Single-crystal samples of the peptide N-acetyl-D,L-valine (NAV) with the exchangeable amide and carboxyl hydrogens replaced by deuterons were investigated by deuterium NMR spectroscopy. The electric field gradient (EFG) and chemical shift (CS) tensors for the amide and the carboxyl hydrogens, both of which are involved in intermolecular hydrogen bonds, were determined. The relationship of these tensors to the structure of NAV is discussed. The orientations of the EFG and the CS tensors of the amide hydrogen in a protein can provide information about the polypeptide backbone structure. For NAV the eigenvector of the largest eigenvalue of the EFG tensor coincides with the NH bond direction, as found by X-ray crystallography, within 2°, and the eigenvector of the intermediate eigenvalue with the normal (n peptide) to the peptide plane within 1°. The directions of largest and smallest shielding are similarly aligned with NH and n peptide respectively. However, the deviations are considerably larger, namely 9° and 11°, respectively. The anisotropy of the CS tensor is determined to be 13.4 +− 2.7 ppm, confirming that the amide hydrogen is involved in a weak hydrogen bond
2H-nuclear magnetic resonance was applied to single crystals of the clathrate of Dianin’s compound with ethanol as a guest. The hydroxyl groups of both the host lattice and the guest were deuterated. The temperature dependences of the 2H spectra and of the quadrupole coupling tensors of the host lattice deuterons provide evidence that these deuterons carry out jumps between two unequally populated sites. These jumps may be visualized as approximate rotations of the hydroxyl groups about the C–O bonds. The occupancies of the two sites were measured and found to follow a Boltzmann distribution at T>145 K. At lower temperatures the minority sites are overpopulated. The temperature dependence of the jump rate was determined by line shape analyses and relaxation time measurements. From the absence of dipolar fine structure in the 2H spectra recorded for specially chosen crystal orientations it is concluded that groups of six hydroxyl units carry out their rotational jumps in a concerted manner.
In the clathrates of Dianin's compound the guests are trapped in cages formed by six host molecules. Two ethanol molecules can be enclosed by each cage. Deuteron nuclear magnetic resonance spectra of single crystals of Dianin's compound with ethanol-OD, ethanol-l,l-D2 and ethanol-2,2,2-D 3 as a guest were recorded in the temperature range 10K < T< 310K. Only at T < 25 K the guests are found to be immobile enough to allow the determination of the bond directions of the different deuterons from the unique eigenvectors of the quadrupole coupling (qc) tensors. The measured qc tensors of the hydroxyl and of the methylene deuterons are used to determine the orientations of the ethanol molecules as a whole. These can occupy six distinct sites. The two ethanol molecules in a given cage are linked by a weak hydrogen bond. The dynamics of the ethanol guests is elucidated by line shape analyses and by deuteron two-dimensional exchange spectroscopy. It is found to be rather complex, consisting of three different types of large-angle reorientations of the entire ethanol molecules, of internal reorientations of the methyl groups about their local C3 axes and of additional librational and torsional motions. The average equilibrium orientations of the ethanol molecules change with increasing temperature. The observed dynamics implies that the host lattice of the clathrate possesses a considerable degree of flexibility, in particular it must enable the two ethanol molecules within a given cage to exchange places, or to diffuse from one cage to the next.
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