Research School of Chemistry, Australian National University, Canberra 2600, Australia.
Department of Life Sciences, Imperial College, London SW7 2AZ, United Kingdom.
Biochim Biophys Acta Bioenerg. 2020 Oct 1;1861(10):148248. doi: 10.1016/j.bbabio.2020.148248. Epub 2020 Jun 19.
Far-red light (FRL) Photosystem II (PSII) isolated from Chroococcidiopsis thermalis is studied using parallel analyses of low-temperature absorption, circular dichroism (CD) and magnetic circular dichroism (MCD) spectroscopies in conjunction with fluorescence measurements. This extends earlier studies (Nurnberg et al 2018 Science 360 (2018) 1210-1213). We confirm that the chlorophyll absorbing at 726 nm is the primary electron donor. At 1.8 K efficient photochemistry occurs when exciting at 726 nm and shorter wavelengths; but not at wavelengths longer than 726 nm. The 726 nm absorption peak exhibits a 21 ± 4 cm electrochromic shift due to formation of the semiquinone anion, Q. Modelling indicates that no other FRL pigment is located among the 6 central reaction center chlorins: P, P Chl, Chl, Pheo and Pheo. Two of these chlorins, Chl and P are located at a distance and orientation relative to Q so as to account for the observed electrochromic shift. Previously, Chl was taken as the most likely candidate for the primary donor based on spectroscopy, sequence analysis and mechanistic arguments. Here, a more detailed comparison of the spectroscopic data with exciton modelling of the electrochromic pattern indicates that P is at least as likely as Chl to be responsible for the 726 nm absorption. The correspondence in sign and magnitude of the CD observed at 726 nm with that predicted from modelling favors P as the primary donor. The pros and cons of P vs Chl as the location of the FRL-primary donor are discussed.
远红（FRL）光光系统 II（PSII）从嗜热颤绿藻中分离出来，使用低温吸收、圆二色性（CD）和磁圆二色性（MCD）光谱学的平行分析以及荧光测量进行研究。这扩展了早期的研究（Nurnberg 等人，2018 年《科学》360（2018）1210-1213）。我们确认吸收在 726nm 的叶绿素是原初电子供体。在 1.8K 时，当在 726nm 和更短的波长处激发时，会发生有效的光化学；但在波长大于 726nm 时不会发生。726nm 吸收峰由于半醌阴离子 Q 的形成而出现 21±4cm 的电致变色位移。建模表明，在 6 个中心反应中心叶绿素中没有其他 FRL 色素：P、P Chl、Chl、Pheo 和 Pheo。这些叶绿素中的两个，Chl 和 P，位于相对于 Q 的距离和方向，以解释观察到的电致变色位移。以前，基于光谱学、序列分析和机制论证，Chl 被认为是原初供体的最可能候选者。在这里，对光谱数据与电致变色模式的激子建模的更详细比较表明，P 至少与 Chl 一样有可能负责 726nm 吸收。在 726nm 处观察到的 CD 与建模预测的 CD 在符号和幅度上的一致性有利于 P 作为原初供体。讨论了 P 与 Chl 作为 FRL-原初供体位置的优缺点。
