Density Functional Theory Study of β-Hairpins in Antiparallel β-Sheets, a New Classification Based upon H-Bond Topology

We present a new classification of beta-turns specific to antiparallel beta-sheets based upon the topology of H-bond formation. This classification results from ONIOM calculations using B3LYP/D95** density functional theory and AM1 semiempirical calculations as the high and low levels, respectively. We chose acetyl(Ala)(6)NH2 as a model system as it is the simplest all-alanine system that can form all the H-bonds required for a beta-turn in a sheet. Of the 10 different conformations we have found, the most stable structures have C-7 cyclic H-bonds in place of the C-10 interactions specified in the classic definition. Also, the chiralities specified for residues i + 1 and i + 2 in the classic definition disappear when the structures are optimized using our techniques, as the energetic differences among the four diastereomers of each structure are not substantial for 8 of the 10 conformations.