Ab Initio Studies of Amino Acid Conformations. 1. The Conformers of Alanine, Serine, and Cysteine

Abstract

High-level ab initio methods (up to MP2/6-31+G*) have been used to characterize the gas phase conformations of alanine, serine, and cysteine. A wide range of possible structures (36 for alanine and 324 for serine and cysteine) was surveyed at the AMI level, and then the geometries of the unique conformers were refined at the 3-21G(*) and 6-31G* levels. At the highest theoretical level, 10 conformers were located for alanine, 51 for serine, and 42 for cysteine. The AMI level provides a poor description of the relative energies. Calculations at the 3-21G(*) level represent a significant improvement, but some bonding schemes are poorly characterized. Better values are obtained at the HF/6-31G* ..