Charge transfer processes are ubiquitous in biology. The bacterial photochemical reaction center (RC) from Rhodobacter sphaeroides constitutes an ideal model system for understanding how the protein structure affects the photoinduced electron transfer in membrane proteins as a number of crystal structures from native and mutant samples are available. Furthermore, the RC protein contains several different cofactors or chromophores, which are located throughout the volume of the RC protein. These individual chromophores constitute a series of intrinsic molecular probes that conveniently allow monitoring localized structural changes when examined by spectroscopic methods. In this work, high hydrostatic pressure optical barospectroscopy is used to obtain new insights into the mechanisms that govern the nano-scale electron transport in bacterial RCs.

In different membrane-embedded and detergent-isolated RC samples it is universally detected by picosecond time-resolved fluorescence that compression of the RC protein at ambient temperature under pressures reaching 1 GPa leads to significant (several-fold) acceleration of the primary electron transfer rate. By steady state absorption and fluorescence, Raman, and CD spectroscopy evidence was obtained for a number of local reorganizations in the binding site of the primary electron donor, a dimer of bacteriochlorophyll a molecules (special pair), between the atmospheric pressure and 0.6 GPa in different samples. The effects are generally reversible in a sense that the initial spectral characteristics of the samples are recovered upon the pressure release. Basic analysis of the experimental data suggests that the observed increase of the primary electron transfer rate is a combined effect of an enhancement of the driving force for electron transfer and of modification of the relative geometry of the electron donor and acceptor sites. Either of these factors applied separately does not provide a satisfactory account of the experimental data. In progress is a more advanced analysis, which considers the complex internal structure of the primary electron donor. Even minor variations of the electron donor geometry may result in significant changes in its electronic structure, which among other effects is prone to induce an important mixing between the intra-dimer singlet exciton and charge-transfer states. According to ref. 1, the pressure-induced deformation of the special pair structure is most likely anisotropic, involving interfacial compression and shear of the special pair.