Photosynthetic oxygen evolution: net charge transients as inferred from electrochromic bandshifts are independent of proton release into the medium.

The manganese containing center of the oxygen evolving complex accumulates four oxidizing equivalents in the four stepped water oxidation cycle. Based on experiments on electrochromic absorption transients and the reduction rate of the primary electron donor, P680, it has been speculated that the oscillations of these variables reflect the net charge of the center as calculated from the difference between electron abstraction and proton release into the medium. We compared proton release with electrochromism in thylakoids and core particles, and under variation of the rate of proton release. We found no equivalent of the variations of the extents and the rates of proton release in electrochromism. The oscillatory pattern of the latter reflects the topological properties of the stepped charge storage relative to the position and orientation of electrochromically responsive pigments rather than responding to proton release from the periphery.