Redox study of electron donation to P-680 in Photosystem II

Flash-induced absorption changes at 820 nm were studied as a function of redox potential in Tris-extracted Photosystem II oxygen-evolving particles and Triton subchloroplast fraction II particles. The rereduction kinetics of P-680⁺ in both preparations showed biphasic recovery phases with half-times of 42 and 625 μs at pH 4.5. The magnitude of the 42 μs phase of P-680⁺ rereduction was strongly dependent on the redox potential of the medium. This absorption transient, attributed to electron donation from D₁ (the secondary electron donor in oxygen-inhibited chloroplasts), titrated as a single redox component with a midpoint potential of +240 ± 35 mV. The experimentally determined midpoint potential was found to be independent of pH over the tested range 4.5-6.0. In contrast, the magnitude of the 625 μs phase of P-680⁺ rereduction was independent of redox potential between +350 and +100 mV. These results are interpreted in terms of a model in which an alternate electron donor with E_m ≈ 240 mV, termed D₀, serves as a rapid donor (t _{1 2} ≤ 2 μs) to P-680⁺ in Tris-extracted and Triton-treated Photosystem-II preparations. According to this model, the slower electron donor, D₁, is functional only when D₀ becomes oxidized.