Ab initio conformational analysis of N-formyl L-alanine amide including electron correlation

The conformational properties of N-formyl L-alanine amide (ALA) were investigated using RMP2/6-311G** ab initio gradient geometry optimization. One hundred forty four structures of ALA were optimized at 30 degrees grid points in its phi (N-C(alpha)), psi (C(alpha)-C') conformational space. Using cubic spline functions, the grid structures were then used to construct analytical representations of complete surfaces, in phi,psi -space, of bond lengths, bond angles, torsional sensitivity and electrostatic atomic charges. Analyses show that, in agreement with previous studies, the right-handed helical conformation, alpha (R), is not a local energy minimum of the potential energy surface of ALA. Comparisons with protein crystallographic data show that the characteristic differences between geometrical trends in dipeptides and proteins, previously found for ab initio dipeptide structures obtained without electron correlation, are also found in the electron-correlated geometries. In contrast to generally accepted features of force fields used in empirical molecular modeling, partial atomic charges obtained by the CHELPG method are found to be not constant, but to vary significantly throughout the phi,psi -space. By comparing RHF and MP2 structures, the effects of dispersion forces on ALA were studied, revealing molecular contractions for those conformations, in which small adjustments of torsional angles entail large changes in non-bonded distances. (C) 2001 Elsevier Science B.V. All rights reserved.