Computational study of inelastic neutron scattering vibrational spectra of water clusters and their relevance to hydration water in proteins

Background Inelastic neutron scattering (INS) vibrational spectra for hydration water in proteins can be obtained from spectral differences, but their interpretation has mainly been limited to comparisons with various forms of ice at high hydration levels without making use of available structural information from neutron protein crystallography. Methods The INS vibrational spectra of free and partially constrained water clusters (up to n = 17) were calculated with DFT methods using published energy-minimized structures. Results Reference is made to neutron diffraction studies of hydrated proteins, which contain a wealth of structural information both on individual water molecules and small clusters in the inner “shell” in order to select representative clusters to serve as models for bound, rather than free clusters as they would occur in a protein. Conclusions INS spectra of the water librational region calculated for a combination of model bound clusters provide a qualitative account of the essentially featureless experimental spectra on water in proteins at very low hydration levels, but do indicate that the well-known rise in intensity near 500 cm^{− 1} is connected to increasing numbers of four-coordinate water molecules in larger clusters. General significance The combination of structural information of hydration water from neutron protein crystallography with much more sophisticated computational methods than used herein should lead to a much more detailed picture of the hydration of proteins. This article is part of a Special Issue entitled “Science for Life” Guest Editor: Dr. Austen Angell, Dr. Salvatore Magazù and Dr. Federica Migliardo.