Evaluation of the influence of anisotropic indirect nuclear spin-spin coupling tensors on effective residual dipolar couplings for model peptides.

Residual dipolar couplings (RDCs) observed between nuclear spins in molecules in partially oriented media have become a valuable source of information for NMR spectroscopists seeking to structurally characterize biological macromolecules. Examination of the form of the direct (D) and indirect (J) nuclear spin-spin coupling Hamiltonians indicates that all observed RDCs contain an unknown contribution from the anisotropic part of J (Delta J) in addition to the direct dipolar contribution, DPQ. Here, we evaluate the influence of DeltaJ on RDCs through a series of DFT calculations on model peptides. Very small corrections to one-bond RDCs measured between heavy atoms in peptides and proteins are recommended: +0.51% for N-C' spin pairs, and +0.45% for C alpha-C' spin pairs. The corrections to RDCs involving at least one proton are negligible. This latter point is likely to be equally applicable to nucleic acids and oligosaccharides in addition to peptides and proteins. Finally, the orientations of the J(N, C') and J(C alpha, C') tensors in the molecular framework are reported for glycylglycine.