Peptide models - XXIV: An ab initio study on N-formyl-L-prolinamide with trans peptide bond. The existence or non-existence of alpha(L) and epsilon(L) conformations

N-formyl-L-prolinamide was subjected to geometry optimization at three leve ls of theory: HF/3-21G, HF/6-31G (d) and B3LYP/6-31G (d). At all three leve ls of computation the global minimum was gamma(L) (inverse gamma-Turn) back bone conformation with two ring-puckered forms "UP" and "DOWN". At HF/3-21G level of theory three backbone conformations were found gamma(L), epsilon( L), and alpha(L). At higher levels of theory the epsilon(L), and alpha(L) c onformations disappeared. The ''UP'' and ''DOWN'' ring-puckered forms, in t he gamma(L) backbone conformation, led to practically identical vibrational spectra at the B3LYP/6-31G (d) level of theory. (C) 1999 Elsevier Science B.V. All rights reserved.