Multiple-quantum nuclear magnetic resonance spectroscopy

A nuclear magnetic resonance (NMR) event is popularly viewed as the flip of a single spin in a magnetc field, stimulated by the absorption or emission of only one quantum of radio-frequency energy. Nevertheless, resonances between nuclear spin states that differ by more than one unit in the Zeeman quantum number also can be induced in systems of coupled spins by suitably designed sequences of radio-frequency pulses. Pairs of states excited in this way oscillate coherently at the frequencies of the corresponding multiple-quantum transitions and produce a response that may be monitored indirectly in a two-dimensional time-domain experiment. The pattern of multiple-quantum excitation and response, influenced largely by the concerted interactions of groups of coupled nuclei, simplifies the NMR spectrum in some instances and provides significant new information in others. Applications of multiple-quantum NMR extend to problems in many different areas, ranging from studies of the structure and function of proteins and nucleic acids in solution to investigations of the arrangements of atoms in amorphous semiconductors. The specific spectroscopic techniques are varied as well and include methods designed, for example, to simplify spectral analysis for liquids and liquid crystals, eliminate inhomogeneous broadening, study interatomic connectivity in liquid-state molecules, identify clusters of atoms in solids, enhance the spatial resolution in solid-state imaging experiments, and probe correlated molecular motions.     

Two-dimensional nuclear magnetic resonance spectroscopy

Great spectral simplification can be obtained by spreading the conventional one-dimensional nuclear magnetic resonance (NMR) spectrum in two independent frequency dimensions. This so-called two-dimensional NMR spectroscopy removes spectral overlap, facilitates spectral assignment, and provides a wealth of additional information. For example, conformational information related to interproton distances is available from resonance intensities in certain types of two-dimensional experiments. Another method generates 1H NMR spectra of a preselected fragment of the molecule, suppressing resonances from other regions and greatly simplifying spectral appearance. Two-dimensional NMR spectroscopy can also be applied to the study of 13C and 15N, not only providing valuable connectivity information but also improving sensitivity of 13C and 15N detection by up to two orders of magnitude.     

High-resolution nuclear magnetic resonance of solids

The development of line-narrowing techniques, such as magic-angle spinning (MAS) and high-power decoupling, has led to powerful high-resolution nuclear magnetic resonance approaches for solid samples. In favorable cases (for instance, where high abundances of protons are present) cross polarization (CP) provides a means of circumventing the time bottleneck caused by inefficient spinlattice relaxation in many solids. The combined CP-MAS approach for carbon-13 with proton decoupling has become a popular and routine experiment for organic solids. For many nuclides with spin quantum number /> (1/2) the central nuclear magnetic resonance transition can be employed in high-resolution experiments that involve rapid sample spinning. A continuing stream of advances holds great promise for the use of high-resolution techniques for the characterization of solids by a wide range of nuclides.     

Force detection of nuclear magnetic resonance

Micromechanical sensing of magnetic force was used to detect nuclear magnetic resonance with exceptional sensitivity and spatial resolution. With a 900 angstrom thick silicon nitride cantilever capable of detecting subfemtonewton forces, a single shot sensitivity of 1.6 x 10(13) protons was achieved for an ammonium nitrate sample mounted on the cantilever. A nearby millimeter-size iron particle produced a 600 tesla per meter magnetic field gradient, resulting in a spatial resolution of 2.6 micrometers in one dimension. These results suggest that magnetic force sensing is a viable approach for enhancing the sensitivity and spatial resolution of nuclear magnetic resonance microimaging.     

Tumor detection by nuclear magnetic resonance

Spin echo nuclear magnetic resonance measurements may be used as a method for discriminating between malignant tumors and normal tissue. Measurements of spin-lattice (T(1)) and spin-spin (T(2)) magnetic relaxation times were made in six normal tissues in the rat (muscle, kidney, stomach, intestine, brain, and liver) and in two malignant solid tumors, Walker sarcoma and Novikoff hepatoma. Relaxation times for the two malignant tumors were distinctly outside the range of values for the normal tissues studied, an indication that the malignant tissues were characterized by an increase in the motional freedom of tissue water molecules. The possibility of using magnetic relaxation methods for rapid discrimination between benign and malignant surgical specimens has also been considered. Spin-lattice relaxation times for two benign fibroadenomas were distinct from those for both malignant tissues and were the same as those of muscle.     

High-resolution nuclear magnetic resonance of inorganic solids

Recent improvements in instrumentation and technique now permit the observation of high-resolution nuclear magnetic resonance spectra of many nuclei in inorganic solids. The application of nuclear magnetic resonance to the study of the structures of materials of interest in chemistry, earth science, and materials science are discussed together with a prognosis for future work.     

Polywater: proton nuclear magnetic resonance spectrum

In the presence of water, the resonance of the strongly hydrogenbonded protons characteristic of polywater appears at 5 parts per million lower applied magnetic field than water. Polywater made by a new method confirms the infrared spectrum reported originally.     

Positron tomography and nuclear magnetic resonance imaging

Noninvasive imaging methods for medical diagnosis and biological investigations, have evolved from qualitative radiological techniques to quantitative methods of measuring biochemical and physiological processes in human body. In particular, new developments in emission tomography, nuclear magnetic resonance imaging, and in vivo spectroscopy offer new horizons for medical research and clinical activities. These methods and their potentials are reviewed and contrasted.     

Some developments in nuclear magnetic resonance of solids

Nuclear magnetic resonance (NMR) spectroscopy continues to evolve as a primary technique in the study of solids. This review briefly describes some developments in modern NMR that demonstrate its exciting potential as an analytical tool in fields as diverse as physics, chemistry, biology, geology, and materials science. Topics covered include motional narrowing by sample reorientation, multiple-quantum and overtone spectroscopy, probing porous solids with guest atoms and molecules, two-dimensional NMR studies of chemical exchange and spin diffusion, experiments at extreme temperatures, NMR imaging of solid materials, and low-frequency and zero-field magnetic resonance. These developments permit increasingly complex structural and dynamical behavior to be probed at a molecular level and thus add to our understanding of macroscopic properties of materials.     

Quantum statistical corrections to dynamic nuclear magnetic resonance

A quantum statistical treatment of the chemical exchange between molecular eigenstates or conformations revealed previously unsuspected dynamic terms in the spin Hamiltonian operator that describes fast exchange. These terms resulted from the effect of nuclear spin on rotational and vibrational relaxation. With the traditional theory, an interpretation of new carbon-13 nuclear magnetic resonance measurements of the chemical shift of methylcyclohexane in solution showed fast-exchange equilibrium constants that were inconsistent with the slow-exchange free-energy difference and were spread over a range of 30 percent for the various carbon-13 positions. Modeling of the new terms indicated that they have the correct magnitude and temperature dependence to reconcile these inconsistencies.     

Recognition of cancer in vivo by nuclear magnetic resonance

Pulsed nuclear magnetic resonance has been used to differentiate in vivo between normal mouse tail tissue and a malignant transplanted melanoma, S91, located on the tail. The tumor displayed a nuclear (proton) spin-lattice relaxation time of approximately 0.7 second contrasted with the simultaneously measured normal tail tissue relaxation time of approximately 0.3 second.     

Studying enzyme mechanism by 13C nuclear magnetic resonance

High-resolution carbon-13 nuclear magnetic resonance (NMR) spectra of enzyme-inhibitor and enzyme-substrate complexes provide detailed structural and stereochemical information on the mechanism of enzyme action. The proteases trypsin and papain are shown to form tetrahedrally coordinated complexes and acyl derivatives with a variety of compounds artificially enriched at the site or sites of interest. These results are compared with the structural information derived from x-ray diffraction. Detailed NMR studies have provided a clearer picture of the ionization state of the residues participating in enzyme-catalyzed processes than other more classical techniques. The dynamics of enzymic catalysis can be observed at sub-zero temperatures by a combination of cryoenzymology and carbon-13 NMR spectroscopy. With these powerful techniques, transient, covalently bound intermediates in enzyme-catalyzed reactions can be detected and their structures rigorously assigned.     

High-resolution nuclear magnetic resonance spectra of selectively deuterated staphylococcal nuclease

An analog of staphylococcal nuclease has been prepared in which all amino acids, except the six following, are fully deuterated: tryptophan; methionine; tyrosine, in ring positions 2 and 6; histidine, in ring position 2; aspartic acid and asparagine, beta-methylene; and glutamic acid and glutamine, gamma-methylene. The analog has a much simpler high-resolution nuclear magnetic resonance spectrum than the fully protonated enzyme. The effects of calcium ion and of the inhibitor 3', 5'-thymidine diphosphate on the spectrum of the analog were readily detected.     

Biological phosphonates: determination by phosphorus-31 nuclear magnetic resonance

Advanced methods of phosphorus-31 nuclear magnetic resonance spectroscopy provided a method whereby biological phosphonates and phosphates can be determined on simple lipid fractions of biological origin. The spectra consist of two easily distinguished resonance bands; one corresponds to families of phosphonates, and the other corresponds to families of orthophosphates.     

Protein structure determination in solution by nuclear magnetic resonance spectroscopy

Knowledge of three-dimensional protein structures is one of the foundations of protein design and protein engineering. Nuclear magnetic resonance spectroscopy was recently introduced as a second method for protein structure determination, in addition to the well-established diffraction techniques with protein single crystals. This new approach enables one to carry out detailed structural studies of proteins in solution and other noncrystalline states, which may be similar or identical to the physiological environment, and promises new insights into the dynamics of protein molecules and the protein-folding problem.     

Effects of pulse shaping in laser spectroscopy and nuclear magnetic resonance

Pulsed excitation fields are routinely used in most laser and nuclear magnetic resonance (NMR) experiments. In the NMR case, constant amplitude (rectangular) pulses have traditionally been used; in laser spectroscopy the exact pulse shape is often unknown or changes from shot to shot. This article is an overview of the effects of radio-frequency and laser pulse shapes and the instrumental requirements for pulse shaping. NMR applications to selective excitation, solvent suppression, elimination of phase roll, and reduced power dissipation are discussed, as are optical applications to soliton generation, velocity selective excitation, and quantitative population transfer.     

Liquid water in frozen tissue: study by nuclear magnetic resonance

Nuclear magnetic resonance (NMR) spectroscopy was used to examine the behavior and extent of liquid water in postrigor-frozen tissue of cod at temperatures below 0 degrees C. A liquid-water phase persists in the tissue down to about -70 degrees C; the extent of the phase decreases rapidly between 0 degrees and -10 degrees C and slowly at lower temperatures. That the NMR absorption peak of the liquid water increases in width, with decreasing temperature, suggests loss of mobility or structuring of the phase. A technique for introducing geometrically uniform cores of muscle into the probe of the high-resolution spectrometer permits quantitative determinations of liquid water.     

High-resolution nuclear magnetic resonance spectroscopy in a circularly polarized laser beam

Nuclei in a fluid subjected to a continuous wave circularly polarized light beam are predicted to experience a static magnetic field proportional to E(+/-) x E(+/-), where E(+/-) is the electric vector of the right or left circularly polarized wave and the dot denotes a time derivative. The field strongly depends on the local electronic structure and is present in all atoms. For an intensity of 10 watts per square centimeter propagating in the direction of the field of a magnetic resonance spectrometer, the general theory presented here predicts shifts of +/- 4 x 10(-8) hertz for protons and +/- 10(-5) hertz for fluorine-19. Larger shifts are predicted if the laser frequency is near an optical absorption.     

Carbon-13 nuclear magnetic resonance study of osmoregulation in a blue-green alga

The process of osmoregulation in a unicellular blue-green alga, Synechococcus sp., has been studied by natural-abundance carbon-13 nuclear magnetic resonance spectroscopy of intact cells and cell extracts. 2-O-alpha-D-Glucopyranosylglycerol was identified as the major organic osmoregulatory solute. This demonstrates the presence of a major osmoregulatory solute in a blue-green alga and is also an example of an osmoregulatory role for glucosylglycerol.     

Natural abundance carbon-13 nuclear magnetic resonance spectra of human serum lipoproteins

Human serum lipoproteins have been studied by Fourier transform nuclear magnetic resonance of carbon-13 in natural abundance. Spectra of highdensity, low-density, and very-low-density lipoproteins were recorded and partly assigned. The prominent features of these spectra reflect the qualitative and quantitative composition of the lipid moiety of these complexes. The results suggest that carbon-13 nuclear magnetic resonance will be a useful technique for studies of the structural and dynamic parameters of lipoproteins.