Primary photochemistry of iron-depleted and zinc-reconstituted reaction centers from Rhodopseudomonas sphaeroides

Abstract

The primary photochemistry of Fe-depleted and Zn-reconstituted reaction centers from Rhodopseudomonas sphaeroides R-26.1 was studied by transient absorption spectroscopy and compared with native, Fe2+-containing reaction centers. Excitation of metal-free reaction centers with 30-ps flashes produced the initial charge-separated state P+I-(P+BPh-, where P is the primary donor and BPh is bacteriopheophytin) with a yield and visible/near-infrared absorption difference spectrum indistinguishable from that observed in native reaction centers. However, the lifetime of P+I- was found to increased approximately 20-fold to 4.2 .+-. 0.3 ns (compared to 205 ps in native reaction centers), and the yield of formation of the subsequent state .**GRAPHIC**. (QA is the primary quinone acceptor) was reduced to 47 .+-. 5% (compared to essentially 100% in native reaction centers). The remaining 53% of the metal-free reaction centers were found to undergo charge recombination during the P+I- lifetime to yield both the ground stage (28 .+-. 5%) and the triplet state PR (25 .+-. 5%). Reconstitution of Fe-depleted reaction centers with Zn2+ restored the "native" photochemistry. Possible mechanisms responsible for the reduced decay rate of P+I- in metal-free reaction centers are discussed.