Abstract

The photosynthetic reaction center from purple photosynthetic bacteria is a membrane-bound protein-pigment complex that serves as an excellent model for studying biological energy conversion. This energy conversion takes place by electron transfer reactions, which occur within the protein and are often coupled to conformational changes that influence the lifetime of the charge-separated state. In order to identify these light-induced conformational changes, near the bacteriochlorophyll dimer, wild type and 11 different mutants of reaction centers from Rhodobacter sphaeroides were studied. Upon 1 min illumination the recovery of the charge-separated states, characterized by steady-state and transient optical spectroscopy, was nearly an order of magnitude slower in one group of mutants (including the wild type) than in mutants carrying the Leu to His mutation at the L131 position. The slower recovery, unlike in the mutants carrying His at the L131 position, was accompanied by a substantial decrease of the electrochromic absorption changes associated with the QY bands of the nearby bacteriochlorophyll monomers, plus a large proton release at pH 6, and a decrease up to 79 mV of the oxidation potential of the dimer during the illumination. The results in the mutants carrying His at the L131 position are modeled as arising from the loss of a proton conducting pathway from the dimer to the solvent, which inhibits the formation of the long-lived charge-separated state. On the other hand, combination of the light-induced conformational changes and lipid binding near accessory bacteriochorophyll pigment under optimized conditions resulted in unprecedented 5 orders of magnitude increase in lifetime of the charge-separated state, which sheds light on a new potential application of the reaction center in energy storage as a light-driven biocapacitor. Moreover, these conformational changes near the dimer can also be blocked by Mn2+ binding. The metal ion binding induced a significant ~ 100 mV increase in the oxidation potential of the dimer and inhibition of formation of the long-lived charge-separated state similar to mutants carrying Leu to His mutation at L131 position. The elevation of oxidation potential of the dimer upon Mn2+ binding can make reaction center protein gain some specific functional features of much more complicated photosystem II.