Diller Anna, Roy Esha, Gast Peter, van Gorkom Hans J, de Groot Huub J M, Glaubitz Clemens, Jeschke Gunnar, Matysik Jörg, Alia A
Gorlaeus Laboratoria, Leiden Institute of Chemistry, Einsteinweg 55, 2300 RA Leiden, The Netherlands.
Proc Natl Acad Sci U S A. 2007 Jul 31;104(31):12767-71. doi: 10.1073/pnas.0701763104. Epub 2007 Jul 24.
In natural photosynthesis, the two photosystems that operate in series to drive electron transport from water to carbon dioxide are quite similar in structure and function, but operate at widely different potentials. In both systems photochemistry begins by photo-oxidation of a chlorophyll a, but that in photosystem II (PS2) has a 0.7 eV higher midpoint potential than that in photosystem I (PS1), so their electronic structures must be very different. Using reaction centers from (15)N-labeled spinach, these electronic structures are compared by their photochemically induced dynamic nuclear polarization (photo-CIDNP) in magic-angle spinning (MAS) NMR measurements. The results show that the electron spin distribution in PS1, apart from its known delocalization over 2 chlorophyll molecules, reveals no marked disturbance, whereas the pattern of electron spin density distribution in PS2 is inverted in the oxidized radical state. A model for the donor of PS2 is presented explaining the inversion of electron spin density based on a tilt of the axial histidine toward pyrrole ring IV causing pi-pi overlap of both aromatic systems.
在自然光合作用中，串联运行以驱动电子从水传输到二氧化碳的两个光系统在结构和功能上相当相似，但运行电位差异很大。在这两个系统中，光化学均始于叶绿素a的光氧化，但光系统II（PS2）中的光化学氧化中点电位比光系统I（PS1）中的高0.7电子伏特，因此它们的电子结构必定有很大差异。利用来自经（15）N标记的菠菜的反应中心，通过魔角旋转（MAS）核磁共振测量中的光化学诱导动态核极化（光-CIDNP）来比较这些电子结构。结果表明，除了已知在2个叶绿素分子上的离域外，PS1中的电子自旋分布未显示明显扰动，而PS2中氧化自由基状态下的电子自旋密度分布模式则是反转的。提出了一个PS2供体模型，该模型基于轴向组氨酸向吡咯环IV倾斜导致两个芳香体系的π-π重叠来解释电子自旋密度的反转。
