Dipole–Dipole Interactions in Hydration Layer

Abstract

As described in Chap. 7, hydration water molecules on protein surfaces form networks of hydrogen bonds. Due to the directionality of hydrogen bonds, the orientation of hydration water molecules is restrained by polar protein atoms and adjoining hydration water molecules. In this regard, molecular dynamics simulation revealed coherent patterns of dipole moments in hydration water molecules over protein surfaces. Dipole–dipole interactions among water molecules and protein atoms likely have a predominant contribution in electrostatic interactions in the vicinity of protein surfaces. Dynamic Stokes shift of fluorescence emission from photo-excited tryptophan side chain is measured to investigate the temporal variation of hydration dipoles experimentally. By applying the linear response theory to a molecular dynamics trajectory simulating the relaxation of photo-excited tryptophan, the observed relaxation would be attributed to the orientational relaxation in the dipole–dipole interactions between hydration water molecules and tryptophan side chains.