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Network of hydrogen bonds in Pro-Ala-Pro and Pro-Phe-Pro diamides: a first principles study of Ala-->Phe point mutation in proline environment.

Intramolecular hydrogen bonding in the Pro-Ala-Pro and Pro-Phe-Pro tripeptides has been characterized using Bader's atoms in molecule (AIM) analyses of relevant electron density topologies. The properties of hydrogen bonds with corresponding ring strains were investigated. Good correlations along the decrease in electron densities at ring critical points were examined from five- to ten-membered hydrogen-bound ring sizes; seven-membered rings being the most energetically favored. AIM analysis confirms the logical conclusion that the molecule has to become very compact to form as many hydrogen bonds as possible. The relatively large hydrogen bond stabilization attributed to the pronounced network of interactions comes at the "energetic expense" of a relatively large internal repulsion due to the compactness of the structures. The net balanced result was a very modest increase in the zero point corrected conformation energy (DeltaE(ZPEC)). These findings aid in establishing hydrogen bonding rules in reductionist "bottoms-up" approaches to peptide and protein folding.