A SUPEREXCHANGE MECHANISM FOR THE PRIMARY CHARGE SEPARATION IN PHOTOSYNTHETIC REACTION CENTERS
A SUPEREXCHANGE MECHANISM FOR THE PRIMARY CHARGE SEPARATION IN PHOTOSYNTHETIC REACTION CENTERS
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DOI：
10.1016/s0005-2728(89)80081-7
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发表时间：
1989-12-07
期刊：
影响因子：
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通讯作者：
OGRODNIK, A
中科院分区：
文献类型：
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作者：
BIXON, M;JORTNER, J;OGRODNIK, A
我们分析了细菌叶绿素二聚体（P）的电子激发单重态（1 P *）通过细菌光合反应中心的A分支与细菌脱镁叶绿素（H）的初级电荷分离的超交换模型，该过程由辅助细菌叶绿素（B）介导。初始1 P * BH和最终P+BH-态之间的超交换电子相互作用的主要贡献来自与能量高于1 P * 的中间电子态P+P-H的混合。超交换电子相互作用是V = VPB VBH/δ E，其中VPB和VBH分别是1 P *BH与P+B-H和P+B-H与P +BH-的电子耦合，而δ E是垂直能量差。非绝热电子转移速率与V2 F成比例，其中F是核Franck-Condon因子，其由（自由）能隙Δ G = -2000 cm-1（介质重组能Δ G）确定。（... < 2500 cm-1）和介质特征频率ω。. apprxeq. 100 cm-1。间接信息的成分的有效电子耦合V. apprxeq. 25 cm-1是从VBH/VPB比率推断的，VBH/VPB比率是从分子间重叠近似结合活化的顺序通道和利用对初级电子转移的动力学的动力学约束计算的，这导致VPB ≥ 25 cm-1。60 cm-1，VBH ≥360 cm-1和δ E ≥ 0. 1100 cm-1。我们讨论了几种物理现象和可观测量，即，在超交换机制的框架下，电场对瞬时荧光、A分支上电荷分离的单向性和初级径向对中的磁相互作用的影响。电场（ε）荧光量子产率（Yf（ε））的依赖性对于75 K下的各向同性样品，预测Yf（ε）= 5 mV/. ANG）/ Yf（0）= 1.39和Yf（ε）= 1.39。= 9 mV/. ANG）/ Yf（0）= 3.5。在恒定场下的荧光偏振数据（Lockhart，D. J.，Goldstein，R. F.和Boxer，S.G.（1988）J.Chem.Phys.89，1408-1415）可以很好地用能量参数来解释。= 1600 cm-1和Δ G = -2000 cm-1以及值ψ。= 61 °。A分支上电荷分离的单向性主要源于结构对称性破缺，它影响电子耦合，而核贡献的贡献已被证明是很小的。在T = 80 K时电子转移速率的预测比率k（A）/k（B）= 82（+190; -70）与最近的实验结果k（A）/k（B）≥一致。25在这个温度下最后，我们检查了初级P+H-自由基对的磁相互作用，建立了单重态能量位移和三重态能量位移与初级电子转移速率k和三重态复合速率kT之间的相互关系，其中P+H-的单重态-三重态分裂为J = α k-β kT，其中系数α k = 0。和β取决于能量参数和Franck-Condon因子。超交换机制内的J的估计依赖于一个假设的配置松弛和基本的取消效果的合并。
We analyse the superexchange model for the primary charge separation from the electronically excited singlet state (1P*) of the bacteriochlorophyll dimer (P) to the bacteriopheophytin (H) across the A branch of the bacterial photosynthetic reaction centers, which is mediated by the accessory bacteriochlorophyll (B). The dominant contribution to the superexchange electronic interaction between the initial 1P* BH and the final P+BH- states originates from the mixing with the mediating electronic state P+P-H, the energy of which is above 1P*. The superexchange electronic interaction is V = VPBVBH/.delta.E, where VPB and VBH are the electronic couplings of 1P*BH with P+B-H and P+B-H with P+BH-, respectively, while .delta.E is the vertical energy difference. The nonadiabatic electron-transfer rate is proportional to V2F, where F is the nuclear Franck-Condon factor, which is determined by the (free) energy gap .DELTA.G = -2000 cm-1, the medium reorganization energy .lambda. (.lambda. < 2500 cm-1) and the medium characteristic frequency .omega. .apprxeq. 100 cm-1. Indirect information on the constituents of the effective electronic coupling V .apprxeq. 25 cm-1 was inferred from the ratio VBH/VPB calculated from the intermolecular overlap approximation in conjunction with an activated sequential channel and the utilization of kinetic constraints on the dynamics of the primary electron transfer, which result in VPB .gtoreq. 60 cm-1, VBH .gtoreq. 360 cm-1 and .delta.E .gtoreq. 1100 cm-1. We discuss several physical phenomena and observables, i.e., electric field effects on the prompt fluorescence, the unidirectionality of charge separation across the A branch and magnetic interactions in the primary radial pair in the framework of the superexchange mechanism. The electric field (.epsilon.) dependence of the fluorescence quantum yield (Yf(.epsilon.)) for isotropic samples at 75 K predicts Yf(.epsilon. = 5 mV/.ANG.)/Yf(0) = 1.39 and Yf (.epsilon. = 9 mV/.ANG.)/Yf(0) = 3.5. The fluoresence polarization data at constant field (Lockhart, D.J., Goldstein, R.F. and Boxer, S.G. (1988) J. Chem. Phys. 89, 1408-1415) can be well accounted for in terms of the energetic parameters .lambda. = 1600 cm-1 and .DELTA.G = -2000 cm-1 together with the value .psi. = 61.degree. for the angle between the dipole P+H- and the transition moment of P. The unidirectionality of the charge separation across the A branch originates predominantly from structural symmetry breaking, which affects the electronic coupling, while the contribution of the nuclear contribution has been shown to be small. The predicted ratio of the electronic transfer rates k(A)/k(B) = 82(+190; -70) at T = 80 K is consistent with the recent experimental result k(A)/k(B) .gtoreq. 25 at this temperature. Finally we examined magnetic interactions of the primary P+H- radical pair, establishing the interrelationship between the singlet energy shifts and the triplet energy shift with the primary electron transfer rate, k, and the triplet recombination rate kT whereupon the single-triplet splitting of P+H- is J = .alpha.k-.beta.kT where the coefficients .alpha. and .beta. depend on energetic parameters and Franck-Condon factors. The estimates of J within the superexchange mechanism rests on the incorporation of an assumed configurational relaxation and essential cancellation effects.