The use of 1JC alpha H alpha coupling constants as a probe for protein backbone conformation

Simple pseudo-3D modifications to the constant-time HSQC and HCACO experiments are described that allow accurate (+/- 0.5 Hz) measurement of one bond JC alpha H alpha coupling constants in proteins that are uniformly enriched with 13C. An empirical phi,psi-surface is calculated which describes the deviation of 1JC alpha H alpha from its random coil value, using 203 1JC alpha H alpha values measured for residues in the proteins calmodulin, staphylococcal nuclease, and basic pancreatic trypsin inhibitor, for which phi and psi are known with good precision from previous X-ray crystallographic studies. Residues in alpha-helical conformation exhibit positive deviations of 4-5 Hz, whereas deviations in beta-sheet are small and, on average, slightly negative. Data indicate that 1JC alpha H alpha depends primarily on psi, and that 1JC alpha H alpha may be useful as a qualitative probe for secondary structure. Comparison of 1JC alpha H alpha coupling constants measured in free calmodulin and in its complex with a 26-amino-acid peptide fragment of myosin light-chain kinase confirm that the calmodulin secondary structure is retained upon complexation but that disruption of the middle part of the 'central helix' is even more extensive than in free calmodulin.