Decoupling between translation al and rotation al motions of water in the proximity of a protein molecule*

The interaction between water and bio macromolecules is of fundamental interest in biophysics and biochemistry. By performing neutron scattering on a perdeuterated protein at various hydration levels, we characterized the dynamics of water in each molecular layer surrounding the biomolecule . We found that the translation al motion of the interfacial water is slowed down more significantly by protein than its rotation, and the retardation effect extends to the second hydration layer for translation while being limited in the first layer for rotation. Molecular dynamics simulation revealed that the observed translation rotation decoupling in hydration water results from that the translation of water is more correlated over space and highly restrained by the spatial confinement on protein. More importantly, water molecules around the structurally most stable protein residues exhibit the most retarded translation while maintaining their rotational freedom. Restraining translation of water around these residues protect the protein stability while maintaining the rotational mobility renders local flexibility to the biomolecule.