S-state cycle

Figure 16.7 The S-state cycle model of 02 generation. (From Voet and Voet, 2004. Reproduced with permission from John Wiley Sons., Inc.)...

Figure 39 Schematic representation of the S-state cycle operating in the water oxidation process of photosynthesis together with the oxidation states of the manganese atoms of the Mn4 cluster in the different S-states...

Scheme 5 The S-state cycle of the oxygen-evolving center in photosystem II [114]. 

An important stage in the OEC model development based on this C-shaped cluster is shown in Pig. 19 [118]. This description established the role of the neutral tyrosyl radical (Yz—O ) derived from the Tyrl61 residue on the D1 polypeptide as the primary oxidant of the Mu4 cluster. In addition, the Yz OH/Yz 0 redox system provides a means of coupling proton transfer to electron transfer during the S-state cycle. It has been argued that these H+/e exchanges are accomplished exclusively by hydrogen atom transfer on... 

Miscellaneous Applications of EPR to PS II. - A number of authors used EPR to detect changes of the S-state cycle and the tyrosine radicals and correlated this with other functional aspects of the OEC and PS II. In particular, the prominent S2 state MLS and the tyrosine signals have been used for these investigations. A representative fraction of such work and some other EPR applications from the last years are discussed below. 

Figure 11.13 One of the suggested forms of S-state cycle describing oxygen evolution from manganese centre...

Dau H, Haumann M. Time-resolved X-ray spectroscopy leads to an extension of the classical S-state cycle model of photosynthetic oxygen evolution. Photosynth Res 2007 92 327-43. 

Accumulation of oxidizing equivalents on the donor side of PSII and water splitting the S-state cycle (Figure 3.4.2D). For water oxidation and 02 generation to occur, four electrons must be removed from two water molecules. As the RC only generates one electron hole per photon absorption, the PSII complex must store... 

Fig. 8. Model for the period-of-four oscillation of manganese oxidation, net charge formation, and water deprotonation. The S-state cycle is driven by four photons initiating electron transfer from P680 to Q, with P680 being stepwise reduced by tyrosine Yz. indicates pH-independent net charge of an S-state. "50,260 ns" indicates biphasic kinetics. Figure from Schlodder and Witt (1999) Stoichiometry of proton reiease from the cataiytic center in photosynthetic water oxidation. J Biol Chem 274 30391.

A New Model of Oxygen Evolution Involving Hydrogen-Atom Abstraction by Yz during the S-State Cycle 391... 

Mn, plus Ca ). These 15 positive charges are precisely balanced by 15 negative charges from the five 0x0- and two carboxylato-ligands plus CF. The overall electroneutrality in the Mn-cluster is also preserved during the S-state cycle (see below). 

The manganese cluster remains electrically neutral through the S-state cycle, rather than accumulating positive charges. Since the process operates in a low dielectric medium, this satisfies the virtual requirement that the reactions occur under electrically neutral conditions ... 

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