AB-INITIO STUDIES OF AMINO-ACID CONFORMATIONS .1. THE CONFORMERS OF ALANINE, SERINE, AND CYSTEINE

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 AM1 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 AM1 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* level, but to obtain reasonably accurate relative energies, correlation corrections are required and calculations at the MP2/6-31+G*//HF/6-31G* level give values in good accord with a series of test calculations at the MP2/6-31+G*//MP2/6-31+G* level. Ab initio rotational constants and dipole moments are reported for all the conformers. The results are compared to previous studies of amino acids and are analyzed in terms of intramolecular hydrogen-bonding interactions.