Quantum mechanical study of secondary structure formation in protected dipeptides.

An extensive computational study of the conformational preferences of three capped dipeptides: Ac-Xxx-Phe-NH(2), Xxx = Gly, Ala, Val is reported. On the basis of local second-order Møller-Plesset perturbation theory (LMP2) and DFT computations we were able to identify the experimentally observed conformers as gamma(L)-gamma(L)(g-) and beta-turn I(g+) in Ac-Gly-Phe-NH(2), and Ac-Ala-Phe-NH(2), and as the closely related gamma(L)(g+)-gamma(L)(g-) and beta-turn I(a,g+) in Ac-Val-Phe-NH(2). In contrast to the experimental observation that peptides with bulky side chain have a propensity for beta-turns, we show that in Ac-Val-Phe-NH(2) the minimum energy structure corresponds to the experimentally non detected beta-strand.