Primary charge separation

Holzapfel W, Finkele U, Kaiser W, Oesterhelt D, Scheer H, Stilz H U and Zinth W 1989 Observation of a bacteriochlorophyll anion radical during the primary charge separation in a reaction center Chem. Rhys. Lett. 160 1-7... 

Figure 3,17. Level structure for the primary charge separation in photosynthetic reaction centers, (a) unistep superexchange dynamics (b) two-step sequential dynamics (Bixon and Jortner, 1999) Reproduced with permission.

The relevance of adiabatic electron transfer to the primary charge separation reaction has been the subject of considerable discussion, mainly due to the observation of undamped low-frequency nuclear motions associated with the P state (see Section 5.5). More recently, sub-picosecond time-scale electron transfer has been observed at cryogenic temperatures, driven either by the P state in certain mutant reaction centres (see Section 5.6) or by the monomeric BChls in both wild-type and mutant reaction centres (see Section 5.7). These observations have led to the proposal that such ultra-fast electron transfer reactions require strong electronic coupling between the co-factors and occur on a time-scale in which vibrational relaxation is not complete, which would place these reactions in the adiabatic regime. Finally, as discussed in Section 2.2, evidence has been obtained that electron transfer from QpJ to Qg is limited by nuclear rearrangement, rather than by the driving force for the reaction. 

Bixon, M., Jortner, J., Michel-Beyerle, M. E., and Ogrodnik, A., 1989, A superexchange mechanism for the primary charge separation in photosynthetic reaction centers. Biochim. Biophys. Acta, 977 2739286. 

Creighton, S., Hwang, J. K., Warshel, A., Parson, W. W., and Norris, J., 1988, Simulating the dynamics of the primary charge separation process in bacterial photosynthesis. Biochemistry, 27 7749781. 

Observation of a bacteriochlorophyll anion radical during the primary charge separation in a reaction center. Chem. Phys. Lett., 160 ln7. 

Kirmaier, C., Holten, D., and Parson, W. W., 1985a, Temperature and detection-wavelength dependence of the picosecond electron transfer kinetics measured in Rhodopseudomonas sphaeroides reaction centers Resolution of new spectral and kinetic components in the primary charge-separation process. Biochim. Biophys. Acta, 810 33n48. 

Van Brederode, M. E., Van Stokkum, I. H. M., Katilius, E., Van Mourik, F., Jones, M. R., and Van Grondelle, R., 1999b, Primary charge separation routes in the BchkBphe heterodimer reaction cenb es of Rhodobacter sphaeroides. Biochemistry, 38 7545n7555. 

Upon light excitation of dark-adapted PS II, the primary charge separation takes place, forming P-680 and Pheo. This probably happens in a small number of picoseconds. Electron transfer from Pheo to occurs in a few hundred picoseconds, stabilizing the separated charges [112]. If is already reduced the (P-680 Pheo ) radical pair can still be formed, although perhaps with a low quantum yield (see Ref. 145), but now it lasts for a few nanoseconds [142] and gives rise to some recombination luminescence or, at low temperature, populates the triplet state of P-680 [141], which itself decays with a of around 1 ms [166]. 

The three-state (three-potential-energy surface) problem is of interest for redox chains, chemical triad model systems, DNA electron transfer, and the primary charge separation in photosynthesis. As there are two energy-gap fluctuations in these reactions, and the fluctuations are not simply related to each other (in contrast to the case of two-electron transfer in two-center systems, vide infra), the problem is intrinsically two-dimensional. Marchi et al. [54], Zusman and Beratan [55], and Okada and Bandyopadhyay [56] have analyzed the nature of these potential energy surfaces and the electron-transfer kinetics. In the steady-state approximation for species 2,... 

We have seen the Z-scheme for the two photosystems in green-plant photosynthesis and the electron carriers in these photosystems. We have also described how the photosystems of green plants and photosynthetic bacteria all appear to function with basically the same sort ofmechanisms of energy transfer, primary charge separation, electron transfer, charge stabilization, etc., yet the molecular constituents of the two reaction centers in green plants, in particular, are quite different from each other. Photosystem I contains iron-sulfur proteins as electron acceptors and may thus be called the iron-sulfur (FeS) type reaction center, while photosystem 11 contains pheophytin as the primary electron acceptor and quinones as the secondary acceptors and may thus be called the pheophytin-quinone (0 Q) type. These two types of reaction centers have also been called RCI and RCII types, respectively. 

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