Invited Paper NT+BI-WeM11
The Coupling between Hydration-Water and Protein Dynamics as Studied by Neutron Scattering

Wednesday, October 17, 2007, 11:20 am, Room 618

The dynamics of proteins is influenced by motions of water molecules at the protein-solvent interphase. However, details about the dynamical coupling remain to be elucidated. Neutron scattering is particularly well-adapted to study macromolecular motions on the ns-ps time scale and their coupling to hydration-water dynamics. Indeed, elastic incoherent neutron scattering is sensitive to hydrogen/deuterium isotope labelling with the scattering cross-section of hydrogen being about 40 times larger than that of deuterium. Consequently, studying a completely deuterated protein hydrated in H₂O gives access to the dynamics of hydration water. Conversely, an identically prepared sample of hydrogenated protein hydrated in D₂O yields information on protein dynamics only, thus enabling a direct comparison between hydration water and protein motions. We studied the coupling between hydration-water and protein dynamics in a biological membrane (purple membrane (PM)) and a soluble, globular protein (maltose binding protein (MBP)) by measuring mean square displacements of hydrogen atoms in the temperature range from 20 to 300 K. Hydration-water in both PM and MBP undergoes a dynamical transition at 200 K, evidenced as a break in atomic mean square displacements as a function of temperature (Wood, Frölich, Plazanet, Kessler, Moulin, Härtlein, Gabel, Oesterhelt, Zaccai & Weik, unpublished results). In the case of PM, this dynamical transition corresponds to the onset of long-range translational diffusion of water molecules as evidenced by neutron diffraction.¹ When atomic mean square displacements of hydration-water molecules become as large as those of protein atoms, a dynamical transition appears at 250 K in PM and at 230 K in MBP. Our results shed new light on the coupling between hydration-water and protein motions and suggest that they are coupled at room temperature, yet decoupled at cryo-temperatures.

¹Weik, M., Lehnert, U. and Zaccai, G. (2005) Liquid-like water confined in stacks of biological membranes at 200 k and its relation to protein dynamics. Biophys J., 89, 3639-3646.