Evidence for the electrochromic identification of the change of charges in the four oxidation steps of the photoinduced water cleavage in photosynthesis

After dark adaptation of oxygen-evolving photosystem (PS) II complexes, oscillatory absorption changes (stable >0.5 s) with a period of 4 were previously detected at 514 nm, probably due to electrochromic band shifts of carotenoids [(1984) FEBS Lett. 176, 83-87]. To support this interpretation, we looked for corresponding phenomena in the 650–714 nm range. Here, oscillations were also observed with a difference spectrum giving evidence for an electrochromic band shift of the chlorophyll a absorption band. The oscillation pattern 0:0:+:+ corresponds to those at 514 nm, supporting the former conclusion that the states S2 and S3 have a surplus charge compared to S0 and S1. This is possible if the electron release pattern 1:1:1:1 is accompanied by an intrinsic proton release pattern 1:0:1:2 for the transition S0 → S1 → S2 → S3 → S0.