Underlying source of the relation between polypeptide conformation and strength of NH···O hydrogen bonds

Previous calculations have revealed that the NH···O bond between a dipeptide NH and a proton acceptor is considerably weaker when the dipeptide is in its extended C5 conformation than in the C7 structure. The present work uses quantum chemical methods to determine the underlying reason for this different behavior. With regard to the pertinent NH group, the two dipeptide conformers show very little difference in total electron density or in the character of the low-lying vacant molecular orbitals which might act as sinks for electron density from the proton acceptor. There is also little evidence that a weak intramolecular H-bond within the C5 structure is able to influence the ability of the NH group to participate in an intermolecular bond. Energy decomposition analysis points toward electrostatic interaction as the chief factor in the different behavior of the C5 and C7 structures. Further analysis reveals that the proximity of the carbonyl O atom of the dipeptide, with its partial negative charge, toward the NH group changes the character of the electrostatic potential in the C5 geometry, making NH a less attractive target for an incoming proton acceptor.