Photosystem electron donors

Figure 12.8 The Z scheme, an overview of the flow of electrons during the light-dependent reactions of photosynthesis. ED and EA refer to the electron donors and acceptors of the two photosystems, respectively...

An alternative approach, using semiconductors as light-driven electron donors, has been demonstrated in model systems (Gratzel 1982 Nikhandrov et al. 1988). These are more stable than the photosystems, but show lower photochemical conversion efficiencies owing to short-circuiting of reducing equivalents. The presently used... 

The question of the molecular basis for the S states has existed since the original proposal by Kok and coworkers. As first formulated, the S state designation referred to the oxidation state of the O2-evolving center which could, in principle, include all of photosystem II and its associated components. Indeed, there are a number of redox-active components on the electron-donor side of photosystem II in addition to the Mn complex, such as the tyrosine radical that gives rise to EPR signal, and cytochrome b jg. 

When the primary electron donation pathway in photosystem II is inhibited, chlorophyll and p-carotene are alternate electron donors and EPR signals for Chl+ and Car+ radicals are observed.102 At 130 GHz the signals from the two species are sufficiently resolved to permit relaxation time measurements to be performed individually. Samples were Mn-depleted to remove the relaxation effects of the Mn cluster. Echo-detected saturation-recovery experiments were performed with pump pulses up to 10 ms long to suppress contributions from cross relaxation and spin or spectral diffusion. The difference between relaxation curves in the absence of cyanide, where the Fe(II) is S = 0, and in the presence of cyanide, where the Fe(II) is S = 2, demonstrated that the relaxation enhancement is due to the Fe(II). The known distance of 37 A between Fe(ll) and Tyrz and the decrease of the relaxation enhancement in the order Tyrz > Car+ > Chl+ led to the proposal of 38 A and > 40A for the Fe(II)-Car+ and Fe(II)-Chl+ distances, respectively. Based on these distances, locations of the Car+ and Chl+ were proposed. 

The reaction center of plant photosystem II, P680, passes electrons to plastoquinone, and the electrons lost from P680 are replaced by electrons from H20 (electron donors other than H20 are used in other organisms). 

It is established that the primary electron acceptor in photosystem 1 is the molecule ferredoxin, while in photosystem 2 it is a quinone. The identity of the primary electron donor in photosystem 2 is still unknown the oxidation of water must take place by electron transfer to this primary donor, X. 

Photosystem 1 is basically similar to the photosynthesizing system of bacteria just discussed. The difference between PSl and the photosystem of bacteria lies mainly in the fact that, instead of bacteriochlorophyll P890, the photochemical active centre of PSl contains chlorophyll a as a primary electron donor having the peak in the differential absorption spectrum at 700 nm and thus denoted as P700. In PS2 the primary donor of electrons is a chlorophyll molecule P680 with the peak in the differential optical spectrum at 680 nm. Photosystems 1 and 2 are located close to each other. Between them there is an electron transport chain containing molecules of plasto-quinones and cytochromes. 

You add a nonphysiological electron donor to a suspension of chloroplasts. When you illuminate the chloroplasts, the donor becomes oxidized. How can you determine whether this process involves both photosystems I and II (In principle, the donor can transfer electrons either to some component on the 02 side of photosystem II or to a component between the two photosystems.)... 

A closely related organized microheterogeneous system has been composed of a Si02 colloid on which a Pd catalyst is immobilized, and the photosystem includes eosin, Eo2 (2), as photosensitizer, iV,A -dibenzyl-4,4 -(3,3 -dimethyl)bipyridinium, BMV2+ (15), as electron relay and TEOA as electron donor [137]. In a... 

Photosensitized generation of hydrido-metal complexes in aqueous media provides a general route for H2-evolution, hydrogenation of unsaturated substrates (i.e. olefins, acetylenes), or hydroformylation of double bonds, see Scheme 2. Co(II) complexes, i.e. Co (II)-fn s-bipyridine, Co(bpy) +, or the macrocyclic complex Co(II)-Me4[14]tetraene N4, act as homogeneous H2-evolution catalysts in photosystems composed of Ru(bpy) + (or other polypyridine (Ru(II) complexes) as photosensitizers and triethanolamine, TEOA, or ascorbic acid, HA-, as sacrificial electron donors [156,157], Reductive ET quenching of the excited photosensitizer... 

The water-soluble Wilkinson-type catalyst chlorotris(diphenylphosphinoben-zene-m-sulfonate)rhodium(I), RhQfdpm) (19), acts as catalyst for H2-evolution [158], hydrogenation and hydroformylation [159]. In a photosystem composed of Ru(bpy)i+ as photosensitizer, ascorbic acid, HA, as electron donor and RhCl(dpm)3, hydrogen evolution proceeds with a quantum efficiency corresponding to (p = 0.033. In the presence of ethylene or acetylene, hydrogen evolution is blocked and hydrogenation of the unsaturated organic substrates predominates. Table 6 summarizes the quantum yields for H2-evolution and... 

TEOA, as electron donor yields upon illumination in the presence of C02 and the Ru-colloid, methane as major photoproduct, cp = 4 x 10 4, and ethylene and ethane at lower yields, cp = 7.5x 10 5 and cp = 4x 10 5 respectively. In this photosystem, reductive ET quenching of excited Ru(bpz) + yields the reduced photoproduct Ru(bpz)j (E° = —0.86 V vs. SCE) that mediates the reduction of COz to methane and hydrocarbon oligomers (Fig. 30a). Interestingly, the reduced photoproduct Ru(bpz)J although thermodynamically capable, does not effect H2-evolution from the system. On the other hand, a series of photosystems composed of Ru(bpy) + as photosensitizer, TEOA as sacrificial electron donor and different bipyridinium electron acceptors (23)-(26) exhibit non-specificity, and... 

Other Co(II)-complexes that were applied in the photosensitized reduction of C02 to CO (and concomitant H2-evolution) include Co(II)-ethylene glycol dimethyl ether complexes [178], and different tetraaza-macrocyclic Co(II)-complexes such as 27,28. A closely related system, where Ni(II)-tetraaza macrocycle (29) substitutes the cobalt homogeneous complexes in the photosystem including Ru(bpy) + as photosensitizer and ascorbic acid as electron donor, has been reported by Tinnemans [181] and Calvin [182],... 

C02-fixation to formate is catalyzed by formate dehydrogenase, ForDH. Photogenerated MV+ mediates the reduction of C02 to formate [200]. Other bipyridinium radicals, such as JV,j V -dimethyl-2,2 -bipyridinium or JV,Ar -trime-thylene-2,2 -bipyridinium radical cation act also as charge carriers for ForDH. The photosystem that Was utilized for generation of MV+ and 002-fixation includes Ru(bpy)f+ as photosensitizer, cysteine as sacrificial electron donor and MV2+ as electron acceptor. The net photosynthetic process accomplished in this photosystem (Fig. 40) corresponds to the reduction of 0O2 to formate by cysteine, see Eq. (70). This is an endoergic transformation by ca. 12.5 kcal mol-1. 

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