Département de Chimie-Biologie, Section de Chimie et Biochimie, et Groupe de Recherche en Biologie Végétale, Université du Québec à Trois-Rivières, Trois-Rivières (Québec), G9A5H7, Canada.
J Phys Chem B. 2009 Nov 19;113(46):15392-8. doi: 10.1021/jp9071586.
The temperature dependence of electron transport through photosystem II (PSII), measured as oxygen evolution, was investigated in thylakoid membranes irradiated with white light of 450 micromol of photons/(m(2) x s). The experiments were performed in steady-state conditions at temperatures between 273 and 303 K. The results show discontinuities, or thermal transitions, in the temperature-response curves of oxygen evolution. The experimental data was examined with the Marcus theory of electron transfer modified to take into account the oxygen evolution discontinuities. For this purpose, the Gibbs free energy of activation of the electron transfer reaction, DeltaG(o), is replaced in the classical Marcus equation with the expression DeltaH(o) - TDeltaS(o), where H(o) and DeltaS(o) are respectively the enthalpy and entropy of activation, and T is the temperature in kelvin. The result of the derivation is a summation of j Gaussian functions, or states, OE = 69 250 summation operator(j){(V(DA)(4)/lambdaT)(1/2) exp[-(T(max) - T)(2)/2Tsigma(o)(2)]}(j) (eq 1), where OE is expressed in micromol oxygen evolution.(mg Chl x h)(-1), and V(DA) is the electronic coupling matrix element between electron donor (D) and acceptor (A) wave functions, lambda the reorganization free energy, k(B) the Boltzmann constant, T(max) = (DeltaH(o) + lambda)/DeltaS(o), sigma(o) = (2k(B)lambda/DeltaS(o2))(1/2), and sigma = T(1/2)sigma(o) is the standard deviation of the Gaussian band. The mathematical simulations revealed the presence of six thermal transitions, or Gaussian bands with maxima at 275.3, 281.2, 286.4, 291.4, 297.1, and 302.4 K. The resolution of the Gaussian bands is about 0.55 owing to multiple band superpositions. The theoretical analyses showed that (i) the oxygen evolution in PSII is essentially dependent on V(DA)(2)/(lambdaT)(1/2) in the pre-exponential term of the modified Marcus equation (eq 1), and (ii) the reorganization energy, lambda, decreases exponentially with increasing temperature, and is also dependent on the electron donor-acceptor distance. It is concluded that the temperature dependence of the large enhancement of oxygen evolution observed in this work originates, at least partly, in heat-induced structural rearrangements in the photosystem II reaction center.
在 450 微摩尔光子/(m(2) x s)的白光照射下，研究了通过光系统 II (PSII)的电子传递对温度的依赖性，该传递通过氧气的产生来衡量。实验在 273 至 303 K 的温度下的稳态条件下进行。结果表明，氧气产生的温度响应曲线存在不连续或热跃迁。实验数据用电子转移的马库斯理论进行了检查，该理论经过修改后考虑了氧气产生的不连续性。为此，在经典马库斯方程中，用表达式 DeltaH(o) - TDeltaS(o)替换电子转移反应的 Gibbs 自由能激活值 DeltaG(o)，其中 H(o)和 DeltaS(o)分别是焓和熵的激活值，T 是开尔文温度。推导的结果是 j 个高斯函数的总和，或状态，OE = 69 250 个算子(j){(V(DA)(4)/lambdaT)(1/2) exp[-(T(max) - T)(2)/2Tsigma(o)(2)]}(j) (eq 1)，其中 OE 以微摩尔氧气产生表示。(mg Chl x h)(-1)，V(DA)是电子供体 (D)和受体 (A)波函数之间的电子偶合矩阵元，lambda 是重组自由能，k(B)是玻尔兹曼常数，T(max) = (DeltaH(o) + lambda)/DeltaS(o)，sigma(o) = (2k(B)lambda/DeltaS(o2))(1/2)，sigma = T(1/2)sigma(o) 是高斯带的标准偏差。数学模拟揭示了六个热跃迁的存在，或具有最大值的高斯带，分别为 275.3、281.2、286.4、291.4、297.1 和 302.4 K。由于多个带的叠加，高斯带的分辨率约为 0.55。理论分析表明，(i) PSII 中的氧气产生本质上取决于修改后的马库斯方程(eq 1)的前指数项中的 V(DA)(2)/(lambdaT)(1/2)，以及 (ii) 重组能 lambda 随温度呈指数下降，并且还取决于电子供体-受体距离。结论是，本工作中观察到的氧气产生的大幅增强对温度的依赖性至少部分源自光系统 II 反应中心的热诱导结构重排。
