Large variations in one-bond 13Cα-13Cβ J couplings in polypeptides correlate with backbone conformation

One-bond (1)J(C alpha C beta) scalar couplings, measured in the protein ubiquitin, exhibit a strong dependence on the local backbone conformation. Empirically, the deviation from the (1)J(C alpha C beta) value measured in the corresponding free amino acid, can be expressed as Delta(1)J(C alpha C beta) = 1.3 + 0.6 cos(psi - 61 degrees) + 2.2 cos[2(psi - 61 degrees)] - 0.9 cos[2(phi + 20 degrees)] +/- 0.5 Hz, where phi and psi are the intraresidue polypeptide backbone torsion angles obtained from ubiquitin's X-ray structure. The relation between (1)J(C alpha C beta) and backbone torsion angles is confirmed by density functional theory (DFT) calculations on the peptide analogue Ace-Ala-NMe.