Hou Harvey J M, Mauzerall David
Department of Chemistry, Gonzaga University, 502 East Boone Avenue, Spokane, Washington 99258, USA.
J Am Chem Soc. 2006 Feb 8;128(5):1580-6. doi: 10.1021/ja054870y.
We have previously reported the enthalpy and volume changes of charge separation in photosystem I from Synechocystis 6803 using pulsed photoacoustics on the microsecond time scale, assigned to the electron-transfer reaction from excited-state P(700) to F(A/B) iron sulfur clusters. In the present work, we focus on the thermodynamics of two steps in photosystem I: (1) P(700) --> A(1)(-)F(X) (<10 ns) and (2) A(1)(-)F(X) --> F(A/B)(-) (20-200 ns). The fit by convolution of photoacoustic waves on the nanosecond and microsecond time scales resolved two kinetic components: (1) a prompt component (<10 ns) with large negative enthalpy (-0.8 +/- 0.1 eV) and large volume change (-23 +/- 2 A(3)), which are assigned to the P(700) --> A(1)(-)F(X) step, and (2) a component with approximately 200 ns lifetime, which has a positive enthalpy (+0.4 +/- 0.2 eV) and a small volume change (-3 +/- 2 A(3)) that are attributed to the A(1)(-)F(X) --> F(A/B)(-) step. For the fast reaction using the redox potentials of A(1)F(X) (-0.67 V) and P(700) (+0.45 V) and the energy of P(700) (1.77 eV), the free energy change for the P(700) --> A(1)(-)F(X) step is -0.63 eV, and thus the entropy change (TDeltaS, T = 25 degrees C) is -0.2 +/- 0.3 eV. For the slow reaction, A(1)(-)F(X) --> F(A/B)(-), taking the free energy of -0.14 eV [Santabara, S.; Heathcote, P; Evans, C. W. Biochim. Biophys. Acta 2005, 1708, 283-310], the entropy change (TDeltaS) is positive, +0.54 +/- 0.3 eV. The positive entropy contribution is larger than the positive enthalpy, which indicates that the A(-)F(X) to F(A/B)(-) step in photosystem I is entropy driven. Other possible contributions to the measured values are discussed.
我们之前曾报道过，利用微秒时间尺度上的脉冲光声技术，测定了集胞藻6803光系统I中电荷分离的焓变和体积变化，这些变化归因于从激发态P(700)到F(A/B)铁硫簇的电子转移反应。在本工作中，我们聚焦于光系统I中两个步骤的热力学：(1) P(700) --> A(1)(-)F(X)（<10 ns）和(2) A(1)(-)F(X) --> F(A/B)(-)（20 - 200 ns）。通过对纳秒和微秒时间尺度上的光声波进行卷积拟合，解析出两个动力学成分：(1) 一个快速成分（<10 ns），具有大的负焓变（-0.8 +/- 0.1 eV）和大的体积变化（-23 +/- 2 ų），这归因于P(700) --> A(1)(-)F(X)步骤；(2) 一个寿命约为200 ns的成分，具有正焓变（+0.4 +/- 0.2 eV）和小的体积变化（-3 +/- 2 ų），这归因于A(1)(-)F(X) --> F(A/B)(-)步骤。对于使用A(1)F(X)（-0.67 V）和P(700)（+0.45 V）的氧化还原电位以及P(700)的能量（1.77 eV）的快速反应，P(700) --> A(1)(-)F(X)步骤的自由能变化为-0.63 eV，因此熵变（TDeltaS, T = 25℃）为-0.2 +/- 0.3 eV。对于缓慢反应，A(1)(-)F(X) --> F(A/B)(-)，采用-0.14 eV的自由能[Santabara, S.; Heathcote, P; Evans, C. W. Biochim. Biophys. Acta 2005, 1708, 283 - 310]，熵变（TDeltaS）为正，+0.54 +/- 0.3 eV。正的熵贡献大于正的焓，这表明光系统I中从A(-)F(X)到F(A/B)(-)的步骤是由熵驱动的。还讨论了对测量值的其他可能贡献。
