ENERGY MINIMIZATION STUDIES ON ALPHA-TURNS

Using a grid search technique, the entire conformational space of a sy stem of four linked peptide units (tetrapeptide) was scanned to pick o ut geometrically possible 5-->1 type hydrogen-bonded conformations def ined as an alpha-turn. The energy minimization of these conformations led to 23 distinct minimum energy conformations (MECs) falling in 13 d ifferent classes. The presence of beta and gamma turn type hydrogen bo nds along with 5-->1 type hydrogen bond gave conformational variabilit y in a given class. The occurrence of bifurcated hydrogen bonding netw ork was a characteristic feature of most of the MECs. In many prototyp e MECs non-glycyl residues such as Ala and Pro could be accommodated. Comparison of MECs with the ex-turn examples that are observed in prot eins showed that the conformationally worked out MECs occurred in isol ation in proteins, with the cc-helical ex-turn being distinctly the mo st predominant. (C) 1998 European Peptide Society and John Wiley & Son s, Ltd.