Depolarizing Effects in Hydrogen Bond Energy in 3-Helices Revealed by Quantum Chemical Analysis.

Hydrogen-bond (H-bond) energies in 3-helices of short alanine peptides were systematically examined by precise DFT calculations with the negative fragmentation approach (NFA), a modified method based on the molecular tailoring approach. The contribution of each H-bond was evaluated in detail from the 3-helical conformation of total energies (whole helical model, WH model), and the results were compared with the property of H-bond in α-helix from our previous study. The H-bond energies of the WH model exhibited tendencies different from those exhibited by the α-helix in that they depended on the helical position of the relevant H-bond pair. H-bond pairs adjacent to the terminal H-bond pairs were observed to be strongly destabilized. The analysis of electronic structures indicated that structural characteristics cause the destabilization of the H-bond in 3-helices. We also found that the longer the helix length, the more stable the H-bond in the terminal pairs of the WH model, suggesting the action of H-bond cooperativity.