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denoted PD1, PD2, ChlD1, and ChlD2. These are assigned to the charge separation apparatus of PS‐II, called P680 because of the absorption maximum at 680 nm observed for the cationic radical. Transfer of excitation energy to this set of chromophores results in a charge‐separated state. Our knowledge of the possible nature of initial excited states and localization of charge separation among the core chromophores under various conditions is incomplete [72], but it is accepted that within picoseconds charge separation stabilizes in a state that can be described as [P680•+ PheoD1•−] [73]. The electron hole is principally localized on chlorophyll PD1 [74], as opposed to being equally distributed in the PD1PD2 pair, presumably as a combined result of the pigment conformation and the effect of the protein matrix [75–77]. Electron transfer on the acceptor side subsequently occurs within hundreds of picoseconds from PheoD1•− to the non‐exchangeable plastoquinone QA. Electron transfer from QA− to the final electron acceptor plastoquinone QB is much slower compared with the previous electron transfer steps, up to a millisecond for the second reduction that creates the plastoquinol (QBH2) that will be released to carry the two electrons further down the chain as depicted in Figures 3.2 and 3.6. The nonheme iron and its (bi)carbonate ligand mediate and modulate electron transfer between the QA and QB sites without Fe participating directly in redox chemistry [78–81].
The charge separation site of PS‐II is interfaced with the water oxidation site via a redox‐active tyrosine (D1‐Tyr161) known as YZ. While the enzyme is poised in the [P680•+ QA•−] charge‐separated state, YZ is oxidized by P680•+ within tens of nanoseconds. The redox‐active YZ is tightly hydrogen‐bonded to the imidazole side group of histidine D1‐His190 [82], which in turn is hydrogen‐bonded to the conserved [83] asparagine D1‐Asn298. Formation of the tyrosyl radical is thought to be coupled to proton shift from the phenolic proton of YZ to His190 and possibly to further proton translocation from His190 to Asn298 [84–86]. The tyrosyl radical YZ• is reduced directly by the Mn4CaOx cluster of the OEC in the micro‐ to millisecond time scale. Successive oxidations of the OEC by the YZ• radical formed after each light‐driven charge separation event lead to accumulation of electron holes (oxidizing equivalents) at the manganese cluster. Four holes are stored at the OEC before it can catalyze the four‐electron oxidation of water into dioxygen. The details of the catalytic cycle of the OEC will be discussed in the next section of this chapter.
Another redox‐active tyrosine (D2‐Tyr160, YD) is found in a position homologous to YZ (see Figure 3.7), but that branch does not contain a water oxidation site. YD presumably participates in regulatory and protective mechanisms of PS‐II, such as influencing the charge distribution among the chlorophylls of P680•+ or resetting the OEC to its resting state at night [75, 87–92]. Like YZ, the YD tyrosine is hydrogen‐bonded to a histidine residue (D2‐His189), but otherwise it is located in a hydrophobic region as opposed to the water‐rich environment of YZ and displays slower redox kinetics compared with YZ [93–96]. A single water molecule present within a phenylalanine‐rich cavity adjacent to YD and which can occupy either a proximal or a distal position with respect to the phenolic side chain is suggested to regulate the redox behavior of YD. [97, 98]
The central design principle of the electron transfer cascade is that the thermodynamic properties of each redox‐active component and the kinetics of electron transfer contribute to stabilization of charge‐separated states, ensuring high quantum yield [99] and directionality of electron transfer. The fast increase in the distance between electron and hole suppresses recombination reactions, but at the same time the multiple steps involved in the process lead to a decrease in free energy differences, reducing the total efficiency. PS‐II successfully couples processes that occur in time scales spanning several orders of magnitude, but it is important to note that under normal operating conditions the enzyme has a lifetime of less than half an hour. Damage originates principally in formation of triplet‐state chlorophyll, whose reaction with triplet dioxygen creates highly reactive, hence damaging singlet dioxygen [100]. The functionality of PS‐II is restored through highly efficient repair mechanisms [101–107].
Research into artificial molecular charge‐separating systems has a long history [39, 104–108]. The central challenge in artificial constructs is to stabilize the charge‐separated state long enough that it can perform redox reactions. For the charge‐separated state to be kinetically competent, it has been realized early on that species comprising at least three components, i.e. triads instead of simple electron donor–acceptor dyads, are required. A representative example of such a system is the molecular carotenoid–porphyrin–fullerene (C–P–C60) triads [39, 109]. In this case light excitation leads first to formation of an excited singlet state localized on the central light‐absorbing porphyrin dye (C–P*–C60). The initial excited state then relaxes to a charge‐separated C–P•+–C60•− state. Charge recombination between the porphyrin and the fullerene is outcompeted by efficient hole transfer to the carotene, leading to the C•+–P–C60•− state with a quantum yield of 95% [109]. The spatial separation of charges in this state contributes to lifetimes in the scale of tens to hundreds of nanoseconds in solution or microseconds in a glass matrix [109, 110]. Even more complicated molecular constructs have been reported that incorporate their own antenna systems and photoprotection units [111, 112]. The use of components based on transition metal ions, particularly ruthenium photosensitizers that can be directly linked to manganese‐based oxidation catalysts, also has a long history and is an active field of research [113–116]. A thorough overview of many additional molecular systems for photoinduced electron transfer is provided in the review by El‐Khouly et al. [12] The challenges in this field, at least in terms of molecular systems discussed in the present chapter, remain the achievement of robustness, kinetic competence of charge‐separated states, and coupling of the one‐electron chemistry with accumulation of oxidizing equivalents so that concerted multi‐electron transformations can be achieved.
3.5 Water Oxidation
Light harvesting, excitation energy transfer, and charge separation are functions shared by all types of photosynthetic organisms. What is special about oxygenic photosynthesis is the use of water as the ultimate electron donor by PS‐II. Water oxidation takes place at an active site, the OEC, that harbors an inorganic oxo‐bridged Mn4Ca cluster. The cluster is readily assembled from Mn2+ and Ca2+ in solution through a process known as photoassembly [66, 117]. The OEC is successively oxidized by the YZ tyrosyl radical (D1‐Tyr161), storing up to four oxidizing equivalents before releasing dioxygen. The storage/catalytic cycle of the OEC is described by the Kok–Joliot cycle of Si states (i = 0–4), with S0 being the lowest oxidation state of the OEC and S4 the highest state that evolves dioxygen (Figure 3.8). Among those states S1 is the resting state of PS‐II, i.e. the state to which the enzyme reverts if left in the dark. YZ and the Mn cluster are in close spatial and electronic contact. Along each Si → Si+1 transition, the intermediates SiYZ• formed when P680•+ oxidizes YZ have a finite lifetime at low temperature and can be studied by electron paramagnetic resonance (EPR) spectroscopy [86, 118–125]. Studies of these intermediates provide information about the tyrosyl radical itself, its interaction with the manganese cluster, the spin state of the cluster, and changes in hydrogen bonding and protonation occurring during the S‐state transition. Storing the four oxidizing equivalents before performing the four‐electron water oxidation provides a low‐energy pathway for