The dynamical coupling between proteins and their hydration water is important for the understanding of macromolecular function in a cellular context. In the case of membrane proteins, the environment is heterogeneous, composed of lipids and hydration water, and the dynamical coupling might be more complex than in the case of the extensively studied soluble proteins. Here, we examine the dynamical coupling between a biological membrane, the purple membrane (PM), and its hydration water by a combination of elastic incoherent neutron scattering, specific deuteration, and molecular dynamics simulations. Examining completely deuterated PM, hydrated in H2O, allowed the direct experimental exploration of water dynamics. The study of natural abundance PM in D2O focused on membrane dynamics. The temperature-dependence of atomic mean-square displacements shows inflections at 120 K and 260 K for the membrane and at 200 K and 260 K for the hydration water. Because transition temperatures are different for PM and hydration water, we conclude that ps-ns hydration water dynamics are not directly coupled to membrane motions on the same time scale at temperatures <260 K. Molecular-dynamics simulations of hydrated PM in the temperature range from 100 to 296 K revealed an onset of hydration-water translational diffusion at Ϸ200 K, but no transition in the PM at the same temperature. Our results suggest that, in contrast to soluble proteins, the dynamics of the membrane protein is not controlled by that of hydration water at temperatures <260 K. Lipid dynamics may have a stronger impact on membrane protein dynamics than hydration water. molecular dynamics simulations ͉ neutron spectroscopy ͉ dynamical transition ͉ purple membrane ͉ bacteriorhodopsin P roteins are animated by a multitude of motions occurring on various length and time scales. In a current model, a protein is not characterized by a single three dimensional structure but, rather, by a large number of conformations, so-called conformational substates, that interconvert via molecular motions (1). Natively unfolded proteins reflect this conformational heterogeneity to an extreme degree. The developing concept of a free-energy landscape has put some order into the complex world of protein motions (1). The energy landscape is a highdimensional space, in which each conformational substate is defined by the coordinates of all atoms in the protein. The energy landscape is organized in a hierarchy of levels; the top level containing only a few substates that interconvert by relatively slow large-scale movements (2). Each substate at this level contains, itself, a larger number of substates, separated by smaller barriers that are sampled by more rapid motions. At the third level, the number of substates becomes very large and the barriers very small, and motions at this level have been proposed to take place on the ps-ns time scale (2). Which of the motions on different levels are crucial for biological function and whether and how they are correlated with each other are chall...

SrFeO(2.5) and SrCoO(2.5) are able to intercalate oxygen in a reversible topotactic redox reaction already at room temperature to form the cubic perovskites Sr(Fe,Co)O(3), while CaFeO(2.5) can only be oxidized under extreme conditions. To explain this significant difference in low temperature oxygen mobility, we investigated the homologous SrFeO(2.5) and CaFeO(2.5) by temperature dependent oxygen isotope exchange as well as by inelastic neutron scattering (INS) studies, combined with ab initio (DFT) molecular dynamical calculations. From (18)O/(16)O isotope exchange experiments we proved free oxygen mobility to be realized in SrFeO(x) already below 600 K. We have also evidence that low temperature oxygen mobility relies on the existence of specific, low energy lattice modes, which trigger and amplify oxygen mobility in solids. We interpret the INS data together with the DFT-based molecular dynamical simulation results on SrFeO(2.5) and CaFeO(2.5) in terms of an enhanced, phonon-assisted, low temperature oxygen diffusion for SrFeO(3-x) as a result of the strongly reduced Fe-O-Fe bond strength of the apical oxygen atoms in the FeO(6) octahedra along the stacking axis. This dynamically triggered phenomenon leads to an easy migration of the oxide ions into the open vacancy channels and vice versa. The decisive impact of lattice dynamics, giving rise to structural instabilities in oxygen deficient perovskites, especially with brownmillerite-type structure, is demonstrated, opening new concepts for the design and tailoring of low temperature oxygen ion conductors.