CHARGE RECOMBINATION KINETICS OF PHOTOSYNTHETIC REACTION CENTERS IN WATER-IN-OIL PHOSPHOLIPID ORGANOGELS

Photosynthetic reaction centers from Rhodobacter sphaeroides were reconstituted in phospholipid vesicles, extracted in hexane and incorporated in organogels. In each reconstituted system, absorption and circular dichroism spectra for the reaction center revealed the absence of significant protein denaturation and maintenance of the relative geometries of the bacteriochlorophyll molecules. The light-dark spectra showed that the photochemical activity of the reaction center was retained in all the systems examined. The kinetics of charge recombination between the primary photo-oxidized electron donor (P+) of the reaction center and the primary (Q(A)(-)) and secondary (Q(B)(-)) quinone accepters were investigated by time-resolved absorption Spectroscopy. Kinetic analysis of flash-generated P+ decay demonstrated that the light-induced charge separation involved Q(B). In all the lipidic systems examined (proteoliposomes, hexane extract, organogel), the recombination kinetics of the charge separated state P(+)Q(B)(-) did not fit a single exponential decay. To account for the multiphasic kinetics observed, multiple conformational states of the reaction center (possibly determined by local heterogeneity of the lipidic systems) are considered. An alternative (or additional) factor, affecting the recombination kinetics of the P(+)Q(B)(-) state, is the exchange interaction of the excess quinone with the Q(B) binding site.