Photoselection in ESP Spectra of Photosynthetic Reaction Centers

The first relatively stable stage of photoinduced electron transfer in reaction centers (RCs) of many photosynthetic bacteria is P+Q A - , where P is the so — called primary donor (dimer of bacteriochlorophyll molecules), and Q A - primary acceptor, a molecule of quinone. As early as 1977 [1], it was reported that when the normally present magnetic interaction between Q A - and paramagnetic Fe2+ — ion is disrupted, an electron spin — polarised (ESP) photoinduced signal may be detected using time — resolved EPR. This signal was subsequently attributed to the state P+Q A - , which since 1987 [2] is considered as a spin — correlated radical pair (SCRP). Simulation of experimental ESP spectra of P+Q A - using the SCRP model, will provide information on the structural organisation of RC and on interactions between its cofactors. Recently we observed that the shape of the ESP signal of P+Q A - in Zn2+ — substituted RCs of Rb. sphaeroides R26 strongly depends on the wavelength of the excitation laser flash. This dependence arises due to the effect of photoselection, i.e., selective excitation of certain orientations of RCs in the sample by plane — polarised laser light. When taking photoselection into account, the SCRP model satisfactorily describes the quite different ESP signals for light polarised parallel and perpendicular to the magnetic field of the spectrometer.