Raszewski Grzegorz, Renger Thomas
Institut für Chemie und Biochemie (Kristallographie), Freie Universität Berlin, Fabeckstrasse 36a, 6, D-14195 Berlin, Germany.
J Am Chem Soc. 2008 Apr 2;130(13):4431-46. doi: 10.1021/ja7099826. Epub 2008 Mar 8.
A structure-based modeling and analysis of the primary photophysical reactions in photosystem II (PS-II) core complexes is presented. The modeling is based on a description of stationary and time-resolved optical spectra of the CP43, CP47, and D1-D2-cytb559 subunits and whole core complexes. It shows that the decay of excited states in PS-II core complexes with functional (open) reaction centers (RCs) is limited by the excitation energy transfer from the CP43 and CP47 core antennae to the RC occurring with a time constant of 40-50 ps at room temperature. The chlorophylls responsible for the low energy absorbance bands in the CP43 and CP47 subunits are assigned, and their signatures in hole burning, fluorescence line narrowing, and triplet-minus-singlet spectra are explained. The different locations of these trap states in the CP43 and CP47 antennae with respect to the reaction center lead to a dramatic change of the transfer dynamics at low temperatures. The calculations predict that, compared to room temperature, the fluorescence decay at 77 K should reveal a faster transfer from CP43 and a much slower and highly dispersive transfer from CP47 to the RC. A factor of 3 increase in the fastest decay time constant of fluorescence that was reported to occur when the RC is closed (the plastoquinone QA is reduced) is understood in the present model by assuming that the intrinsic rate constant for primary electron transfer decreases from 100 fs-1 for open RCs to 6 ps-1 for closed RCs, leading to a reduction of the primary electron acceptor PheoD1, in 300 fs and 18 ps, respectively. The model suggests that the reduced QA switches the photosystem into a photoprotective mode in which a large part of the excitation energy of the RC returns to the CP43 and CP47 core antennae, where the physiologically dangerous triplet energy of the chlorophylls can be quenched by the carotenoids. Experiments are suggested to test this hypothesis. The ultrafast primary electron transfer inferred for open RCs provides further support for the accessory chlorophyll ChlD1 to be the primary electron donor in photosystem II.
本文介绍了基于结构的光系统II（PS-II）核心复合物初级光物理反应的建模与分析。该建模基于对CP43、CP47和D1-D2-cytb559亚基以及整个核心复合物的稳态和时间分辨光谱的描述。结果表明，在室温下，具有功能性（开放）反应中心（RC）的PS-II核心复合物中激发态的衰减受限于从CP43和CP47核心天线到RC的激发能量转移，其时间常数为40 - 50皮秒。确定了CP43和CP47亚基中负责低能量吸收带的叶绿素，并解释了它们在空穴烧蚀、荧光线窄化和三线态减单重态光谱中的特征。这些陷阱态在CP43和CP47天线中相对于反应中心的不同位置导致低温下转移动力学的显著变化。计算预测，与室温相比，77 K时的荧光衰减应显示从CP43的转移更快，而从CP47到RC的转移更慢且高度分散。在本模型中，通过假设初级电子转移的本征速率常数从开放RC的100飞秒⁻¹降至封闭RC的6皮秒⁻¹，导致初级电子受体去镁叶绿素D1分别在300飞秒和18皮秒内减少，从而理解了据报道当RC关闭（质体醌QA被还原）时荧光最快衰减时间常数增加3倍的现象。该模型表明，还原的QA将光系统切换到光保护模式，其中RC的大部分激发能量返回CP43和CP47核心天线，在那里叶绿素的生理危险三线态能量可被类胡萝卜素淬灭。建议进行实验来检验这一假设。为开放RC推断的超快初级电子转移为辅助叶绿素ChlD1作为光系统II中的初级电子供体提供了进一步支持。
