Asymmetric ¹³C NMR multiplet relaxation and dipolar-CSA cross correlation for glycine ¹³C_α methylenes in peptides

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.