Natural abundance ~(13)C REDOR coupled to a singly ~(15)N-labeled nucleus: simultaneous determination of interatomic distances in crystalline ammonium [~(15)N]L-glutamate monohydrate

摘要

REDOR technique was applied to natural abundance 13C nuclei coupled to a singly labeled 15N nucleus to determine the 13C, 15N interatomic distances simultaneously in crystalline ammonium (15N] L-glutamate monohydrate (1). Consequently. the interatomic C-N distances between !5N and IJC=O, 13Ca. 13CIJ' 13C'Y' and IJCS carbon nuclei for 1 were determined with a precision of j:O.15 A, after the experimental conditions such as the location of samples in the rotor, length of 1T pulse etc. were carefullyoptimized. IJC-REDOR factors for three spin system, (~SISo)cNIN2, and the sum of1wo isolated 2-spin system, (~SISo)" = (~SISO)CNI + (AS/So)CN2, were further evaluated by the REDOR measurements on isotopically diluted 1 in a controlled manner. Subsequently, the intra- and intermolecular C-N distances were separated by searching the minima in the contour map of root mean square deviation (RMSD) between the theoretically and experimentally obtained (~SISo)" values against two interatomic distances, TC-NI and TC-N2. When the intramolecular C-N distance (TC-NI) of the particular carbon nucleus is substantially shorter than the intermolecular one (TC-N2)' C~N distances within a single molecule were obtained with an accuracy of j:O.06 A as in the cases of C=O, Ca and CIJ carbon nuclei. C-N distances between the molecule in question and the nearest neighboring molecules can be also obtained, although accuracy was lower. On the contrary, it was difficult to determine the interatomic distances in the same molecule when the intermolecular dipolar contribution is larger than the intramolecular one as in the case of Cs carbon nucleus.