Peptide models XX. Aromatic side-chain–backbone interaction in phenylalanine-containing diamide model system. A systematic search for the identification of all the ab initio conformers of N-formyl-L-phenylalanine-amide

Abstract

Phenylalanine is the simplest among the four natural amino acid residues that have aromatic side chains. The ab initio conformational analysis performed at the RHF/3-21G level on a phenylalanine-containing diamide model system (N-Formyl-L-Phe-NH2) revealed 19 different structures. Single-point energy calculations were performed using RHF/6-31+G* and DFT(B3LYP)/6-311++G** levels for all conformers. The inverse (γL) and the normal (γD) gamma turn, the extended (βL), the left-handed helical [Formula: see text], and the inverse polyproline II [Formula: see text] backbone conformers each have three (g+, a, and g−) side-chain (χ1,) rotamers. The [Formula: see text] and the [Formula: see text] type main-chain conformers have only two side-chain orientations, respectively. No minima have been found for the conformational building unit of the right-handed helical [Formula: see text] and for the polyproline II [Formula: see text] structures. The present ab initio conformational analysis for For-L-Phe-NH2 is a unique example in which a systematic and complete conformational set was established for a diamide system with an aromatic side chain. Analytic vibrational frequency calculations were established for all stationary points found as minima on the potential energy surface. These data may be used in the future as reference conformers in more detailed vibrational and (or) chemical shielding calculations or during the structural analysis of peptides and proteins by X-ray or NMR techniques. Keywords: ab initio peptide conformers, all RHF/3-21G structures of For-L-Phe-NH2, backbone – side-chain interaction in phenylalanine-containing peptide models.