Evaluation of the dipeptide approximation in peptide modeling by ab initio geometry optimizations of oligopeptides

HF/4-21G ab initio gradient geometry optimizations were performed on model tri- and hexapeptides to examine the validity and limitations or the widely used dipeptide approximation in empirical peptide conformational analyses. For the molecules N-formyl-Ala-Ala-amide, N-formylpentaglycine amide, and N-formylpentaalanine amide, several conformations were investigated, including repeated C7eq, repeated C5, helical, and bend forms. The results show that the order or low-energy regions obtained for dipeptides is significantly changed by long-range interactions in the hexapeptides