Water oxidation Photosynthesis

In photosynthesis, water oxidation is accomplished by photosystem II (PSII), which is a large membrane-bound protein complex (158-161). To the central core proteins D1 and D2 are attached different cofactors, including a redox-active tyro-syl residue, tyrosine Z (Yz) (158-162), which is associated with a tetranuclear manganese complex (163). These components constitute the water oxidizing complex (WOC), the site in which the oxidation of water to molecular oxygen occurs (159, 160, 164). The organization is schematically shown in Fig. 18. 

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...

The protons/electrons produced in water oxidation at a photoanode side of a PEC device could be used (on the cathode side) to reduce C02 to alcohols/hydrocarbons (CH4, CH30H, HC00H, etc.). In this way, an artificial leaf (photosynthesis) device could be developed [11]. While nanocarbon materials containing iron or other metal particles show interesting properties in this C02 reduction [106], it is beyond the scope of this chapter to discuss this reaction here. It is worthwhile, however, to mention how nanocarbon materials can be critical elements to design both anode and cathode in advanced PEC solar cells. Nanocarbons have also been successfully used for developing photocatalysts active in the reduction of C02 with water [107]. 

Fig. 3.2b Electron transport in an artificial photosynthesis scheme. M = light sensitizer, M = a water oxidation site, and A = a reduction site.

The potential level of the 02 evolving site of the photosynthesis (see Fig. 1) ranging around 0.82 V shows that a four-electron process occurs in it. The water oxidation site of the photosynthesis contains more than four Mn ions interacting with each other, thus leading to the four-electron reaction of water to give 02, Such a multielectron reaction leads to the generation of H2 from proton reduction as described later in chapter 4 on water photolysis. 

Figure 20-7 Simplified representation of the photoreactions in photosynthesis. The oxidation of water is linked to the reduction of NADP by an electron-transport chain (dashed line) that is coupled to ATP formation (photophosphorylation).

The oxygen formed clearly comes from H20 and not from C02, because photosynthesis in the presence of water labeled with lgO produces oxygen labeled with 180, whereas carbon dioxide labeled with 180 does not give oxygen labeled with 180. Notice that the oxidation of the water produces two electrons, and that the formation of NADPH from NADP requires two electrons. These reactions occur at different locations within the chloroplasts and in the process of transferring electrons from the water oxidation site to the NADP reduction site, adenosine diphosphate (ADP) is converted to adenosine triphosphate (ATP see Section 15-5F for discussion of the importance of such phosphorylations). Thus electron transport between the two photoprocesses is coupled to phosphorylation. This process is called photophosphorylation (Figure 20-7). 

Another approach to wards photocatalysis is to use dy as a sensitizer instead of a semiconductor as in photosynthesis. It is not the aim of this book to cover all the aspects of the sensitized photochemical conversion system, but typical sensitized systems for photocatalytic reactions of water are described in Chapter 18 The concept of a photochemical conversion system using a sensitizer and water oxidation/reduction catalysts is mentioned in Chapter 19, accompanied by a discussion on the sensitization of semiconductors. 

Although catalytic water oxidation (dark reaction) is the first and important reaction of the electron flow in the photosynthesis represented by Fig. 19.1 whereby water is used as the source of electrons provided to the whole system, its catalyst and reaction mechanism are not yet established.10-13) In the photosynthesis Mn-protein complex works as a catalyst for the difficult four-electron oxidation of two molecules of water to liberate one 02 molecule (Eq. (19.2)). It is inferred that at least four Mn ions are involved in the active center, but its structure is not yet completely elucidated. 

Molecular catalyst for water oxidation has been attracting a great deal of attention not only as a model for the photosynthetic catalyst but also as a component in an artificial photosynthesis. Many structural models have been synthesized and investigated as the photosynthetic Mn complex. Tetrakis(2,2 -bipyridine)(di /i -oxo)di-Mn complex lu) and tetrakis(2,2 -bipyridine)( jJ. -oxo)di-Ru complex (2, Meyer s complex)14) are typical examples, but many of them failed to show high activity for water oxidation. 

The invention of aerobic photosynthesis, the light-driven oxidation of water to oxygen, stands as one of the pivotal evolutionary innovations in the history of life on Earth. The process is carried out only at the oxygen-evolving complex (OEC) of PSII in plants and algae, as well as in cyanobacteria. Despite the biological uniqueness of water oxidation to 02, several of the core proteins of PSII have homologues in the so-called type I and type II anaerobic photosynthetic reaction... 

Yagi M, et al. Molecular catalysts for water oxidation toward artificial photosynthesis. Photochem Photobiol Sci. 2009 8(2) 139-47. 

Messinger J. Evaluation of different mechanistic proposals for water oxidation in photosynthesis on the basis of Mn40xCa structures for the catalytic site and spectroscopic data. Phys Chem Chem Phys. 2004 6(20) 4764 71. 

Zaharieva I, Najafpour MM, Wiechen M, Haumann M, Kurz P, Dau H. Synthetic manganese-calcium oxides mimic the water-oxidizing complex of photosynthesis functionally and structurally. Energy Environ Sci. 2011 4(7) 2400-8. 

Inoue H, Shimada T, Kou Y, et al. The water oxidation bottleneck in artificial photosynthesis how can we get through it An alternative route involving a two-electron process. ChemSusChem 2011 4 173-9. 

Light energy conversion and water-oxidation systems in photosynthesis... 

By far the most important role of manganese in nature is its direct involvement in the photocatalytic, four-electron oxidation of water to dioxygen in green plant photosynthesis, an essential process for the maintenance of life. Pirson, in 1937, first discovered the requirement of manganese in photosynthesis by showing that plants grown in a Mn-deficient medium lost their water oxidation capacity (184). During the next four decades, several researchers showed that two photosystems, photosystem I (PSI) and photosystem II (PSII), were involved in photosynthesis and that 02 evolution and Mn were localized at PSII (for a review, see Ref. 185). 

We begin our discussion of electron flow in photosynthesis with the water oxidation step ... 

R13 Dismukes, G.C. (1986) The organization and function of manganese in the water-oxidizing complex of photosynthesis. In Manganese in Metabolism and Enzyme Function (Wedler, F.C. and Schram, V.L., eds.), Academic Press, New York, in the press. 

---