Ab initio force fields of alanine dipeptide in four non-hydrogen bonded conformations

Abstract

We have previously calculated the force fields and dipole-moment derivatives of the L-alanyl dipeptide, CH3CONHCHCH2CONHCH3, in the C5, C7eq, and C7ax conformations with intramolecular NH...OC hydrogen bonding. We have now extended these studies with similar ones on four open, non-hydrogen-bonded conformers optimized by Scarsdale et al. the [beta]2, [alpha]R, [alpha]L and [alpha]' structures. The force constants were derived using the 4-21 Gaussian basis set and were scaled as before. The present results help to isolate the effects of conformation on the force fields and dipole derivatives, and also provide further insight into the effects of hydrogen bonding by comparison with the previous results. Additional effects can also be seen, such as those of the non-planarity of the peptide group, and the influence on the CO bond when the adjacent NH forms a hydrogen bond, and vice versa.