Relaxation processes in proteins range in time from picoseconds to seconds. Correspondingly, biological electron transfer (ET) could be controlled by slow protein relaxation. We used the Langevin stochastic approach to describe this type of ET dynamics. Two different types of kinetic behavior were revealed, namely: oscillating ET (that could occur at picoseconds) and monotonically relaxing ET. On a longer time scale, the ET dynamics can include two different kinetic components. The faster one reflects the initial, nonadiabatic ET, whereas the slower one is governed by the medium relaxation. We derived a simple relation between the relative extents of these components, the change in the free energy (DeltaG), and the energy of the slow reorganization Lambda. The rate of ET was found to be determined by slow relaxation at -DeltaG < or = Lambda. The application of the developed approach to experimental data on ET in the bacterial photosynthetic reaction centers allowed a quantitative description of the oscillating features in the primary charge separation and yielded values of Lambda for the slower low-exothermic ET reactions. In all cases but one, the obtained estimates of Lambda varied in the range of 70-100 meV. Because the vast majority of the biological ET reactions are only slightly exothermic (DeltaG > or = -100 meV), the relaxationally controlled ET is likely to prevail in proteins.

Amphiphilic copolymers composed of styrene and maleic acid (SMA) monomers caused a major methodical breakthrough in the study of membrane proteins. They were found to directly release phospholipids and membrane proteins both from artificial and natural lipid bilayers, yielding stable water-soluble discoidal SMA/lipid particles (SMALPs) of uniform size. Although many empirical studies indicate the great potency of SMALPs for membrane protein research, the mechanisms of their formation remain obscure. It is unknown which factors account for the very assembly of SMALPs and govern their uniform size. We have developed a coarse-grained (CG) molecular model of SMA copolymers based on the MARTINI CG force field and used it to probe the behavior of SMA copolymers with varying composition/charge/concentration in solution as well as their interaction with lipid membranes. First, we found that SMA copolymers tend to aggregate in solution into clusters, which could account for the uniform size of SMALPs. Next, molecular dynamics (MD) simulations showed that periodic SMA copolymers with styrene/maleic acid ratios of 2:1 ([SSM] n ) and 3:1 ([SSSM] n ) differently interacted with lipid bilayers. While clusters of 2:1 SMA copolymers induced membrane poration, the clusters of 3:1 SMA copolymers extracted lipid patches from the membrane yielding SMALP-like structures. Extraction of lipid patches was also observed when we simulated the behavior of 3:1 copolymers with varying lengths and statistical distribution of styrene and MA units. Analysis of MD simulation trajectories and comparison with experimental data indicate that the formation of SMALPs requires copolymer molecules with a sufficient number of units made of more than two sequential styrene monomers.

Crossing the membrane/water interface is an indispensable step in the transmembrane proton transfer. Elsewhere we have shown that the low dielectric permittivity of the surface water gives rise to a potential barrier for ions, so that the surface pH can deviate from that in the bulk water at steady operation of proton pumps. Here we addressed the retardation in the pulsed proton transfer across the interface as observed when light-triggered membrane proton pumps ejected or captured protons. By solving the system of diffusion equations we analyzed how the proton relaxation depends on the concentration of mobile pH buffers, on the surface buffer capacity, on the form and size of membrane particles, and on the height of the potential barrier. The fit of experimental data on proton relaxation in chromatophore vesicles from phototropic bacteria and in bacteriorhodopsin-containing membranes yielded estimates for the interfacial potential barrier for H(+)/OH(-) ions of approximately 120 meV. We analyzed published data on the acceleration of proton equilibration by anionic pH buffers and found that the height of the interfacial barrier correlated with their electric charge ranging from 90 to 120 meV for the singly charged species to>360 meV for the tetra-charged pyranine.