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

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 increase 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 P⁺Q(A)^- (Q(A) 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 state (28 ± 5%) and the triplet state P(R) (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.