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 Em ≈ 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.
All Science Journal Classification (ASJC) codes
- Cell Biology