H. Charych, J. O. Nagy, W. Spevak, M. D. Bednarski, Science 261, 585 (1993)

Science http://www.sciencemag.org

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SELF-ASSEMBLY OF PHASE-SEGREGATED LIQUID CRYSTAL STRUCTURES

The following points are made by Takashi Kato (Science 2002 295:2414):

1) Molecules in the liquid crystalline (LC) state, although still mobile, have an orientational ordering that depends on external conditions, such as temperature or electric fields (1,2). The best known example would be the thermotropic LC materials, which exhibit different phases as a function of temperature, that are used in displays (3). In contrast to these high-tech materials, biological self-assembled systems consist of a variety of discrete molecules forming heterogeneous and hierarchical structures (structures within structures) on various scales of length ranging from the nanometer to the micrometer scale. For example, lipids exhibit lyotropic LC states in which phase formation depends on solution concentration (4). Biological cell membranes are formed through self-assembly of lipids with other biomolecules such as proteins and steroids.

2) Although the intrinsic ordering within a LC material can be controlled by external parameters, the ordering can also be controlled internally through weak interactions to create more complex structures that add functionality to the material. One approach is to obtain self-assembled structures from a variety of different molecules (2,5). Another is to form phase-segregated structures of "block" molecules of longer polymers and smaller molecules on the nanometer and micrometer scale (2). The control of intermolecular interactions (noncovalent interactions) within these structures is critical because several competing effects are at work.

3) In the formation of the self-assembled LC structures, specific intermolecular interactions such as hydrogen bonding and ionic interactions play key roles. A notable example is the formation of supramolecular liquid crystals (5). In the case of these materials, well-defined structures such as rods and disks are made through self-assembly of two or more complementary molecular components by hydrogen bonding. For example, benzoic acids and pyridines form supramolecular rod-like complexes exhibiting thermally stable nematic and smectic (layered) LC phases (5). Moreover, identical molecules self-assemble into unconventional LC complexes. Studies of these materials have initiated and stimulated the development of supramolecular polymers.

4) In summary: Additional functionality can be incorporated into liquid crystalline materials by using phase segregation and self-assembly. Intermolecular interactions such as hydrogen bonding and ionic interactions play key roles in the formation of these complex structures. One-, two-, and three-dimensional phase-segregated structures on various scales of length are formed by self-assembly of a variety of partially incompatible molecules. Such structures can enhance anisotropic properties such as ionic conductivity.

References (abridged):

1. D. Demus, J. W. Goodby, G. W. Gray, H.-W. Spiess, V. Vill, Eds., Handbook of Liquid Crystals (Wiley-VCH, Weinheim, Germany, 1998)

2. J. W. Goodby, Curr. Opin. Solid State Mater. Sci. 4, 361 (1999)

3. J. H. Haaren and D. Broer, Chem. Ind. 1998, 1017 (1998)

4. C. E. Fairhurst, S. Fuller, J. Gray, M. C. Holmes, G. J. T. Tiddy, in (1), vol. 3, chap. 7.

5. T. Kato, Struct. Bonding (Berlin) 96, 95 (2000)

Science http://www.sciencemag.org

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OBSERVATION OF SURFACE AND BULK PHASE TRANSITIONS IN NEMATIC LIQUID CRYSTALS

The following points are made by M.I. Boamfa et al (Nature 2003 421:129):

1) The behavior of liquid crystal (LC) molecules near a surface is of both fundamental and technological interest: it gives rise to various surface phase-transition and wetting phenomena(1-5), and surface-induced ordering of the LC molecules is integral to the operation of LC displays.

2) In a confined geometry, interactions between LC molecules and the surfaces of the confining walls give rise to different LC behavior at the surface and in the bulk. Upon undergoing the IN transition the delicate balance of intermolecular interactions tends to favor the LC surface interaction in the nematic phase, whereas the LC LC interaction tends to dominate in the isotropic phase. The new nematic phase therefore occurs first at the surface, a phenomenon known as "wetting"(1,2). The IN transition is relatively weak thermodynamically, so large pre-transitional effects take place before the transition. The interaction of these pre-ordered LC molecules with the surface is known as "pre-wetting"(2). In a magnetic field the diamagnetic LC molecules undergo a torque that forces them to orient relative to the field direction. Although at the level of individual molecules the magnetic field effect is negligibly small in the isotropic phase, the collective behavior of the mesophase causes the molecular contribution to add up constructively, resulting in a macroscopic orientation effect.

3) The authors report the observation of a pure isotropic nematic (IN) surface phase transition -- clearly separated from the bulk IN transition -- in a nematic LC on a substrate. Differences in phase behavior between surface and bulk are expected(1-4), but have hitherto proved difficult to distinguish, owing in part to the close proximity of their transition temperatures. The authors have overcome these difficulties by using a mixture of nematic LCs: small, surface-induced composition variations lead to complete separation of the surface and bulk transitions, which the authors then study independently as a function of substrate and applied magnetic field.

4) The authors find the surface IN transition to be of first order on surfaces with a weak anchoring energy and continuous on surfaces with a strong anchoring. The authors demonstrate that the presence of high magnetic fields does not change the surface IN transition temperature, whereas the bulk IN transition temperature increases with field. The authors attribute this to the interaction energy between the surface and bulk phases, which is tuned by magnetic-field-induced order in the surface-wetting layer.

References (abridged):

1. Cahn, W. J. Critical point wetting. J. Chem. Phys. 66, 3667-3670 (1977)

2. Nakanishi, H. & Fisher, M. E. Multicriticality of wetting, prewetting, and surface transitions. Phys. Rev. Lett. 49, 1565-1568 (1982)

3. Sheng, P. Phase transition in surface-aligned nematic films. Phys. Rev. Lett. 37, 1059-1062 (1976)

4. Sluckin, T. J. & Poniewierski, A. Novel surface phase transition in nematic liquid crystals. Phys. Rev. Lett. 55, 2907-2910 (1985)

5. Yokohama, H. et al. Boundary dependence of the formation of new phase at the isotropic-nematic transition. Mol. Cryst. Liq. Cryst. 99, 39-52 (1983)

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