Mokvist Fredrik, Sjöholm Johannes, Mamedov Fikret, Styring Stenbjörn
Molecular Biomimetics, Department of Chemistry-Ångström, Uppsala University, Ångström Laboratory , P.O. Box 523, S-751 20 Uppsala, Sweden.
Biochemistry. 2014 Jul 8;53(26):4228-38. doi: 10.1021/bi5006392. Epub 2014 Jun 27.
We have earlier shown that all electron transfer reactions in Photosystem II are operational up to 800 nm at room temperature [Thapper, A., et al. (2009) Plant Cell 21, 2391-2401]. This led us to suggest an alternative charge separation pathway for far-red excitation. Here we extend these studies to a very low temperature (5 K). Illumination of Photosystem II (PS II) with visible light at 5 K is known to result in oxidation of almost similar amounts of YZ and the Cyt b559/ChlZ/CarD2 pathway. This is reproduced here using laser flashes at 532 nm, and we find the partition ratio between the two pathways to be 1:0.8 at 5 K [the partition ratio is here defined as (yield of YZ/CaMn4 oxidation):(yield of Cyt b559/ChlZ/CarD2 oxidation)]. The result using far-red laser flashes is very different. We find partition ratios of 1.8 at 730 nm, 2.7 at 740 nm, and >2.7 at 750 nm. No photochemistry involving these pathways is observed above 750 nm at this temperature. Thus, far-red illumination preferentially oxidizes YZ, while the Cyt b559/ChlZ/CarD2 pathway is hardly touched. We propose that the difference in the partition ratio between visible and far-red light at 5 K reflects the formation of a different first stable charge pair. In visible light, the first stable charge pair is considered to be PD1+QA-. In contrast, we propose that the electron hole is residing on the ChlD1 molecule after illumination by far-red light at 5 K, resulting in the first stable charge pair being ChlD1+QA-. ChlD1 is much closer to YZ (11.3 Å) than to any component in the Cyt b559/ChlZ/CarD2 pathway (shortest ChlD1-CarD2 distance of 28.8 Å). This would then explain that far-red illumination preferentially drives efficient electron transfer from YZ. We also discuss mechanisms for accounting for the absorption of the far-red light and the existence of hitherto unobserved charge transfer states. The involvement of two or more of the porphyrin molecules in the core of the Photosystem II reaction center is proposed.
我们之前已经表明，在室温下，光系统II中的所有电子转移反应在800纳米处仍可运行[Thapper, A., 等人(2009年)《植物细胞》21卷，2391 - 2401页]。这使我们提出了一种用于远红光激发的替代电荷分离途径。在此，我们将这些研究扩展到了极低温度(5K)。已知在5K下用可见光照射光系统II(PS II)会导致YZ以及Cyt b559/ChlZ/CarD2途径氧化的量几乎相似。这里使用532纳米的激光闪光再现了这一情况，并且我们发现在5K时两条途径之间的分配比为1:0.8[此处分配比定义为(YZ/CaMn4氧化产率):(Cyt b559/ChlZ/CarD2氧化产率)]。使用远红光激光闪光的结果则大不相同。我们发现在730纳米处分配比为1.8，在740纳米处为2.7，在750纳米处大于2.7。在此温度下，在750纳米以上未观察到涉及这些途径的光化学现象。因此，远红光照射优先氧化YZ，而Cyt b559/ChlZ/CarD2途径几乎未受影响。我们提出，5K时可见光和远红光之间分配比的差异反映了不同的首个稳定电荷对的形成。在可见光中，首个稳定电荷对被认为是PD1+QA-。相比之下，我们提出在5K下用远红光照射后，电子空穴位于ChlD1分子上，导致首个稳定电荷对为ChlD1+QA-。ChlD1与YZ(11.3 Å)的距离比与Cyt b559/ChlZ/CarD2途径中的任何组分的距离都更近(ChlD1与CarD2的最短距离为28.8 Å)。这就可以解释远红光照射优先驱动从YZ进行高效电子转移。我们还讨论了用于解释远红光吸收以及迄今未观察到的电荷转移态存在的机制。有人提出光系统II反应中心核心中的两个或更多卟啉分子参与其中。
