CALCULATIONS OF ONE-BOND, 2-BOND AND 3-BOND NUCLEAR SPIN-SPIN COUPLINGS IN A MODEL PEPTIDE AND CORRELATIONS WITH EXPERIMENTAL-DATA

We present ab initio calculations of the Fermi contact term and experimental correlations of six coupling constants, (3)J(HNH alpha), (1)J(C alpha H alpha), (2)J(C'H alpha), (1)J(C alpha N), (2)J(C alpha N) and (1)J(C'N), in a peptide as functions of the backbone dihedral angles, phi and psi. Given estimates of experimental uncertainties, we find semiquantitative experimental correlations for (3)J(HNH alpha), (1)J(C alpha N) and (2)J(C alpha N), qualitative correlations for (1)J(C alpha H alpha) and (2)J(C'H alpha), but no experimental correlations of practical utility for (1)J(C'N), owing to its complex dependence on at least four dihedral angles. Errors in the estimation of dihedral angles from X-ray crystallographic data for proteins, which result from uncertainties in atom-to-atom distances, place substantial limitations on the quantitative reliability of coupling constant calculations fitted to such data. In the accompanying paper [Edison, A.S. et al., J. Biomol. NMR, 4, 543-551] we apply the results of the coupling constant calculations presented here to the estimation of phi and psi angles in staphylococcal nuclease from experimental coupling constants.