Matysik J, Gast P, van Gorkom H J, Hoff A J, de Groot H J
Leiden Institute of Chemistry, Gorlaeus Laboratoria, Einsteinweg 55, P.O. Box 9502, 2300 RA Leiden, The Netherlands.
Proc Natl Acad Sci U S A. 2000 Aug 29;97(18):9865-70. doi: 10.1073/pnas.170138797.
We report (13)C magic angle spinning NMR observation of photochemically induced dynamic nuclear spin polarization (photo-CIDNP) in the reaction center (RC) of photosystem II (PS2). The light-enhanced NMR signals of the natural abundance (13)C provide information on the electronic structure of the primary electron donor P(680) (chlorophyll a molecules absorbing around 680 nm) and on the p(z) spin density pattern in its oxidized form, P(680)(.+). Most centerband signals can be attributed to a single chlorophyll a (Chl a) cofactor that has little interaction with other pigments. The chemical shift anisotropy of the most intense signals is characteristic for aromatic carbon atoms. The data reveal a pronounced asymmetry of the electronic spin density distribution within the P(680)(.+). PS2 shows only a single broad and intense emissive signal, which is assigned to both the C-10 and C-15 methine carbon atoms. The spin density appears shifted toward ring III. This shift is remarkable, because, for monomeric Chl a radical cations in solution, the region of highest spin density is around ring II. It leads to a first hypothesis as to how the planet can provide itself with the chemical potential to split water and generate an oxygen atmosphere using the Chl a macroaromatic cycle. A local electrostatic field close to ring III can polarize the electronic charge and associated spin density and increase the redox potential of P(680) by stabilizing the highest occupied molecular orbital, without a major change of color. This field could be produced, e.g., by protonation of the keto group of ring V. Finally, the radical cation electronic structure in PS2 is different from that in the bacterial RC, which shows at least four emissive centerbands, indicating a symmetric spin density distribution over the entire bacteriochlorophyll macrocycle.
我们报告了在光系统II（PS2）反应中心（RC）中光化学诱导动态核自旋极化（光化学诱导动态核极化，photo-CIDNP）的（13）C魔角旋转核磁共振观测结果。天然丰度（13）C的光增强核磁共振信号提供了关于原初电子供体P（680）（吸收波长约为680 nm的叶绿素a分子）的电子结构及其氧化形式P（680）（+）中p（z）自旋密度模式的信息。大多数中心带信号可归因于与其他色素相互作用很小的单个叶绿素a（Chl a）辅因子。最强信号的化学位移各向异性是芳香族碳原子的特征。数据揭示了P（680）（+）内电子自旋密度分布的明显不对称性。PS2仅显示一个宽而强的发射信号，该信号归属于C-10和C-15次甲基碳原子。自旋密度似乎向环III移动。这种移动很显著，因为对于溶液中的单体Chl a自由基阳离子，自旋密度最高区域在环II周围。这引发了关于该星球如何利用Chl a大环芳香族循环为自身提供分解水并产生氧气气氛的化学势的第一个假设。靠近环III的局部静电场可以使电荷和相关自旋密度极化，并通过稳定最高占据分子轨道来提高P（680）的氧化还原电位，而颜色变化不大。例如，该场可由环V酮基的质子化产生。最后，PS2中的自由基阳离子电子结构与细菌RC中的不同，细菌RC显示至少四个发射中心带，表明在整个细菌叶绿素大环上自旋密度分布对称。
