Va. Daragan et Kh. Mayo, ASYMMETRIC C-13 NMR MULTIPLET RELAXATION AND DIPOLAR-CSA CROSS-CORRELATION FOR GLYCINE C-13(ALPHA) METHYLENES IN PEPTIDES, Chemical physics letters, 206(1-4), 1993, pp. 393-400
Chemical shift anisotropy (CSA) can be an important contribution to nu
clear spin relaxation. This is particularly the case for C-13 at high
magnetic field strength. Normally, CSA is treated as an often discarde
d correction in NMR relaxation data analysis. Here, CSA is exploited i
n the C-13 multiplet effect of methylene in ethanol, glycine, and synt
hetic peptides which have been selectively C-13-enriched in terminal a
nd internal glycine positions. Simple inversion-recovery experiments d
emonstrate large differences between relaxation rates of left and righ
t outer lines of C-13 triplet NMR spectra at 90 and 150 MHz. Model ana
lyses are performed where dipolar and dipolar-CSA relaxation mechanism
s are taken into account. Both the restricted rotational diffusion mod
el and the model-free approach are able to describe the experimental s
ign and amplitude of the dipolar-CSA cross correlation at some orienta
tion of the CSA tensor with respect to the molecular frame. The popula
r wobbling-in-a-cone model, however, gives the opposite sign of this c
ross-correlation function under all conditions examined.