Super-exchange mechanism

Although no evidence has been found that the monomeric bacteriochlorophyll unit, which is intercalated between the special pair and the pheophytin (see Fig. 1), actually acquires an electron, it should be noted that also this unit may act as a mediator via a super-exchange mechanism (Kuznetsov and Ulstrup, 1982 Miller and Beitz, 1981), which is conceptually related to through-bond interaction. 

The lack of well-resolved kinetic data in support of the formation of [P -Ba -BOa] presented a dilemma, since the monomeric BChl is in van der Waals contact with the primary donor, whereas BO is further removed at a distance of 17 A. Rudolf Marcus examined the available kinetic data and reviewed two alternative mechanisms proposed for the reduction of the intermediate electron carrier, BChl. One was the super-exchange mechanism of electron transfer that implicates the existence of a virtually populated [P BA ]-state in the mediation of electron transfer between P and BOa and the other a two-step electron transfer to BO that can be kinetically resolved by an intermediary [P BA"]-state. In view of the lack of resolution of such a state in the data obtained by Martin and Breton , Marcus estimated that the putative [P Ba -BOa] state would have an upper lifetime limit of -1 ps. Of course, undertaking measurements of such a brief kinetic event in the neighborhood of a 3-ps event would demand substantially improved measuring techniques and procedures. With this in mind, Holzapfel et al. extended their femtosecond measurements to look for the intermediary [P -BA"]-state. Their measurements entailed the use of short excitation pulses (60/v), appropriate wavelengths for selective excitation of the primary electron donor P, high time resolution (-100 fs), and sufficiently low excitation intensity to avoid double photon excitation and consequently nonlinear processes. As the results summarized below show, these measurements provided new evidence for the existence of a kinetically resolvable, though extremely transient, intermediary [P -Ba -BCJa] state. 

The precursor of photosynthetic electron transfer is the lowest singlet excited state of the pigment complex P. It is generally accepted that upon population of this state, P disappears and the radical pair P Hl is formed in about 3 ps at room temperature (Martin et al.,1986, Woodbury et al.,1985, Holzapfel et al.,1990, Kirmaier and Holten, 1988) and somewhat faster, down to 700 fs, at low temperature (Fleming et al., 1988). However the precise nature of this reaction is subject to debate, especially the role of Bl, which is located between P and Hl (for a review see Martin and Vos, 1992). It has been proposed that P B is a virtual intermediate in a super-exchange mechanism (Michel-Beyerle et al., 1988, Warshel et al., 1988). Based on the observation of transients faster than P decay, Zinth and coworkers proposed that P l is a real intermediate, which decays with a faster rate than it is formed (Holzapfel et al., 1989, 1990). 

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