E. O. Stejskal

Magic-angle spinning and polarization transfer in proton-enhanced NMR

Abstract High-speed spinning at the magic angle can significantly modify the rate of polarization transfer from abundant to rare spins in proton-enhanced NMR experiments on solids, if the spinning speed is greater than or comparable to the static dipole-dipole interaction among abundant spins in the rotating frame. In adamantane, this fact can be strikingly demonstrated experimentally. When the spinning speed is much less than dipolar coupling among abundant spins, the effect of spinning on polarization transfer is not dramatic, regardless of the nature of the static dipolar coupling between rare and abundant spins. A semiquantitative theoretical analysis is presented which describes the principal features of these phenomena in terms of the amplitude and frequency modulation of the dipolar interactions between and among the two spin systems. This analysis of the effect of coherent motion (spinning) on polarization transfer also provides some qualitative insight into how incoherent random molecular motion affects transfer in cross-polarization experiments.

Prospects for carbon-13 nuclear magnetic resonance analysis of solid fossil-fuel materials

Abstract Because of excessive line broadening in solids due to magnetic dipole—dipole interactions and chemical shift anisotropies, and because of long spin-lattice relaxation times, standard continuous wave (CW) or pulse Fourier transform techniques do not normally yield structurally informative 13 C n.m.r. spectra from solid samples. However, the techniques of dipolar decoupling, cross polarization and magic-angle spinning show great promise for routine 13 C n.m.r. studies of solids. Applications of these techniques to analysis of anthracite, lignite and algal coal samples, and to oil shale and kerogen samples, are discussed. It is shown that cross-polarization spectra obtained with dipolar decoupling display chemical shift anisotropy which interferes with attempts to distinguish the resonances of aromatic and aliphatic carbons. However, with magic-angle spinning the distinction can be made. Prospects and potential difficulties with applications of these techniques are discussed.

Comparisons of quadrature and single-phase fourier transform NMR

Abstract Reflections of lines in time-averaged Fourier transform NMR spectra obtained by imperfect quadrature detection can be removed either by 90° rf phase shifting of the excitation pulse together with simple data routing, or by phase and amplitude manipulations of the two imperfect free induction decays after completion of data accumulation. The sensitivity of the resulting reflection-free spectra is comparable to that of spectra obtained by single-phase detection using a rf crystal filter to remove aliased noise. However, since the quadrature experiment involves data sampling rates half as fast as the comparable single-phase experiment, distortions associated with the finiterecovery time of the spectrometer areless severe. Even for fast-recovery spectrometers, asymmetric spectral distortions in Fourier transform experiments involving moderately broad lines can occur. In quadrature detection schemes, these distortions are less severe since they are not folded through the entire spectral width as occurs in single-phase experiments.

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

Magic‐angle carbon‐13 NMR study of CO2 adsorbed on some molecular sieves

Carbon‐13 NMR spin‐lattice relaxation times of isotopically enriched CO2 adsorbed on a variety of channel and cavity molecular sieves have been obtained. At room temperature, the spin‐lattice relaxation of adsorbed CO2 is dominated by paramagnetic impurities in commercial molecular sieves with critical cage dimensions less than about 8 A, but by spin rotation in some sieves with larger cage dimensions. Asymmetric 13C line shapes are generally observed for CO2 adsorbed in these molecular sieves. This asymmetry is, for the most part, independent of the presence of paramagnetics, but is instead generated by the local microscopic magnetic susceptibility heterogeneities of the host sieve. Substantial narrowing of the asymmetric lines can be achieved by magic‐angle spinning. For CO2 adsorbed on Na+‐mordenite, spinning experiments at 3 kHz reveal only a single symmetric line, consistent with the great majority of CO2 molecules in the system being physi‐ rather than chemisorbed.

Carbon-13 nuclear magnetic resonance measurement of oil composition in single viable soybeans

High-resolution, natural abundance13C NMR spectra of intact, viable soybeans have been obtained by Fourier transform techniques. The spectra are interpreted in terms of the relative concentrations of the major fatty acids present in the soybean triglycerides. This nondestructive analysis is sufficiently fast and accurate to permit the selection of individual soybeans for use in a genetic breeding program designed to reduce the undesirably high linolenic acid content of soybean oil.

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.

Analysis of double cross-polarization rates in solid proteins

Abstract The double cross-polarization experiment involving the sequential transfer of nuclear polarization among 1H-, 15N-, and 13C-spin systems can be used to detect the occurrence of 13C-15N bonds in solid proteins. However, the complexity of most proteins requires simplifying approximations to make possible the determination of the concentrations of those 13C-15N bonds. This paper demonstrates that a model involving only two 13C-15N cross-polarization rates (determined from model compounds) and simple statistics is adequate to treat data from heavily 13C- and 15N-labeled proteins in soybean leaves. In the course of examining the applicability of model compounds to this problem, we have used a moment analysis of 15N NMR lineshapes to determine 13C-15N dipolar couplings in two doubly labeled amino acids, glycine and asparagine.

Carbon-13 nuclear magnetic resonance analysis of intact oilseeds

High resolution natural abundance13C nuclear magnetic resonance (NMR) spectra of intact oilseeds have been obtained by Fourier transform techniques. The spectra can be interpreted in terms of the relative concentrations of the major fatty acids in the oilseed. The13C NMR spectra are well resolved despite the fact that1H NMR spectra of the same seeds are poorly resolved. The difference in resolution can be attributed to the simplicity of the13C NMR spectra in which all spin-spin coupling can be removed, in which the separation of lines is increased by a factor of 30, and in which line broadening due to intermolecular dipolar interactions is not important. The13C NMR Fourier transform technique is sufficiently sensitive that a high quality spectrum can be obtained from a single soybean, for example, in about 10 minutes. Similar spectra of single seeds can be obtained in comparable times for corn, castor bean, peanut, sunflower seed, and rapeseed. Because the NMR technique is nondestructive, it can be used to select individual oilseeds for use in breeding programs designed to improve oil quality. By employing some special experimental NMR line narrowing techniques, it also appears feasible to obtain moderately well resolved, natural abundance13C NMR spectra of the immobile, rigid protein, and carbohydrate components of an intact oilseed, as well as the more mobile oil components.

Cross-polarization nmr of N-15 labeled soybeans

Abstract Cross-polarization 15N nmr spectra of 15N-labeled soybean seeds, pods, and leaves have been obtained at 9.12 MHz both with and without high-speed sample rotation at the magic angle. Spectral resolution is sufficient to permit a determination of the relative concentrations of amide and amine nitrogens, as well as of a few specific amino acid residues of proteins in the solid, intact samples. Utilization by soybean of nitrogen from labeled fertilizer in the presence of dinitrogen fixation can be determined from these spectra. A double-cross polarization 13C nmr spectrum of a spinning, 15N-labeled seed has been obtained in which resonances are observed only from these carbons directly bonded to nitrogens. This technique leads to a qualitative estimate of amino-acid composition of the protein which is complementary to that obtained directly from the 15N nmr spectrum.