T1 and T2 relaxations of the 13C nuclei of deuterium-labeled nucleosides

The effect of H-2 on C-13 longitudinal (T-1) and transverse (T-2) relaxation parameters was determined for the first time for diastereospecifically deuterium-labeled nucleosides, which are used as the building blocks for non-uniform isotope labeling for the solution NMR structure determination of the large biologically functional oligo-DNA and -RNA ('NMR window' approach, ref. 7). It emerged that the T-1 and T-2 of the deuterated methine carbon in the diastereospecifically deuterium-labeled nucleoside 9 could be used as the correction term to give the monoexponential decay of C-13 longitudinal and transverse magnetization of the constituent H-1-C-13-H-2 group. The correlation time derived from this corrected T-1 of the methylene carbon corresponds well with the correlation time obtained from deuterium relaxation study. The extreme narrowing limit (omega tau(c) << 1) where dipole-dipole (DD) relaxation of C-13 and quadrupole (Q) relaxation of H-2 are related by T-1(DD)/T-2(DD) approximate to 1 and T-1(Q)/T-2(Q) approximate to 1 was used to demonstrate the above conclusion. The difference in the observable T-1 and T-2 in various methylene and methine-type carbons with either fully protonated or diastereospecifically deuterated nucleosides 1-14 allowed the estimation of the contribution of the alternative relaxation pathways other than DD relaxation. It was found by comparison of the T-1 relaxation of the quaternary carbon with the methine carbon (C-13-H-2) or (C-13-H-1) in compound 2 that the contribution of the intermolecular and intramolecular relaxations of C-13 With protons that are two bonds away is larger than DD(C-13-H-2), and the sum of all these contributions define the T-1 of the methine carbon (C-13-H-2). The observed difference between the experimental T-1 and T-2 of the methine carbon is attributed to the cross-correlation between DD(C-13-H-2) and Q(H-2) relaxation, which is consistent with recent theoretical predictions. For T-2 measurement, the decoupling of deuterium with 0.6-2.5 kHz power during the echo period by WALTZ does not effectively eliminate the DD(C-13-H-2)-Q(H-2) cross-correlation for the methine carbon. The suppression of this DD(C-13-H-2)-Q(H-2) cross-correlation was, however, more effective by applying a 180 degrees deuterium pulse in the middle of the short (0.5 ms) echo period (compare T-2 of 3.91s and 0.39, respectively, at 294 K using these two different decoupling procedures). The comparison of the observed T-1 and T-2 relaxations of the methylene carbon shows that they are indeed very close. The various contributions of the methine carbon relaxation such as DD(C-13-H-2), intermolecular and cross-correlation, DD(C-13-H-1)-Q(H-2), to the relaxation of the methylene carbon were ca. 15% in T-1 and ca. 25% in T-2. (C) 1998 John Wiley & Sons, Ltd.