DEVIATIONS FROM PLANARITY OF THE PEPTIDE-BOND IN PEPTIDES AND PROTEINS

The work described here is the result of a survey of the peptide omega angles in carried out to establish ''ideal'' or ''target'' values for their distribution. We have shown that substantial deviations from pl anarity can be tolerated with a standard deviation in the angle of up to 6 degrees about a mean value for the trans peptide that is less tha n 180 degrees. The distortion can arise from pyramidalization at the a mino nitrogen atom as well as simple twist about the peptide bond. We include an analysis of omega angles in the existing database of protei n structure (PDB) and show that their distributions can depend on the refinement method used, but no correlation with resolution is evident. A surprising finding was a systematic variation of omega in phi psi, space in proteins as well as in the linear and cyclic peptides. This i s particularly manifest as a consistent difference between the mean om ega values in chains of left and right-hand chirality. This dichotomy is observed for all the standard amino acids and is especially strikin g in the absence of secondary structure. The phenomenon is discussed i n the context of theoretical work on peptide analogues, and the implic ations for protein conformation and structure are briefly considered. (C) 1996 Academic Press Limited