Hydrogen bonding in peptide secondary structures

Hydrogen bonding interactions in various peptide secondary structures (β-sheet, 2₇-ribbon, 3₁₀-helix, α-helix, π-helix, β-turn II, and γ-turn) have been investigated in small oligopeptides by quantum chemical calculations at the B3LYP/6-31G** level. Besides the primary O⋯H-N interactions, the optimized structures revealed the importance of N⋯H-N hydrogen bonding in several structures. The effect of substitution on the energy and structural properties was investigated comparing the properties of glycine, alanine, valine, and serine. The aliphatic substituents generally weaken the hydrogen bonds, the strongest effects being observed in crowded valine conformers. Additional hydrogen bonding interactions introduced by the OH group of serine can both strengthen (by polarizing the amide moiety through N⋯H interaction) and weaken (constraining the C=O oxygen by O⋯H-O interaction) the backbone hydrogen bonds. The effect of water as a polarizable medium on the energy properties was assessed by the COSMO model.