Hydration-dependent internal dynamics of macromolecules: a neutron scattering study

We have investigated how the internal dynamics of two different macromolecules is affected by hydration, to understand if there are common features related to the plasticizing role of water. To this purpose, elastic and quasielastic incoherent neutron scattering spectra of AOT (sodium bis(2-ethylhexyl) sulfosuccinate) reverse micelles at different hydration degrees W=[H2O]/[AOT] have been compared with elastic neutron scattering data of lysozyme in deuterated glycerol, as a function of increasing water content h (h=grams of water/grams of lysozyme) in the mixture. The elastic and quasielastic data on AOT micelles provide a coherent description of a hydration-dependent intrinsic micelle dynamics, which is activated above a threshold Wsimilar to1 and approaches to a saturation limit for W>6. This dynamics essentially involves the hydrogen atoms belonging to AOT hydrocarbon tails and appears to be strongly correlated to the fully hydrated AOT head groups mobility. For lysozyme in glycerol, at room temperature the total hydrogen mean square displacement (MSD) <u(2)> strongly rises above hsimilar to0.1 and tends to saturate at higher hydration contents. The striking similarity between the dynamical behaviour of these two systems as a function of their progressive hydration confirms that water molecules generally play a plasticizer role independent of the intrinsic nature of the system they interact with.