Ab initio quantum mechanics analysis of imidazole C-H...O water hydrogen bonding and a molecular mechanics forcefield correction.

While it is well established that classical hydrogen bonds play an important role in enzyme structure, function and dynamics, the role of weaker, but 'activated' C-H donor hydrogen bonds is poorly understood. The most important such case involves histidine which often plays a direct role in enzyme catalysis and possesses the most acidic C-H donor group of the standard amino acids. In the present study, we obtained optimized geometries and hydrogen bond interaction energies for C-H...O hydrogen bonded complexes between methane, ethylene, benzene, acetylene, and imidazole with water at the MP2-FC/6-31++G(2d,2p) and MP2-FC/aug-cc-pVDZ/MP2-FC/6-31++G(2d,2p) levels of theory. A strong linear relationship is obtained between the stability of the various hydrogen bonded complexes and both separation distances for H...O and C----O. In general, these calculations indicate that C-H...O interactions can be classified as hydrogen bonding interactions, albeit significantly weaker than the classical hydrogen bonds, but significantly stronger than just van der Waals interactions. For instance, while the electronic energy of stabilization at the MP2-FC/aug-cc-pVDZ/MP2-FC/6-31++G(2d,2p) level of theory of a water O-H...O water hydrogen bond is 4.36 kcal/mol more stable than the methane C-H...O water interaction, the water-water hydrogen bond is only 2.06 kcal/mol more stable than the imidazole Ce-H...O water hydrogen bond. Neglecting this latter hydrogen bonding interaction is obviously unacceptable. We next compare the potential energy surfaces for the imidazole Ce-H...O water and imidazole Na-H...O hydrogen bonded complexes computed at the MP2/6-31++G(2d,2p) level of theory with the potential energy surface computed using the AMBER molecular mechanics program and forcefields. While the Weiner et al and Cornell et al AMBER forcefields reasonably account for the imidazole N-H...O water interaction, these forcefields do not adequately account for the imidazole Ce-H...O water hydrogen bond. A forcefield modification is offered that results in excellent agreement between the ab initio and molecular mechanics geometry and energy for this C-H...O hydrogen bonded complex.