The primary electron donor

The electron affinity of radicals is considerably greater than the electron affinity of monomers, hence in the presence of monomer" ions, or in the presence of an excess of the primary electron donors, the dimeric radical ions are rapidly converted into dimeric diions,... 

Let us consider the conditions which favor the formation and survival of the dimeric and polymeric radical ions. This might be achieved by keeping the concentration of monomer high, the concentration of monomer" ions low and by removing the radical ions as rapidly as possible from the zone containing the primary electron donors. Moreover, since the radical ions dimerize, their average life time increases as their concentration decreases. The following experiment should probably produce the best results. 

Though accelerating effect of redox mediators is proved, differences in electrochemical factors between mediator and azo dye is a limiting factor for this application. It was reported that redox mediator applied for biological azo dye reduction must have redox potential between the half reactions of the azo dye and the primary electron donor [37], The standard redox potentials for different azo dyes are screened generally between -430 and -180 mV [47],... 

Zero field splitting (zfs) values in photoexcited triplets of primary donor bacteriochlorophyll a in photosynthetic bacteria are much lower than those found for vitro BChla triplets. There is a pronounced difference in kinetics of population and depopulation of the triplet sublevels as well. The differences have been attributed to the effect of BChla dimerization and it is now generally accepted that the primary electron donor in photosynthetic bacteria consists of a BChla dimer (special pair)(l- ). 

The Primary Donor. - The radical-cation P+ In the bRC of purple bacteria and also in PS I the primary electron donors have been identified as (B)Chl dimers and EPR/ENDOR clearly showed that the unpaired electron and the positive charge - is (asymmetrically) distributed in a supermolecular orbital extending over both dimer halves (see sections 2.1,3.1). Dimer formation has the important consequence of charge delocalization and this stabilization of the primary donor radical-cation leads to a decrease of the oxidation potential. A fine tuning of the potential is possible through interactions with the environment, e.g. via H-bonds. 

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. 

The hepatic endoplasmic reticulum possesses oxidative enzymes called mixed-function oxidases or monooxygenase with a specific requirement for both molecular oxygen and a reduced concentration of nicotinamide adenine dinucleotide phosphate (NADPH). Essential in the mixed-function oxidase system is P-450 (Figure 1.12). The primary electron donor is NADPH, whereas the electron transfer involved P-450, a flavoprotein. The presence of a heat-stable fraction is necessary for the operation of the system. 

A further improvement in the photosystem has been considered (Figure 8.3) [97] by utilizing 1,4-dimethoxynapthalene (DMN) as the primary electron donor (towards 1DCA ) and ascorbic add as the sacrificial electron donor. 

P700 is the primary electron donor, A may be a chlorophyll-a, A a quinone and A2 (also called X) a specialised iron-sulphur centre. A and B are bound iron-sulphur centres characterised by absorption at 430nm. 

P680 is the primary electron donor, Ph is pheophytin-a and Q-Fe is a plastoquinone-iron complex. 

The excitation energy for the functioning of the primary electron donor in the PSI RC is delivered via a core antenna made up of chlorophyll molecules bound to the protein. In contrast to most other photosynthetic systems, where the antennae and RCs are located on distinct complexes, PSI has a combined system in which the light-harvesting Chls are associated with the same protein that binds the redox cofactors of the electron transfer in the RC [16]. 

Primary photochemical events in two site-directed mutants YF(M208) and YL(M208) of RC from Blastochloris viridis, in which tyrosine at position M208 is replaced by phenylalanine and leucine, respectively, were investigated with the use of 1H-ENDOR as well as optical absorption spectroscopy (Mue et al., 2000). The residue at M208 is in close proximity to the primary electron donor, P, the (BChl), and the BPh. Analysis of the experimental data revealed two torsional isomers of the 3-acetyl group of... 

Figure 2. Paths of electron transfer in PSII P680, reaction-center chlorophyll that functions as the primary electron donor P680, first excited singlet state ofP680 Pheo, pheophytin QA, primary quinone electron acceptor QB, secondary quinone electron acceptor cyt b559, cytochrome b559 Chlz, redox-active chlorophyll that mediates electron transfer between cytochrome b559 and P680 YD, redox-active tyrosine that gives rise to the dark-stable tyrosine radical Yz, redox-active tyrosine that mediates electron transfer from the Mn complex to P680.

Nabedryk, E., Allen, J. P., Taguchi, A. K. W., Williams, J. C., Woodbury, N. W., and Breton, J., 1993, Fourier transform infrared study of the primary electron donor in chromatophores of Rhodobacter sphaeroides with reaction centers genetically modified at residues Ml 60 and L131. Biochemistry, 32 13879913885. 

All species contain two different chlorophylls, BChl a and an additional light-harvesting pigment, either BChl c, d ox e [40], As in Chloroflexus these light-harvesting pigments are housed in chlorosomes located adjacent to the cytoplasmic membrane [45,46]. In Chlorobium BChl c is associated with a 7.5-kDa polypeptide. (J.M. Olson and P. Roepstorff, unpublished). A small amount of energytransferring BChl a is also found in the chlorosomes [47,48], but most of the BChl a is found in a water-soluble protein associated with the cytoplasmic membrane [45]. The RC (see Fig. 4 and Ref. 36) is similar to that of PS I in cyanobacteria and chloroplasts. The primary electron donor P-840 = +0.25 V) is a BChl... 

Photosynthetic eubacteria are classified as filamentous, green sulfur, gram-positive linked, purple, and cyanobacteria. All contain membrane-bound RCs in which (B)Chl serves as the primary electron donor. The RCs may be divided into two main types RC-1, in which the initial electron acceptor is a (B)Chl molecule and the secondary acceptor is an Fe-S center, and RC-2, in which the initial acceptor is a (B)Ph molecule and the secondary acceptor is a quinone. RC-1 centers are found in green sulfur and gram-positive linked bacteria, while RC-2 centers are found in filamentous bacteria and purple bacteria. Cyanobacteria contain both RC-1 and RC-2 centers in which the chlorophyll is Chi a. BChl a is found in filamentous, green sulfur and purple bacteria, while BChl g is characteristic of the grampositive line. BChl b is found in certain purple bacteria instead of BChl a. 

In the photosynthetic reactions, the primary electron donor P-700 becomes excited to its lowest excited singlet state and reacts by transferring an electron to the primary electron acceptor. The electron is then further transferred among a set of electron carriers arranged in order of increasing redox potentials (Fig. 2). This set of molecules is often viewed as a linear chain, a view which may not be the case in PS 1. A photochemical description of these events would follow the electron path from the first (more primary) acceptor to more remote (secondary) acceptors. This is not possible because of the uncertainties concerning the early acceptors. We shall thus describe the more remote acceptors first and then move closer to the primary photoreaction. 

When the components of the PS II reaction centre are drawn on a redox scale and compared in this way to those of the purple bacterial reaction centre, a remarkable similarity can be seen between the electron acceptors in each system (Fig. 4). The chemical natures of these components are extremely similar, being made up of a complex of two quinones, an iron atom and a pheophytin (a bacteriopheo-phytin in bacteria). The donor side of PS II in the redox scheme is, however, not comparable to that in bacteria. P-680 may appear to be structurally similar to P-870 in bacteria in that it is made up of chlorophyll (bacteriochlorophyll in bacteria) and that is acts as the primary electron donor however, the P-680/P-680+ redox couple is approximately 600-800 mV more oxidizing than the equivalent bacterial redox couple P-870/P-870, = +450 mV). In addition, PS II has an array of high-potential components which make up the 02-evolving enzyme and which are clearly unique to that system. 

The primary electron donor of PS II was detected as a flash-induced absorption change attributable to chlorophyll a oxidation [160]. Its bleaching maximum is close to 680 nm and it is thus designated P-680 (it is also called Chi an). The fluorescence of the light-harvesting chlorophylls interferes with measurements at this wavelength, thus many kinetic studies of P-680 have been done by measuring the smaller broad absorption increase at around 820 nm [161,162]. This broad absorption in the near infrared is probably responsible for the fact that P-680 is a quencher of chlorophyll fluorescence. 

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