Freie Universität Berlin, FB Physik, Arnimallee 14, D-14195 Berlin, Germany.
Acc Chem Res. 2009 Dec 21;42(12):1861-70. doi: 10.1021/ar900225y.
Photosynthesis in plants and cyanobacteria involves two protein-cofactor complexes which are denoted as photosystems (PS), PSII and PSI. These solar-energy converters have powered life on earth for approximately 3 billion years. They facilitate light-driven carbohydrate formation from H(2)O and CO(2), by oxidizing the former and reducing the latter. PSII splits water in a process driven by light. Because all attractive technologies for fuel production driven by solar energy involve water oxidation, recent interest in this process carried out by PSII has increased. In this Account, we describe and apply a rationale for estimating the solar-energy conversion efficiency (eta(SOLAR)) of PSII: the fraction of the incident solar energy absorbed by the antenna pigments and eventually stored in form of chemical products. For PSII at high concentrations, approximately 34% of the incident solar energy is used for creation of the photochemistry-driving excited state, P680*, with an excited-state energy of 1.83 eV. Subsequent electron transfer results in the reduction of a bound quinone (Q(A)) and oxidation of the Tyr(Z) within 1 micros. This radical-pair state is stable against recombination losses for approximately 1 ms. At this level, the maximal eta(SOLAR) is 23%. After the essentially irreversible steps of quinone reduction and water oxidation (the final steps catalyzed by the PSII complex), a maximum of 50% of the excited-state energy is stored in chemical form; eta(SOLAR) can be as high as 16%. Extending our considerations to a photosynthetic organism optimized to use PSII and PSI to drive H(2) production, the theoretical maximum of the solar-energy conversion efficiency would be as high as 10.5%, if all electrons and protons derived from water oxidation were used for H(2) formation. The above performance figures are impressive, but they represent theoretical maxima and do not account for processes in an intact organism that lower these yields, such as light saturation, photoinhibitory, protective, and repair processes. The overpotential for catalysis of water oxidation at the Mn(4)Ca complex of PSII may be as low as 0.3 V. To address the specific energetics of water oxidation at the Mn complex of PSII, we propose a new conceptual framework that will facilitate quantitative considerations on the basis of oxidation potentials and pK values. In conclusion, photosynthetic water oxidation works at high efficiency and thus can serve as both an inspiring model and a benchmark in the development of future technologies for production of solar fuels.
光合作用是植物和蓝藻利用两种蛋白辅因子复合物来完成的，这两种复合物被称为光合作用系统（photosystems，PS），包括 PSII 和 PSI。这两种太阳能转化器已经为地球生命提供了大约 30 亿年的能量。它们通过氧化前者和还原后者，将 H(2)O 和 CO(2)转化为碳水化合物。PSII 在光的驱动下分解水。由于所有利用太阳能的燃料生产技术都涉及水氧化，因此最近人们对 PSII 进行的这一过程产生了浓厚的兴趣。在本报告中，我们描述并应用了一种用于估计 PSII 太阳能转化效率（eta(SOLAR)）的原理：即被天线色素吸收并最终以化学产物形式储存的入射太阳能的分数。对于高浓度的 PSII，大约 34%的入射太阳能用于产生激发态 P680*，其激发态能量为 1.83eV。随后的电子转移导致结合的醌（Q(A)）还原和 Tyr(Z)氧化，时间约为 1 微秒。在这种自由基对状态下，复合损失的稳定性约为 1 毫秒。在这一水平上，最大的 eta(SOLAR)为 23%。在醌还原和水氧化的基本不可逆步骤（由 PSII 复合物催化的最后步骤）之后，最大 50%的激发态能量以化学形式储存；eta(SOLAR)可高达 16%。将我们的考虑扩展到一个优化的光合作用生物体，该生物体利用 PSII 和 PSI 来驱动 H(2)的产生，如果所有来自水氧化的电子和质子都用于 H(2)的形成，太阳能转化效率的理论最大值可高达 10.5%。上述性能数据令人印象深刻，但它们代表的是理论最大值，并没有考虑到完整生物体中降低这些产量的过程，如光饱和、光抑制、保护和修复过程。PSII 的 Mn(4)Ca 配合物中水氧化的催化过电势可能低至 0.3V。为了解决 PSII 的 Mn 配合物中水氧化的具体能量学问题，我们提出了一个新的概念框架，这将有助于根据氧化电位和 pK 值进行定量考虑。总之，光合作用的水氧化效率很高，因此可以作为未来太阳能燃料生产技术的灵感模型和基准。
