Robert A. McKay

Quantitative determination of the concentrations of 13C15N chemical bonds by double cross-polarization NMR

Methode de determination des liaisons chimiques entre 15 N et 13 C dans des composes doublement marques par ces isotopes en utilisant la double polarisation croisee en RMN

Phenylalanine ring dynamics by solid-state 13C NMR

Abstract The partial collapse of dipolar and chemical-shift tensors for carbons, both on and off the ring C2 symmetry axis of l -phenylalanine crystallized from two different solvent systems, confirms that a fraction of sites for one of the crystalline modifications permits high-frequency 180° ring flips. Analysis of the same tensors indicates the presence of other motions for both materials. These motions are reasonably well represented by a model involving restricted isotropic rotational reorientation, a motion which can be associated with looseness of packing in the crystals.

Heteropolypeptides from hydrogen cyanide and water? Solid state15N NMR investigations

Cross-polarization magic-angle spinning15NMR spectra have been used to determine the composition of hydrogen cyanide polymers both before and after treatment with water. The unambiguous presence of secondary amide groups (as in peptide links) has been established by double-cross-polarization studies on the polymers synthesized from equimolar amounts of H13CN and HC15N. The NMR results are consistent with the hypothesis that the original heteropolypeptides on Earth were synthesized directly from hydrogen cyanide and water without the intervening formation of α-amino acids.

Applications of high-resolution 13C and 15N n.m.r. of solids

The combination of cross polarization, dipolar decoupling and magic angle spinning results in liquid-like high-resolution 13C and 15N n.m.r. spectra of a wide variety of solid materials. Structural determinations based on such 13C n.m.r. spectra include the measurement of the extent to which pyrolysed polyacrylonitrile fibres (Orion) retain aliphatic character during the first step of the production of a carbon fibre, the determination of the chemical identity of the cross links formed from an acetyleneterminated polyimide resin, and the characterization of the metabolic products of a bacterial fermentation of wood lignin. All of these non-destructive analyses are performed on intact heterogeneous samples. The high resolution of the carbon experiment can also be exploited by obtaining proton spin-lattice relaxation parameters for chemically different protons in solids. Because of spin diffusion, these parameters are dependent on spatial proximity and so are helpful in measuring the homogeneity of solid blends of polymers such as poly(phenylene oxide) and polystyrene. High-resolution 13C n.m.r. spectra of polymers can also be used for measuring microscopic chain dynamics. 13C rotating-frame relaxation parameters observed for polycarbonate and poly (ethylene terephthalate) are related to the effects on motion of annealing, additives and structural substitutions. Individual relaxation rates are observed for individual carbons, so the behaviour of side groups is cleanly separated from that of the main chain. All of the line-narrowing and sensitivity-enhancing techniques applied to 13C n.m.r. of solids work equally well for 15N n.m.r. Use of 15N rotating-frame and cross-polarization parameters leads to the assessment of the relative concentrations of 13C -15N and 12C -15N pair concentrations in the main chains of multiply labelled proteins. Such measurements can be used to characterize the rate of protein turnover in fully expanded soybean leaves, as well as the details of protein synthesis in cultured soybean cotyledons.

Natural Abundance N-15 NMR of the Solids from the Reaction of HCN and Ammonia

Natural-abundance cross-polarization 15N NMR spectra of the complex solids from the reaction of HCN and ammonia have been obtained at 9.12 MHz with magic-angle sample spinning at 1.5 kHz. The spectra are interpreted in terms of the relative concentrations of nitrile-, aromatic-, amide-, urea-, and ammonia-like nitrogen present. The spectrum of the water-insoluble polymeric fraction of the reaction products shows a diversity of nitrogen functionality as broad as that observed for an isolated bacterial RNA.

HIGH‐RESOLUTION SOLID‐STATE NMR OF GLASSES

High-resolution nuclear magnetic resonance is widely recognized as a powerful tool for studying molecular structure and dynamics. Applications of this technique have traditionally required liquid samples, in which rapid, isotropic motion of the molecules averages the local interactions to produce sharp resonances. But in some situations, it is the properties of a sample in the solid state that are of interest. Glasses, the topic of this conference, provide an important example: N M R studies of the material as a liquid or in solution do not shed any light on the nature of the glassy state. In the past few years, the sensitivity-enhanced cross-polarization (CP) N M R experiment for natural abundance I3C in organic solids has been introduced and combined with magic-angle spinning (MAS) to generate liquid-like high-resolution rare-spin N M R of This combination (CP/MAS) experiment has gained popularity because of its inherent simplicity and because of the valuable information obtainable from the resulting spectra concerning the structure and dynamics of solid materials. With the recent commercial availability of CP/MAS spectrometers, even more widespread application of I3C N M R by solid-state materials scientists is likely. In contrast, although considerable progress has been made in high-resolution H N M R of solids, this experiment has not enjoyed popularity. The multiple-pulse techniques used for line narrowing are difficult, and the results have seldom been of practical utility.6 In this paper, we will illustrate the kinds of information presently being obtained using cross-polarization magic-angle spinning 13C NMR. After briefly reviewing the physical principles associated with the line-narrowing and sensitivity-enhancing techniques, we will show examples, selected from studies of complicated solid systems of practical importance-generally amorphous polymers-which highlight the value of rare-spin N M R in the study of glassy materials.