The energetically favorable cis peptide bond for the azaglycine-containingpeptide: For-AzGly-NH2 model

The conformational preferences of the azaglycine-containing peptide model, For-AzGly-NH2 (1), were investigated with ab initio and DFT methods for the cases when the formyl group has either a trans (1a) or cis (1b) peptide bo nd. Based on the HF/6-31G* potential energy surfaces, the minimum energy co nformations for 1 were characterized. The structures and energetic relation s between the resulting minima for 1 were systematically examined using var ious basis sets (6-31G*, 6-31G**, 6-311G** and 6-31+G**). An important resu lt, that the global minimum of 1b (omega (0)approximate to0 degrees) is mor e stable than that of 1a (omega (0)approximate to 180 degrees), was found. This is comparable to the glycine peptide model, For-Gly-NH2 (2), whose glo bal minimum energy conformation prefers the trans peptide bond to the cis p eptide bond as already known. The resulting minima for 1 demonstrate its ut ility as a valuable biological probe.