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ScienceWeek
CHEMISTRY: ON OBSERVATIONS OF INTERNAL COMBUSTION
NMR spectroscopy: "Nuclear magnetic resonance" refers to the absorption of electromagnetic radiation at a characteristic precise frequency by a nucleus with a nonzero magnetic moment in an external magnetic field. The phenomenon occurs if the nucleus has nonzero spin, in which case it behaves as a small magnet in an external magnetic field. In quantum mechanics, electrons, protons, and neutrons have an intrinsic angular momentum known as "spin", and a magnetic moment parallel or antiparallel to that angular momentum. When electrons are combined together to form an atom or ion, there is a resultant angular momentum which is a combination of the intrinsic spin of the electrons and the angular momentum due to their motion about the nucleus, and this is the "spin" of the atom or ion. Atoms or ions with non-zero spin are magnetic atoms or ions. NMR spectroscopy is the main application of NMR, the technique widely used for chemical analysis and structure determination.
The following points are made by Jeffrey Reimer (Nature 2003 426:508):
1) It was a burning candle that introduced generations of scientists to the discipline of observation. From the glow of the wick to the hues and flickers of the flame, combustion became the point of entry into the world of experimental science. Combustion continues to be a passion for many, who are drawn to the rich interplay of kinetics, thermodynamics, and transport phenomena that describe modern combustion technologies.
2) Despite such allure, in situ experimental observations of combustion remain difficult in environments inaccessible to light. Measurements of reactions within porous media are particularly problematic. For example, many reactors are filled with solid particles to combust fuel catalytically (thereby reducing the combustion temperature and lowering emission of environmentally harmful nitrogen oxides); but the presence of these solids makes experimental access to temperature, pressure and composition inside the reactor very difficult. Looking to the future, nanoscale combustion engines embedded in a silicon chip will not be easily monitored using current combustion diagnostics.
3) Anala et al(1) have demonstrated the potential of nuclear magnetic resonance (NMR), one of the most effective toolkits in experimental science, to study combustion reactions and transport in opaque media. Magnetic resonance methods, both spectroscopic and imaging, rely on the collective observation of the angular momentum of around 10^(18) nuclei. Such huge numbers of nuclei are needed because the energy differences between the various angular-momentum states of individual nuclei -- which are the basis of the measurement -- are small compared with the thermal energy available at room temperature. But this requirement hinders the acquisition of quantitative NMR data in gas-phase systems. To counteract this, Anala et al(1) have exploited a clever technique: xenon gas can be prepared so that individual atoms have angular momenta characteristic of extremely low temperatures, and then delivered to the NMR experiment at ambient temperatures(2); using this "hyperpolarized" xenon overcomes the sensitivity limitations inherent in such measurements.
4) Anala et al(1) were thus able to detect NMR signals from xenon atoms in a methane flame burning inside a porous material (a zeolite molecular sieve). Their measurements reveal differing zones of temperature and pressure within the flame. For example, from the subtle shifts in the many resonance frequencies of the xenon atoms, the authors determined the temperature changes experienced by xenon atoms inside the micropores of the material. They also noted slight changes in pressure experienced by xenon atoms in and above the bed of porous material.(3-5)
References (abridged):
1. Anala, S. et al. J. Am. Chem. Soc. 125, 13298 13302 (2003)
2. Goodson, B. J. Magn. Reson. 155, 157 216 (2002)
3. Ernst, R. R., Bodenhausen, G. & Wokaun, A. Principles of Nuclear Magnetic Resonance in One and Two Dimensions (Oxford Univ. Press, 1987)
4. Natterer, J. & Bargon, J. Prog. Nucl. Magn. Reson. Spectrosc. 31, 293 315 (1997)
5. Augustine, M. P., TonThat, D. M. & Clarke, J. Solid State Nucl. Magn. Reson. 11, 139 156 (1998)
Nature http://www.nature.com/nature
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XENON NMR SPECTROSCOPY AND PROTEIN CONFORMATION CHANGES
The following points are made by S.M. Rubin et al (J. Am. Chem. Soc. 2001 123:8616):
1) Since the observation of a xenon-binding site in the interior of myoglobin in 1965, the interaction of proteins with xenon have been studied with the goal of using this inert gas as a biomolecular probe. Xenon is relatively small and hydrophobic, and it binds weakly to the nonpolar interiors of many proteins with little perturbation of the structure of the protein. The sensitivity of the xenon *chemical shift to its local environment has motivated magnetic resonance studies of xenon in biological systems. More recently, the intense xenon-129 NMR signals attainable with *optical pumping techniques have been used to probe cavities in *lyophilized *lysoenzyme and *lipoxygenase, detect blood oxygenation levels, and identify xenon binding sites in a lipid transfer protein by the *spin-polarization induced *nuclear Overhauser effect.
2) The authors report the dependence of the xenon-129 chemical shift on the specific native conformation of a maltose-binding protein from the bacterium Escherichia coli. This protein is a *periplasmic protein in *gram-negative bacteria that plays a role in active transport and serves as an initial receptor for *chemotaxis. The authors suggest that the ability to discriminate protein conformation via the xenon-129 chemical shift indicates the potential use of xenon-129 NMR for direct assessment of protein functional states and ligand binding events.
J. Am. Chem. Soc. http://pubs.acs.org/JACS
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Notes:
chemical shift: In this context, a change in the normal wavelength of absorption or emission due to a change in chemical environment.
optical pumping: In general, the use of a specific-frequency light source to change energy distributions in atomic or molecular systems from equilibrium distributions to nonequilibrium distributions.
lyophilized: In this context, a "lyophilized" protein is a protein maintained in a freeze-dried state, i.e., a state in which a vacuum has been used to remove water by sublimation.
lysoenzyme: a protein catalyst that breaks down cells and/or macromolecules.
lipoxygenase: In general, any member of a group of enzymes that catalyze the oxidation of polyunsaturated fatty acids.
spin-polarization: A phenomenon in which the unpaired electron spin on one atom or part of a molecule is transferred to another atom by interaction between the unpaired and paired electrons.
nuclear Overhauser effect: In nuclear magnetic resonance spectroscopy, a phenomenon in which saturation of electron resonance by the application of a radio frequency field causes a marked increase in nuclear polarization and in the produced NMR signal.
periplasmic protein: The "periplasmic space" is the space between the outer and inner membranes of gram-negative bacteria.
gram-negative bacteria: Most bacteria can be classified into two types, depending on the chemistry of their outer coat, which chemistry determines whether a bacterium will admit certain dyes into the interior. The classification, according to the differential staining technique, is "gram-negative" vs. "gram-positive", named after the bacteriologist H.C. Gram (1853-1938). Gram-positive bacteria take up a crystal violet stain and turn purple, while gram-negative bacteria exclude the crystal violet and counterstain instead with stains such as safranin, eosin red, or brilliant green. As might be expected, since the technique differentiates the outer coats of bacteria, some antibiotics are effective against one type and not the other type, and vice versa. In general, gram-positive bacteria have a structure consisting of a cytoplasmic core, a plasma membrane, and a rigid external capsule. Gram-negative bacteria, however, have two plasma membranes between the inner cytoplasmic core and the external capsule: the two plasma membranes are separated by a "periplasmic space" packed with various enzymes.
chemotaxis: In general, the term "chemotaxis" refers to any movement of an organism in response to chemical concentration gradients.
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