Macromolecular crowding in biological media is an essential factor for cellular function. The interplay of intermolecular interactions at multiple time and length scales governs a fine-tuned system of reaction and transport processes, including particularly protein diffusion as a limiting or driving factor. Using quasielastic neutron backscattering, we probe the protein self-diffusion in crowded aqueous solutions of bovine serum albumin on nanosecond time and nanometer length scales employing the same protein as crowding agent. The measured diffusion coefficient DðφÞ strongly decreases with increasing protein volume fraction φ explored within 7% ≤ φ ≤ 30%. With an ellipsoidal protein model and an analytical framework involving colloid diffusion theory, we separate the rotational D r ðφÞ and translational D t ðφÞ contributions to DðφÞ. The resulting D t ðφÞ is described by short-time self-diffusion of effective spheres. Protein self-diffusion at biological volume fractions is found to be slowed down to 20% of the dilute limit solely due to hydrodynamic interactions. macromolecular crowding | quasi-elastic neutron scattering | globular proteins T he interior of biological cells is a medium with a macromolecular volume fraction of up to 40%. This crowding crucially affects reaction kinetics and equilibria in the cell (1, 2). Cellular function and structure thus cannot be understood without a systematic understanding of both phase behavior and transport processes in crowded media. Diffusion is the main transport process for systems at low Reynolds numbers, governing many dynamic processes in nature (3). From the perspective of a single tracer molecule, all other molecules act as obstacles. In vivo diffusion coefficients for globular proteins in living cells (4-7) are strongly decreased compared to the in vitro diffusion coefficient in dilute buffer solutions. Systematic measurements of the tracer diffusion of proteins dissolved in concentrated suspensions of crowding agents, i.e., other proteins or polymers, reveal a complex dependence of the slowing down on the combination of tracer molecule and crowding agent (8-10). Furthermore, macromolecular crowding is found to induce subdiffusive behavior in several cases (11,12), being suggested as a slower but more reliable diffusive search process inside the cell (13). This anomalous diffusion process has been found also in theory and simulations (12-15) suggesting a crossover from subdiffusive behavior at small times to diffusive behavior at larger times.Proteins are macromolecules generally with a nonspherical shape and a nonhomogeneous surface charge, showing specific interactions with ligands. Furthermore, proteins not only show global motions like translational and rotational diffusion but also internal and interdomain motions. Therefore, proteins pose a challenge to colloid theory (16,17). In a recent simulation study Ando and Skolnick (4) revealed that using an equivalent-sphere model for macromolecules is a reasonable approximation to describe diffusion. Moreover, ...

The dynamics of proteins in solution includes a variety of processes, such as backbone and side-chain fluctuations, interdomain motions, as well as global rotational and translational (i.e. center of mass) diffusion. Since protein dynamics is related to protein function and essential transport processes, a detailed mechanistic understanding and monitoring of protein dynamics in solution is highly desirable. The hierarchical character of protein dynamics requires experimental tools addressing a broad range of time- and length scales. We discuss how different techniques contribute to a comprehensive picture of protein dynamics, and focus in particular on results from neutron spectroscopy. We outline the underlying principles and review available instrumentation as well as related analysis frameworks.

We report on a joint experimental-theoretical study of collective diffusion in, and static shear viscosity of solutions of bovine serum albumin (BSA) proteins, focusing on the dependence on protein and salt concentration. Data obtained from dynamic light scattering and rheometric measurements are compared to theoretical calculations based on an analytically treatable spheroid model of BSA with isotropic screened Coulomb plus hard-sphere interactions. The only input to the dynamics calculations is the static structure factor obtained from a consistent theoretical fit to a concentration series of small-angle X-ray scattering (SAXS) data. This fit is based on an integral equation scheme that combines high accuracy with low computational cost. All experimentally probed dynamic and static properties are reproduced theoretically with an at least semi-quantitative accuracy. For lower protein concentration and low salinity, both theory and experiment show a maximum in the reduced viscosity, caused by the electrostatic repulsion of proteins. The validity range of a generalized Stokes-Einstein (GSE) relation connecting viscosity, collective diffusion coefficient, and osmotic compressibility, proposed by Kholodenko and Douglas [PRE, 1995[PRE, , 51, 1081 is examined. Significant violation of the GSE relation is found, both in experimental data and in theoretical models, in semi-dilute systems at physiological salinity, and under low-salt conditions for arbitrary protein concentrations.

It has long been realized that cations play a critical role in the readsorption of water into the interlayer region in clay minerals. To explore possible differences in the water dynamics related to the presence of cations in clays, and to examine the dynamics of its surface water, which plays a prominent role in diffusion of water in clay barriers a comparative study was carried out to highlight differences between water dynamics in montmorillonite and halloysite. Whereas montmorillonite has interlayer cations that interact with interlayer water, and which can rehydrate after dehydration at temperature, halloysite has no interlayer cations. Water is found in both interlayers and on the surface of these clay particles. In this study we show that by combining incoherent inelastic neutron scattering (quasi-elastic and elastic fixed window) and neutron spin echo, it was possible to discriminate the dynamics of surface water (by collapsing the interlayer region by heating and rehydrating the surface layer) from interlayer water. The analysis of the elastic fixed window scans in the temperature range 5−300 K revealed an extension of water dynamics in montmorillonite to lower temperatures than in halloysite. These differences suggested mechanisms that cations (Na+ in this case) in the interlayer regions facilitate water mobility allowing interlayer water to be readmitted to montmorillonite. Finally it was shown that the occurrence of magnetic fluctuations, caused by the presence of paramagnetic Fe3+ ions in the crystalline clay lattice, gave rise to a quasi-elastic contribution that disrupted the evaluation of water diffusion computed from such measurements. Therefore previous estimates of water diffusion coefficients might have been overestimated in recent literature.