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 cha racterize the gas phase conformations of alanine, serine, and cysteine . A wide range of possible structures (36 for alanine and 324 for seri ne and cysteine) was surveyed at the AM1 level, and then the geometrie s of the unique conformers were refined at the 3-21G() and 6-31G* lev els. 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, correlatio n corrections are required and calculations at the MP2/6-31+G//HF/6-3 1G level give values in good accord with a series of test calculation s at the MP2/6-31+G//MP2/6-31+G* level. Ab initio rotational constant s and dipole moments are reported for all the conformers. The results are compared to previous studies of amino acids and are analyzed in te rms of intramolecular hydrogen-bonding interactions.