Electron photosynthetic reaction centre

So far we have exclusively discussed time-resolved absorption spectroscopy with visible femtosecond pulses. It has become recently feasible to perfomi time-resolved spectroscopy with femtosecond IR pulses. Flochstrasser and co-workers [M, 150. 151. 152. 153. 154. 155. 156 and 157] have worked out methods to employ IR pulses to monitor chemical reactions following electronic excitation by visible pump pulses these methods were applied in work on the light-initiated charge-transfer reactions that occur in the photosynthetic reaction centre [156. 157] and on the excited-state isomerization of tlie retinal pigment in bacteriorhodopsin [155]. Walker and co-workers [158] have recently used femtosecond IR spectroscopy to study vibrational dynamics associated with intramolecular charge transfer these studies are complementary to those perfomied by Barbara and co-workers [159. 160], in which ground-state RISRS wavepackets were monitored using a dynamic-absorption technique with visible pulses. 

The semiquinone biradical produced in the photocycle of bacterial photosynthetic reaction centres was trapped at 77 K and examined at 9.6, 35, and 94 GHz.16 Simulations of spectra at the multiple resonant frequencies using the simulated annealing method revealed the spatial and electronic structure of the biradical. The value of r was found to be 17.2 + 0.2 A, which is in good agreement with the value of 17.4 0.2 A obtained in an X-ray crystal structure. This study shows the power of high-frequency EPR combined with data obtained at lower frequencies. 

In proteins Dutton et al.[4] concluded by studying an example from photosynthetic reaction centres, that a protein acts by mediating ET in a fixed exponential way without much possibility for Nature to improve the ET properties in evolution, other than choosing the optimum distance between the active ET centres. It has been shown both experimentally and theoretically that 7t systems are better than a systems in transmitting electrons, if the orientation of the former system is favourable. Thus the statement of Dutton et al. [4] could be rephrased by saying that 7t systems or other favourable ET systems cannot be incorporated in a favourable position in a protein, by evolutional... 

Cowan, J.A., Sanders, J.K.M., Beddard, G.S. and Harrison, R.J. 1987. Modelling the photosynthetic reaction centre photoinduced electron transfer in a pyromellitimide-bridged special pair Porphyrin Dimer, J. Chem. Soc., Chem. Commun., 55-58. 

A schematic view of bacterial photosynthetic reaction centre with a chain of electron-carrying metal prosthetic groups is shown in Figure 11.4. The metal ions involved are magnesium (bacteriochlorophylls) and iron (haem and non-haem iron species). 

One of the ongoing mysteries of the OEC is how PSII could have evolved the ability to use water as an electron donor. It would seem that a number of changes, such as formation of a very high-potential oxidant in the charge-separation reaction and incorporation of a four-electron water-oxidation catalyst, would have had to occur simultaneously in order to convert an anaerobic photosynthetic reaction centre into PSII. 

Fleming, G. R., Martin, J. L., and Breton, J., 1988, Rates of primary electron transfer in photosynthetic reaction centres and their mechanistic implications. Nature, 333 1909192. 

Kleinfeld, D., Okamura, M. Y., and Feher, G., 1984, Electron transfer kinetics in photosynthetic reaction centres cooled to cryogenic temperatures in the charge-separated state evidence for light-induced structural changes. Biochemistry, 23 5780n5786. 

Zinth, W., Huppmann, P., Arlt, T., and Wachtveitl, J., 1998, Ultrafast spectroscopy of the electron transfer in photosynthetic reaction centres towards a better understanding of electron transfer in biological systems Phil. Trans. Roy. Soc. Land. A, 356 4659476. 

We now turn to another important influence on ET dynamics, electric fields. It is reasonable that the presence of a strong electric field should influence the dynamics of charge separation processes because the dipole moment associated with the newly formed CS state will interact with the field. This interaction will modify the barrier height for the charge separation process. Indeed, it has been postulated that electric field effects might be the cause of the observed directionality of electron transfer in the photosynthetic reaction centre (see Figure 37). 

Electronic spectra of linear conjugated polyene radical cations are of interest for several reasons. Firstly, such species occur as intermediates in different processes of biological relevance, e.g. the protection of the photosynthetic reaction centre, the charge transfer processes in membranes or in model smdies for photoinduced charge separation. Secondly, they may be involved in the formation of solitons upon doping or photoexcitation of polyacetylene , and finally, they are of theoretical interest because their interpretation requires models which account for non-dynamic correlation. 

The solid-state structure of the photosynthetic reaction centre complexes has inspired several studies of light-induced electron transfer in solid media. A particularly useful medium is provided by porous glass which facilitates rapid electron-transfer reactions without the involvement of polar solvents. Solid matrices suitable for light-induced electron-transfer processes are also provided by silica, zeolites, and clays. A theoretical description has been reported for dealing with the distribution of separation distances between donor and acceptor that is often found in the solid state. ... 

This field was opened up in 1994 when Zysmilich and McDermott reported strong nuclear spin polarizations in solid-state spectra from photosynthetic reaction centres in which the forward electron transfer from the primary charge-separated state to the accepting quinone had been blocked. It has been rapidly expanding since 1996 when they published two further accounts of their results elucidating the phenomena in more detail/ and when a group at Leiden University took up that line of research. 

The photosynthetic reaction centres (RCs) are transmembrane protein-pigment complexes that perform light-induced charge separation during the primary steps of photosynthesis. RCs from purple bacteria consist of three protein subunits, L, M and H, and bind four bacteriochlorophylls, two bacteriopheophytins, two quinones, one non-haem iron and one carotenoid. The elucidation at atomic resolution of the three-dimensional structures of the bacterial RCs from Rhodopseudomonas (Rps.) viridis (1) and Rhodobacter (Rb,) sphaeroides (2-4) has provided impetus for theoretical and experimental work on the mechanism of primary charge separation in the RCs. The structures revealed that the cofactors are bound at the interface between the L and M subunits and are organised around a pseudo C2 symmetry axis. However, the structural symmetry does not result in functional symmetry as the electron transfer proceeds only along the L branch (5). 

Bylina EJ and Wong R (1992) Analysis of spontaneous herbicide resistant revertants derived from Rhodobacter capsulatus in which Ser L223 of the reaction center is replaced with alanine. In Murata N (ed) Research in Photosynthesis, Vol 1, pp 369-372. Kluwer Academic Publishers, Dordrecht Bylina EJ, KirmeierC, McDowell L, Molten D and Yuovan DC (1988) Influence of an amino-acid residue on the optical properties and electron transfer dynamics of a photosynthetic reaction centre complex. Nature 336 182-184 Chamorovsky SK, Zakhorova NI, Remennikov SM, Sabo YA and Rubin AB (1998) The cytochrome subunit structure in the photosynthetic reaction center of Chromatium minutissimum. FEBS Lett 422 231-234... 

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