Peptide models XXII. A conformational model for aromatic amino acid residues in proteins. A comprehensive analysis of all the RHF/6-31+G conformers of For-L-Phe-NH2

Phenylalanine is expected to have conformational features similar to the ot her three naturally occurring aromatic amino acid residues: Tyr, Trp and Hi s. Previous ab initio structure determinations resulted in 19 different con formers of HCO-L-Phe-NH2 at the RHF/3-21G level of theory. The present work summarises the results of a comprehensive analysis incorporating RHF/3-21G , RHF/6-31+G*, RHF/6-31+G*//RHF/3-21G and B3LYP/6-311++G**//RHF/3-21G data as some of the previously established minima have vanished with the larger basis set, three out of the 19 stationary points having migrated during the optimisation. On top of that, the conformational building unit of the righ t-handed helix-like (alpha(L)) conformation and that of the polyproline II (epsilon(L)) conformation are still missing from the E = E(phi,psi) surface . The optimised ab initio structures are also analysed in the context of -P he- conformers taken from a large X-ray database on non-homologous proteins incorporating a total of 158664 amino acid residues. (C) 1998 Elsevier Sci ence B.V. All rights reserved.