Large vortex-like structure of dipole field in computer models of liquid water and dipole-bridge between biomolecules

We propose a framework to describe the cooperative orientational motions of water molecules in liquid water and around solute molecules in water solut ions. From molecular dynamics (MD) simulation a new quantity "site-dipole f ield" is defined as the averaged orientation of water molecules that pass t hrough each spatial position. In the site-dipole field of bulk water we fou nd large vortex-like structures of more than 10 Angstrom in size. Such cohe rent patterns persist more than 300 ps although the orientational memory of individual molecules is quickly lost. A 1-ns MD simulation of systems cons isting of two amino acids shows that the fluctuations of site-dipole field of solvent are pinned around the amino acids, resulting in a stable dipole- bridge between side-chains of amino acids. The dipole-bridge is significant ly formed even for the side-chain separation of 14 Angstrom, which correspo nds to five layers of water. The way that dipole-bridge forms sensitively d epends on the side-chain orientations and thereby explains the specificity in the solvent-mediated interactions between biomolecules.