ELECTRON-TRANSFER IN REACTION CENTERS OF RHODOBACTER-SPHAEROIDES AND RHODOBACTER-CAPSULATUS MONITORED BY FLUORESCENCE OF THE BACTERIOCHLOROPHYLL DIMER

Spectral and kinetic characteristics of fluorescence from isolated rea ction centers of photosynthetic purple bacteria Rhodobacter sphaeroide s and Rhodobacter capsulatus were measured at room temperature under r ectangular shape of excitation at 810 nm. The kinetics of fluorescence at 915 nm reflected redox changes due to light and dark reactions in the donor and acceptor quinone complex of the reaction center as ident ified by absorption changes at 865 nm (bacteriochlorophyll dimer) and 450 nm (quinones) measured simultaneously with the fluorescence. Based on redox titration and gradual bleaching of the dimer, the yield of f luorescence from reaction centers could be separated into a time-depen dent (originating from the dimer) and a constant part (coming from con taminating pigment (detached bacteriochlorin)). The origin was also co nfirmed by the corresponding excitation spectra of the 915 nm fluoresc ence. The ratio of yields of constant fluorescence over variable fluor escence was much smaller in Rhodobacter sphaeroides (0.15 +/- 0.1) tha n in Rhodobacter capsulatus (1.2 +/- 0.3). It was shown that the chang es in fluorescence yield reflected the disappearance of the dimer and the quenching by the oxidized primary quinone. The redox changes of th e secondary quinone did not have any influence on the yield but excess quinone in the solution quenched the (constant part of) fluorescence. The relative yields of fluorescence in different redox states of the reaction center were tabulated. The fluorescence of the dimer can be u sed as an effective tool in studies of redox reactions in reaction cen ters, an alternative to the measurements of absorption kinetics.