WATER HYDROGEN-BOND DYNAMICS CLOSE TO HYDROPHOBIC AND HYDROPHILIC GROUPS

We discuss the analysis of molecular dynamics calculations where we si mulate the kinetics of the breaking and forming of hydrogen bonds in d istinctly different environments: liquid water and concentrated aqueou s solution of DMSO. In our analysis, we consider reactive flux correla tion functions computed for a variety of specific conditions and ident ify rate constants for bond making and breaking in terms of the liquid 's molecular dynamics. According to the proposed mechanism of hydrogen -bond kinetics, hydrogen bonds most frequently break during a process of switching allegiance with a newly formed bond replacing the broken one. Simulations reveal bond dynamics in the mixture to be significant ly slower than in pure water. This is interpreted in terms of reduced likelihood of fluctuations in the hydrogen-bond network, related to th e presence of free hydrogen-bonding sites, which participate in the pr ocess of switching allegiances. Implications of our analysis for futur e experimental work are briefly discussed.