Protein conformation and proton nuclear‐magnetic‐resonance chemical shifts

Abstract

The NMR chemical shifts of the polypeptide backbone protons in basic pancreatic trypsin inhibitor from bovine organs, and the inhibitors E and K from the venom of Dendroaspis polylepis polylepis were analyzed. Using the corresponding shifts in model peptides, the chemical shifts observed in the proteins were decomposed into random-coil shifts and conformation-dependent shifts. Correlations between contributions to the latter term and the polypeptide conformation were investigated by using the crystal structure of the bovine inhibitor. In addition to the well-known ring-current effects, a correlation was found between chemical shifts of amide and C.alpha. protons and the length of the H-bonds formed by these protons with nearby O atoms as acceptor groups. There remain sizeable and as yet unexplained residual conformation shifts. Overall, the present treatment provides a satisfactory qualitative explanation for the outstandingly large shifts of backbone H atoms in these diamagnetic proteins.