Chemical shift anisotropy (CSA) can be an important contribution to nuclear spin relaxation. This is particularly the case for 13C at high magnetic field strength. Normally, CSA is treated as an often discarded correction in NMR relaxation data analysis. Here, CSA is exploited in the 13C multiplet effect of methylene in ethanol, glycine, and synthetic peptides which have been selectively 13C-enriched in terminal and internal glycine positions. Simple inversion-recovery experiments demonstrate large differences between relaxation rates of left and right outer lines of 13C triplet NMR spectra at 90 and 150 MHz. Model analyses are performed where dipolar and dipolar-CSA relaxation mechanisms are taken into account. Both the restricted rotational diffusion model and the model-free approach are able to describe the experimental sign and amplitude of the dipolar-CSA cross correlation at some orientation of the CSA tensor with respect to the molecular frame. The popular wobbling-in-a-cone model, however, gives the opposite sign of this cross-correlation function under all conditions examined.
Bibliographical noteFunding Information:
This work was supported by National Research Council/National Science Foundation International (USSR) Project Development Grant to KHM and benefitted from NMR facilities made available through Grant RR-04040 from the National Institutes of Health. We are indebted to Dr. M. Kloczewiak for the peptide synthesis, and Dr, N. Janes for the time at Bruker AM-360 spectrometer.