PEPTIDE MODELS .20. 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

Phenylalanine is the simplest among the four natural amino acid residu es that have aromatic side chains. The ab initio conformational analys is performed at the RHF/3-21G level on a phenylalanine-containing diam ide model system (N-Formyl-L-Phe-NH2) revealed 19 different structures . Single point energy calculations were performed using RHF/6-31+G an d DFT(B3LYP)/6-311++G* levels for all conformers. The inverse (gamma( L)) and the normal (gamma(D)) gamma turn, the extended (beta(1)), the left-handed helical (alpha(D)[phi approximate to +60 degrees, psi appr oximate to +60 degrees]), and the inverse polyproline II (epsilon(D)[p hi approximate to 60 degrees, psi approximate to 180 degrees]) backbon e conformers each have three (g(+), a, and g(-)) side-chain (chi(1)) r otamers. The delta(L)[phi approximate to 240 degrees, psi approximate to 60 degrees] and the delta(D)[phi approximate to 180 degrees, psi = -60 degrees] type main-chain conformers have only two side-chain orien tations, respectively. No minima have been found for the conformationa l building unit of the right-handed helical (alpha(L)[phi approximate to -60 degrees, psi approximate to -60 degrees]) and for the polyproli ne II (epsilon(L)[phi approximate to -60 degrees, psi approximate to 1 80 degrees]) structures. The present ab initio conformational analysis for For-L-Phe-NH2 is a unique example in which a systematic and compl ete conformational set was established for a diamide system with an ar omatic side chain. Analytic vibrational frequency calculations were es tablished for all stationary points found as minima on the potential e nergy surface. These data may be used in the future as reference confo rmers in more detailed vibrational and (or) chemical shielding calcula tions or during the structural analysis of peptides and proteins by X- ray or NMR techniques.