8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

P2.82Tue 611:23-14:00Understanding water dynamics near topologicallycomplex solutes from simulationAna Vila Verde 1 and Kramer Campen 21 University of Amsterdam, Van ’t Hoff Institute for Molecular Science, PO Box 94157 1090GE, Amsterdam, Netherlands2 AMOLF, Amsterdam, NetherlandsThe properties of solutes in water (e.g. conformational fluctuations of proteins) are known todepend on the properties of the adjoining solvent. Our understanding of this connection is incomplete,partly because the structural and dynamic properties of water near solutes differ frombulk water in a manner that depends on both solute chemistry and topology. Here we report astudy focusing on the least understood of these factors: solute topology. We use classical molecularsimulations in explicit solvent to investigate water near disaccharides because disaccharidesare small enough for detailed study but show the topological and chemical complexity inherentto large biomolecules. We find that the observed slow down of translation and rotation of localwater populations precisely agrees with increases in local hydrophobicity. Moreover, as recentlyobserved in proteins, identical functional groups may interact differently with water depending onthe chemistry and topology of neighboring groups.To understand these observations we investigate the mechanism of hydrogen bond exchange forwaters within the sugar first solvation shell. Recent reports indicate that rotation of water in bulkoccurs through large angular jumps involving bifurcated hydrogen bond intermediates. The rotationalslow down of water near small solutes can be predicted using transition state theory, byaccounting for the decrease in the accessible transition state volume and changes the enthalpy ofthe hydrogen bonds relative to bulk. For our larger solutes we find that accounting for these twofactors is insufficient because of unnintuitive changes in the free energy landscape associated withwater rotation - reduction in the number of available reactant states and broadening of the transitionstate. Simple scaling considerations from bulk fail to capture these changes for all but the simplestsolute topologies, effectively making water dynamics system dependent.82