Electrons transfer in photosynthesis

Includes revelatory details on the light-dependent pathways of electron transfer in photosynthesis, based on newly available molecular structures... 

Figure 5.8 Increasing spatial separation of ions in the sequential steps of electron transfer in photosynthesis. SP = special pair...

N.M. Chernyavskaya and D.S. Chernyavskii, Tunnel Electron Transfer in Photosynthesis, MGU, Moscow, 1977 (in Russian). 

Govindjee G (ed) (1987) Excitation energy and electron transfer in photosynthesis, Martinus NijhofT, Dordrecht... 

The role of mediator molecules in donor-acceptor electron transfer processes is an item of considerable recent interest [73 — 81]. A lot of research has been done on intermediate acceptors in the electron transfer in photosynthesis and theoretical studies bases on the superexchange interaction have been carried out [76 — 82]. In Refs. [83,84], electron transfer in the presence of ordered mediator molecules with arbitrary energy levels in one-dimensional case [83] as well as electron transfer in the presence of one resonant mediator [84] were considered. 

Figure 19.25. Comparison of Photosynthesis and Oxidative Phosphorylation. The light-induced electron transfer in photosynthesis drives protons into the thylakoid lumen. The excess protons flow out of the lumen through ATP synthase to generate ATP in the stroma. In oxidative phosphorylation, electron flow down the electron-transport chain pumps protons out of the mitochondrial matrix. Excess protons from the intermembrane space flow into the matrix through ATP synthase to generate ATP in the matrix.

As a possible in-vitro model for one-electron transfer in photosynthesis, the photochemical reaction between hemin and Chla in pyridine solution has been studied, and it has been shown that the relatively slow reduction and oxidation of iron porphyrins can be accelerated by the presence of Chla by an order of magnitude and that light further increases the rate or reaction. Chla probably forms a rather stable complex with hemin, as is shown by fluorescence quenching experiments [Brody (17)]. 

The first fluorescence and fluorescence quantum yields from the excited state of the radical anion of 1,4-benzoquinone have been reported by Cook et al., and electron transfer from aromatic donors to the S state of chloranil giving the singlet radical ion pair has been observed on the fs/ps time scale (Hubig et al.). A series of porphyrin quinones having variable acceptor strengths of the quinone moiety has been synthesised by Dieks et al. and may be considered to be well-suited as biomimetic model compounds for studying photochemically-induced electron transfer in photosynthesis. 

Iron-containing proteins are those in which iron atoms are bound with sulfur-containing ligands. The number of labile sulfur atoms is usually equal to that of iron atoms which varies between 1 and 18. The molecular mass of these proteins is in the range of 6000 to 750,000 units. Such proteins effect electron transfer in photosynthesis, nitrogen fixation, and respiration in mitochondria. They are capable of transferring electrons under a potential close to that of a reversible hydrogen electrode. Their oxidation-reduction potential is between +0.35 and -0.49 V. 

Equation (25) describes a process of electron transfer from donor molecules to CO2 (for both oxygenic and anoxygenic photosynthesis). Unlike the anaerobic processes described in Section 3, in which the electron transfer is frequently extracellular (between cells), the electron transfer in photosynthesis is intracellular. As such, it can occur completely via biological electron carriers, and without need for free electron carriers such as H2. However, the presence of hydrogenase enzymes in representatives from both classes of phototrophs [2,24,25] makes H2 metabolism possible in both accessory and primary roles. 

Marcus, R. A. and N. Sudn, Biochim. Biophys. Acta, 811,1985, 265-322. (Details of electron transfer in photosynthesis)... 

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