Comparison of Fully Optimized α- and 3₁₀-Helices with Extended β-Strands. An ONIOM Density Functional Theory Study

We compare the structures and energies of β-strands, α-helices, and 3₁₀-helices for capped polyalanines, acetyl(ala)_NNH₂, for values of N from 2 to 18, using completely optimized mixed DFT/AM1 calculations. Non-pairwise additive cooperativity is manifest from the variation of the relative energies, helical strain, dipole moments, and H-bond lengths of both types of helices, but especially for the α-helices. While the gas-phase 3₁₀-helices are more stable for small polyalanines, largely due to the additional H-bond, the α-helices become relatively more stable as the polyalanines increase in size.