Dimeric primary electron donor

In bacterial RCs, the light-driven reactions are initiated by a non-covalently linked dimeric primary electron donor, D, also called a special pair . The existence of such a dimer was first postulated from spin-resonance experiments (Norris et al., 1971). This special pair was identified as a homodimeric bacteriochlorophyll a (Rb. sphaeroides) or b (Rp. viridis) complex. Its position is close to the periplasmic side at the L and M polypeptide interface. In the Rp. viridis RC the dimer is located close to heme 3 of the cytochrome-c subunit. The helices C, D, E, and cd of both, L- and M-subunits, stabilize the... 

Vermeglio, A., Clayton, R.K. Orientation of chromophores in reaction centers of Rhodopseudomonas sphaeroides. Evidence for two absorption bands of the dimeric primary electron donor. Biochim. Biophys. Acta 449, 500-515 (1976)... 

Bacterial reaction centers (RCs) contain three protein subunits (L, M and H), four bacteriochlorophylls (BChl), two bacteriopheophytins (H), two quinones (Q) and one atom of Fe [1]. Accordingly to spectral properties [2] and X-ray analysis [3] two BChls form a dimer, primary electron donor P. Two other BChls (B) and two H form two transmembrane cofactor chains located in L and M protein subunits and terminated by two Qs P-BL-HL-QA and P-BM-HM-QB [3]. Only LA-chain is photochemically active. The excited state P within 3-4 ps at 293K transfers an electron to (BLHL) acceptor complex [4-6]. An electron is further transfered to QA within 200 ps. 

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. 

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. 

Fig. 4.2. Optical and ESR spectra of electron carriers in the RC of purple photosynthetic bacteria. (A and B) Primary electron donor (D,) bacteriochlorophyll dimer. (A) Light-dark optical spectrum (recorded at 30 °C) and (B) ESR spectrum of D, in Rps. sphaeroides. The ESR spectrum is y/2-times narrower than the corresponding spectrum of the Bchl cation, indicating a dimeric structure (from Ref. 3).

In green plants, algae, and cyanobacteria, the primary photochemical events of photosynthesis occur in the protein-pigment complex called photosystem II (PSII). PSII consists of more than ten polypeptide chains and a number of co-factors important for electron transport.(i, 6) The co-factors are believed bound to two homologous polypeptides approximately 32 kD in size (D1 and D2). Photoexcitation of the PSD reaction center drives single electron transfer from the primary electron donor, P, (probably a dimer of chlorophyll a) to the primary electron acceptor, one of two pheophytin a molecules. The reduced pheophytin transfers the electron on to a primary plastoquinone... 

The PS-1 reaction center is remarkably similar to the reaction center in photosynthetic bacteria and to photosystem 11 in green plants with respect to the apparent symmetrical arrangement of the major proteins and the associated pigment molecules and cofactors. For example, the two large heterodimerforming proteins that are encoded by the psaA and psaB genes, in photosystem I, are the counterparts of the L- and M-subunits of the photosynthetic bacterial reaction center and of the D1 and D2 subunits of the PS-11 reaction center. While both the PS-11 and purple bacterial reaction centers use pheophytin and quinones (plastoquinone, ubiquinone, or menaquinone) as the primary and secondary electron acceptors, the PS-1 reaction center is similar to that of green sulfur bacteria and heliobacteria in the use of iron-sulfur proteins as secondary electron acceptors. It may be noted, however, that the primary electron donor in all reaction centers is a dimer of chlorophyll molecules. 

The consistency of the RC indicates that structural or assembly needs dictate a particular packing arrangement of the transmembrane proteins that embed the colactors. The latter consists of a well conserved tetrapyrrole dimer, as the primary electron donor, suggesting that this is an essential requirement for any functional photosystem. Within this constraint, the individual characteristics of the donor can be modified to adjust the changes in the photosynthetic pathway due to dilTerent light conditions, as well as the presence of UV and ionising radiation, that characterized... 

The primary photochemistry in bacterial reaction centers (RCs) involves the light-induced electron transfer from a primary electron donor, D (a bacteriochlorophyll dimer), through a series of electron acceptors (a bacteriopheophytin, ( )a, and a primary quinone, Q ) to a secondary quinone acceptor, Qb (reviewed in ref. 1). The charge transfer is accompanied by protonation of the quinones which is the first step in proton translocation across the bacterial membrane. The electrochemical proton gradient formed across the membrane provides the driving force for ATP synthesis (reviewed in ref. 2). Thus, electron transfer can be viewed as a mechanism for setting up the system to carry out the physiologically important function of proton translocation (2). 

From a proteic point of view, the location of the light-induced reactions in PS1 appears to be very different from that in PS2 or in bacterial RCs. Indeed, in the bacterial RCs the membrane-embedded part of the photosystems that carry the first electron donors and acceptors consists of two proteic subunits of 250-350 amino acids, whereas psa A and psa B are each constituted of 750 amino acids. If one compares the function of the different photosystems, it clearly appears that many of the electron tranfer steps are similar between PS2 and bacterial RCs. In these RC, after excitation of the primary electron donor, the electron rapidly jumps from a chlorophyllic structure (a dimer of BChl in bacterial RCs) to a (bacterio)pheophytin. From the (bacterio)pheophytin the electron is transferred to a quinone then to a second quinone. In PS1, after the excitation of the primary electron donor, the electron jumps rapidly from the primary electron donor P700 (most likely a dimer of Chls) to a chlorophyll (Aq) from Aq it is transferred to the A acceptor, and thence to a series of iron-sulfur clusters (Fx. Fg and Fb) (4). If some structural analogy may be found between all the photosystems. It obviously will concern the proteic features related to the first electronic steps, e.g. those which are located in the local environment of the primary donor and/or primary acceptors of electrons. 

Fig. 11.7 Pump-probe measurements of stimulated emission from (A) the laser dye IR132 in dimethylsulfoxide, and (B) the lowest excited singlet state of the bacteiochlorophyll dimer that serves as the primary electron donor in reaction centers from Rhodobacter sphaeroides [23]. The excitation flash was centered at 830 nm and had a width at half-maximum amplitude (FWHM) of about 16 fs in both cases. The probe pulse had a FWHM of about 80 fs and was crmtered at 900 nm in (A) and 940 nm in (B). The ordinate scales are arhitrary...

The realization that the primary electron donor in bacterial photosynthetic systems is a closely-coupled bacteriochlorophyll dimer has stimulated considerable interest in the photophysics of face-to-face porphyrin dimers [1-3]. Many such synthetic models have been described, including both symmetrical and asymmetrical porphyrin systems. Much of the interest in determining the photophysical properties of these face-to-face porphyrin dimers lies with the possibility that excitation may result in generation of a charge separated state [4]. In fact, the nature of the central metal cations determines the reaction exergonicity for photoinduced charge separation, at least for monophotonic events. Below, we illustrate the current status of this subject by reference to symmetrical (i.e., the same cation in both porphyrins) and asymmetrical (i.e., different cations in each porphyrin) dimers. 

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