Redox potentials bacteriochlorophyll

In the excited singlet state the dimer of bacteriochlorophyll possesses a redox potential of 930 mV, which is sufficient to reduce the intermediate primary acceptor J. The rate of electron transfer from the excited state of bacteriochlorophyll dimer P to J is quite high (k 1011 s-1). The high rate of electron transfer ensures a high quantum yield (0 1) of the charge separation process... 

Thus, most of the data collected on bacterial RC seem to agree on the presence of a bacteriochlorophyll pair which can be oxidized upon illumination. Thermodynamically this donor has been characterized by studying the dependence of photooxidation from the ambient redox potential in Rp. sphaeroides the E -j was found to be 0.44 V and pH independent [14]. 

Fig. 5. EPR spectra associated with the reduced intermediary acceptor i [BO"] formed in the reaction-center/cytochrome compiex and measured at different temperatures and microwave-power levels (A, B and C). (D) Redox titration of the I signal. Samples poised at indicated redox potentials and then illuminated for 3 m before being cooled to 7 K and assayed. See text for details. Figure source Tiede, Prince and Dutton (1976) EPR and optical spectroscopic properties of the electron carrier intermediate between the reaction center bacteriochlorophylls and the primary acceptor in Chromatium vinosum. Biochim Biophys Acta. 449 455, 460.

Zhang L. Y. and Friesner R. A. Ab initio electronic structure calculation of the redox potentials of bacteriochlorophyll and bacteriopheophytin in solution. J. Phys. Chem. 99 (1995) pp. 16479-16482. 

One of the most interesting differences that is known between the primary electron donors of bacteria and photosystem II of plants is the dramatically higher oxidation potential for P in the plant system. The difference in redox potentials of these two donors in vivo is much larger than the difference in the redox potentials of bacteriochlorophyll a and chlorophyll a in organic solvents. 

The calculation of redox potentials for bacteriochlorophyll and bacteriopheophytin molecules described in Ref. 43 at the HF 6-3 Ig level using the PS-GVB solvation module represents the largest tJt initio self-consistent reaction field calculation to date. The calculations performed an a workstation involved up to 889 basis functions and were successful in predicting the correct relative redox energies. 

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. 

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