ON THE STRUCTURES OF FREE GLYCINE AND ALPHA-ALANINE

Correlated-level ab initio calculations, including large basis set MP2, have been performed for several conformers of the neutral forms of the amino acids glycine and cu-alanine. These calculations resulted in accurate geometric structures and relative energies for the conformers considered. The structural results obtained support the rotational constants measured for the lowest-lying two conformers of both glycine and ac-alanine. Energetic and structural results, however, indicate necessary model improvements for existing gas-phase electron diffraction studies of these simplest amino acids. The calculations performed also reveal that, in contrast to what has recently been suggested for this class of compound (R.F. Frey, J. Coffin, S.Q. Newton, M. Ramek, V.K.W. Cheng, F.A. Momany and L. Schafer, J. Am. Chem. Sec., 114 (1992) 5369), correlated-level geometry optimizations can usually be avoided even if nearly quantitative accuracy is sought in relative energy predictions for the conformers.