Asymmetric Electron Transfer in Reaction Centers of Purple Bacteria Strongly Depends on Different Electron Matrix Elements in the Active and Inactive Branches

Abstract

We have re-examined the contribution of electronic matrix elements (V 1 ) between the primary electron donor and the accessory bacteriochlorophylls in the active (A) and inactive (B) branches of bacterial reaction centers (RC) to the unidirectional light-induced electron transfer (ET) (a preliminary report was recently given by Kolbasov and Scherz in Photosynthesis: Mechanisms and Effects; Garab, G., Ed.; Kluwer Academic Publishers: Dordrecht, 1998; Vol. II; pp 719-722). Our calculations showed that V 1 B 2 is probably smaller by 3 orders of magnitude than V 1 A 2 in Rb. sphaeroides and by at least 1 order of magnitude in Rps. Viridis. These phenomena reflect the quantum interference and mutual cancellation of the resonance integrals corresponding to different ET pathways between atoms of P and the accessory bacteriochlorophyll in the inactive branch.The calculated values of V and the corresponding ET rate constants for mutated RC further support this conclusion. Zhang and Friesner (Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 13603-13605), using more elaborate calculations, showed that V 1 A /V 1 B for Rps. Viridis can reach a value of 14 for same reason, indicating that differences in the overlap matrix elements are key factors in the unidirectional electron flow in both organisms.

Ultrafast photo-induced charge transfer unveiled by two-dimensional electronic spectroscopy

“…This localization of the charge density has been explained by specific interactions, such as hydrogen bonds, 24,59 asymmetric interaction with the protein, 21,23 or a special arrangement and interaction of the reaction center pigments. 20,[59][60][61] In the language of the exciton model, all these mechanisms serve to lower the CT states on one side of the complex in a similar way as the counter-ion in our case. Our results show that the presence of an asymmetric interaction facilitates relaxation to the bottom of the band, where the charge is localized on one side of the complex.…”

The First Picoseconds in Bacterial Photosynthesis—Ultrafast Electron Transfer for the Efficient Conversion of Light Energy

“…In addition, details in the structural arrangement of the chromophores on the two branches may contribute to the asymmetry: the temperature factors from structure analysis indicate that the chromophores on the A branch are structurally better defined and the phytyl chains of the chromophores of the A branch are arranged in a way to keep B A and H A in close contact with high electronic coupling. [3,4,43,44] In the mid-1980s a series of ultrafast experiments were performed by J. L. Martin and J. Breton with strongly improved experimental techniques. [45][46][47][48] The authors used a high temporal resolution on the femtosecond timescale, they performed the experiments with excitation at the appropriate wavelengths in the long-wavelength absorption band of the special pair.…”

Theory of delocalised charge transfer​​​​​​​

“…The two branches of the RC can be symmetric, as is the case in heliobacteria 34,74,83 , with identical probability of charge transfer down either; however, in most organisms the RC is asymmetrical, with charge preferably travelling down one branch, called the active branch 34 . This is accomplished by by differences in electronic coupling between SP and the primary charge acceptor on each branch 84 , and energetic asymmetries between the branches, which also has the effect of affecting the charge delocalisation on SP-the charge ranges from fully delocalised across both (B)Chl in P [84][85][86] , to being strongly asymmetrical 87 .…”

“…As the coupling in the monomer may have been weaker than in modern RCs, we take 10 meV as the lower bound for J P . Modern RCs have a coupling between the special pair and the charge acceptor of between 0.7 and 13 meV 84,85,87,161 ; we compensate for the decrease in coupling through dimerisation by multiplying these values by √ 2 to find the range of V X 0 ,X A . We assume CT takes place in the normal or activationless regime (as opposed to the inverted regime), meaning ∆E X 0 ,X A ≥ −λ X 0 ,X A , and that E X 0 ≥ E X A , meaning ∆E X 0 ,X A ≤ 0.…”

“…84) with initial population entirely in state |φ a . At the transition state x = x T , W a = W b ; away from the transition state, |W a (x) −W b (x)| |V ab |.…”