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 S 2 and S 3 have a surplus charge compared to S 0 and S 1 . 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 S 0 → S 1 → S 2 → S 3 → S 0 .