Solvation of Hydrogen Bonded Systems: CH···O, OH···O, and Cooperativity

Abstract

The effect of solvation upon hydrogen bonds is assessed by a number of computational techniques. The SCRF approach and its more refined variants, which treat the solvent as a polarizable continuum, are compared with results obtained via treatment of discrete interactions with individual solvent molecules. The effect of solvation upon CH··O H-bonds is very much like its influence upon conventional OH··O bonds. Results obtained with a continuum model of solvent are not altered much by explicit inclusion of a first hydration sphere. The H-bond energy arising from the CαH group of amino acids is fairly substantial and can even exceed the NH··O H-bond energy in certain conformations of a polypeptide. On the other hand, both H-bonds rapidly weaken when placed in an aqueous environment. Chains composed of CH··O bonds show a comparable degree of cooperativity as do OH··O bonds. When placed in a polarizable medium, the cooperativity lessens along with the strength of the individual H-bonds. The functional side chains of certain amino acids, such as histidine, can form H-bonds simultaneously at a number of sites. Multiple H-bonds follow the usual patterns of positive and negative cooperativity found within one-dimensional H-bonded chains. The calculated solvation energy of the pertinent molecule in polarizable medium is considerably smaller than the combined interaction energy of the molecule and its surrounding water molecules. However, this difference is much smaller when each H-bond energy is computed not in vacuo, but rather within a polarizable medium.