Preferred structures of constrained peptides from achiral α,α-dialkyiated glycyl residues with acyclic side chains.

ABSTRACT

Conformational energy computations of the monopeptides from three achiral α,α− dialkylated glycyl residues with acyclic side chains (namely α,α-dimethyl-; α,α−diethyl-; and a, adi- n-propylglycines) are reported as a function of the relevant N-Cα-C' bond angle. In parallel, experimental studies were performed in the solid state (infrared absorption and X-ray diffraction) and in solution (infrared absorption and proton magnetic resonance) on the corresponding protected homo-peptide series (the former series to the dodecamer, the other two series to the pentamers). The results obtained unequivocally indicate that the preference from a helical to a fully extended conformation increases as side-chain bulkiness increases. The longest homo-peptides from α,α−dimethylglycine form stable 310-helices. A picture of the mode of self-association of the helical structures has also been determined. The results of the theoretical analyses fit well with the experimentally observed conformational properties in the solid state and in chloroform solution.