Silicomolybdate substitutes for the function of a primary electron acceptor and stabilizes charge separation in the photosystem II reaction center complex

Effects of silicomolybdate on the charge recombination between P680⁺ and the reduced pheophytin were studied by absorption and electron paramagnetic resonance spectroscopies in the photosystem II D1/D2/cytochrome b-559 reaction center complex. This preparation lacks the primary and secondary quinone acceptors, Q_A and Q_B, and exhibits the charge recombination which produces the triplet state of P680 to a large extent. In the presence of silicomolybdate, the light-induced triplet signal of P680 was almost completely eliminated at cryogenic temperatures as well as at 4°C. Under these conditions, two types of signals, one reversible and the other irreversible, which are ascribable to P680⁺ and the cation radical of antenna chlorophyll a, respectively, were generated upon illumination at cryogenic temperatures. These results indicate that silicomolybdate, which is known to be an artificial electron acceptor of Q_A, rapidly receives electrons from the reduced pheophytin even at cryogenic temperatures and thus suppresses the radical pair recombination which occurs in the time range of nanosecond. P680⁺ formed by flash excitation in the presence of silicomolybdate relaxed mainly with a long half decay time of 74 ms at 4°C. This indicates that the reduction of P680⁺ by the secondary electron donor, Z, is significantly decreased.