The internal dynamics of bacteriorhodopsin, the light-driven proton pump in the purple membrane of Halobacterium halobium, has been studied by inelastic neutron scattering for various conditions oftemperature and hydration. Light activation can take place when the membrane is vibrating harmonically. The ability of the protein to functionally relax and complete the photocycle initiated by the absorption of a photon, however, is strongly correlated with the onset of low-frequency, large-amplitude anharmonic atomic motions in the membrane. For a normally hydrated sample, this occurs at about 230 K, where a dynamical transition from a lowtemperature harmonic regime is observed. In moderately dry samples, on the other hand, in which the photocycle is slowed down by several orders of magnitude, no transition is observed and protein motions remain approximately harmonic up to room temperature. These results support the hypothesis, made from previous neutron diffraction studies, that the "softness" of the membrane modulates the function of bacteriorhodopsin by allowing or not allowing large-amplitude motions in the protein.The direct environment of a protein has a strong influence on protein internal dynamics and function. Inelastic neutron scattering has unique advantages for the study of thermal motions in proteins. Here we describe such a study of a membrane protein in its natural lipid environment under various external conditions. By varying temperature and hydration, the thermal motions of bacteriorhodopsin (BR) in the purple membrane (PM) of Halobacterium halobium were characterized and correlated with aspects ofprotein function. Previously, thermal dynamics had been studied mainly in small globular proteins by various experimental methods, such as Mossbauer spectroscopy (1), inelastic neutron scattering (2) or optical spectroscopy (3), used to sample their atomic motions, as well by molecular dynamics simulation approaches (4).PM functions as a light-driven proton pump. It contains a single protein of 26 kDa, BR, organized with lipid on a highly ordered two-dimensional lattice. The structure of BR is now known to relatively high resolution for a membrane protein in its natural lipid environment (5). Its main features are seven a-helices, arranged in a bundle around a retinal molecule bound via a Schiff base to a lysine residue in the protein.Upon illumination, PM undergoes a "photocycle" with a frequency of the order of milliseconds (6). The photocycle has been studied extensively and was shown to be greatly influenced by external conditions such as temperature and relative humidity (7-10).The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.The structure and hydration of PM have been studied as a function of temperature and relative humidity by neutron diffraction (11,12). Progressive drying of the membranes showed that the hydration around the lipid...

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Inelastic-neutron-scattering studies of glassy, liquid, and polycrystalline selenium have been performed at temperatures between 100 and 600 K. A self-consistent data evaluation, taking careful account of multiple and multiphonon scattering, shows that a complete interpretation is possible in terms of a temperature-dependent density of vibrational states, together with diffusion broadening of the elastic line. This density of states is used to calculate thermodynamic properties and to show that about one-third of the additional entropy of the liquid is vibrational. The results raise a number of questions concerning current theories of the glass transition.PACS numbers: 64.70.-p, 63.50.+X It has long been realized that a fundamental difference between a glass and a supercooled liquid is the presence in the latter of both conformational and vibrational motion. As an idealized example, motion in the liquid can be represented by an atom vibrating about an equilibrium position which is itself changing with time as a result of diffusion. What is not yet understood, however, is the way in which conformational motion freezes out at the glass transition temperature Tg and the way in which it influences thermodynamic properties. To take the particular example of interest here, there have been no direct measurements of the relative importance of conformational and vibrational contributions to the sudden (but not discontinuous) increase in the heat capacity CpiT) on passing from the glass to the supercooled liquid, * one of the most obvious signatures of Tg, That this increase in C{T) is not simply a "release of degrees of freedom" can be seen in Se^ where CpiT) increases from the classical value of 3/? mole ~ ^ K ~ ^ in the glass to a value 50% higher just above Tg. (Se is a glass with a Tg sufficiently low for easy study, but without bonded H atoms or side groups which complicate the analysis in organic polymers.)Recent mode-coupling theories ^' "^ have attempted to understand conformational motion (also known as relaxation) by describing it in terms of a time-dependent particle-density correlation function. The predictions of these theories have encouraged a number of neutron studies of the dynamics of the glass transition: ^"^ Conformational and vibrational motion are distinguished by time scales of typically 1 ns (varying rapidly with temperature but set by the experiment) and 1 ps, respectively, and show up in quasielastic and spin-echo experiments, on the one hand, and in measurements of the Debye-Waller factor and inelastic scattering, on the other. Most experiments have been concerned with conformational motion above Tg and demonstrate the presence of relaxation with a temperature dependence similar to that shown by the bulk viscosity, and in reasonable agreement with theory. Vibrational motion has been examined less carefully, mainly because detailed analysis of the data is complicated by multiple and multiphonon scattering. In this Letter we adopt a self-consistent, model-independent procedure for data analysis an...

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