X-Pro peptides. A theoretical study of the hydrogen bonded conformations of (α-aminoisobutyryl-L-prolyl)_n sequences

Abstract

Intramolecularly hydrogen bonded conformations of (Aib-Pro)n sequences have been analysed theoretically. Both 4→1 (C₁₀ and 3→1 (C₇ hydrogen bonded regular structures are shown to be stereochemically feasible. Conformational energies for the helical structures have been estimated using classical potential energy methods. Both C₁₀ and C₇ conformations have very similar energies. Pyrrolidine ring puckering has a pronounced effect on the energies, and only Cγ-endo puckered Pro residues can be accommodated. The theoretical calculations using spectroscopic data suggest that the recently proposed novel 3₁₀ helical conformation for benzyloxycarbonyl(Aib-Pro)₄-methyl ester is in solution, is indeed energetically and stereochemically favourable.