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

Abstract

The manganese containing center of the oxygen evolving complex accumulates four oxidizing equivalents in the four stepped water oxidation cycle. Based on experiments on electrcchromic absorption transients and the reduction rate of the primary electron donor, Peso, 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 electrcchromism 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 electrochromicaIIy responsive pigments rather than responding to proton release from the periphery.

Electrostatics and proton transfer in photosynthetic water oxidation

“…The major portion decays with the typical time constant of the electron transfer from the Mn cluster to Y ox Z . Rappaport and colleagues also reported a smaller, more rapidly decaying component, which they attributed to an intra-protein proton transfer around Y Z , a feature that we did not observe in our experiments despite sufficiently high time resolution (Haumann et al 1994;. The overall behaviour, rapid rise and slow partial decay of the electrochromic transient, with the latter following the reduction of Y ox Z by the Mn cluster, point to the lack of charge neutralization, e.g.…”

The manganese complex of photosystem II in its reaction cycle—Basic framework and possible realization at the atomic level

“…More recently Schlodder and Witt detected a pH-independent release of a single proton by microelectrode measurements on PSII core complexes of the cyanobacterium Synechococcus elongatus [115]. From electrochromic measurements it is clear that no charge is accumulated in the S 0 → S 1 transition [118,128,129]. We conclude that the S 0 → S 1 transition likely involves oxidation of the Mn complex as well as a charge-compensating deprotonation of or close to the Mn complex.…”

“…Proton release has been investigated using electrodes or pH-sensitive dyes [105,[115][116][117]. Electrochromic shifts of UV/vis absorption bands (Stark effect) of PSII pigments have provided insights into changes of the net charge of the PSII manganese complex [103,[118][119][120][121][122]. The thus obtained experimental results are crucial for construction and evaluation of mechanistic models and summarized in the following.…”

“…Later non-integer and pH-dependent proton release has been explained by electrostatically triggered pK-shifts of peripheral bases [105,[115][116][117]126,127]. Electrochromic studies suggest that, irrespective of pH, no charge compensating deprotonation occurs at the Mn 4 Ca-complex [103,118,128,129]. The activation energy is low (∼15 kJ/mol) in H 2 O and D 2 O [99] (12 kJ/mol in [130]) and the S 1 → S 2 transition can proceed even at cryogenic temperatures [131][132][133].…”

Computational studies of the O2-evolving complex of photosystem II and biomimetic oxomanganese complexes

“…The catalytic cycle depicted in Figure 11 indicates that the S 1 →S 2 transition involves simply oxidation of Mn (3), without deprotonation [165][166][167][168][169][170][171] and with little structural rearrangements, or changes in Mn-Mn distances, consistently with EXAFS measurements [42]. The most significant change, associated with the S 1 →S 2 oxidation, is that the cluster becomes positively charged due to the lack of deprotonation.…”