Department of Chemistry, Kansas State University, Manhattan, Kansas 66506, United States.
J Phys Chem B. 2012 Mar 29;116(12):3890-9. doi: 10.1021/jp3007624. Epub 2012 Mar 19.
It is widely accepted that the primary electron acceptor in various Photosystem II (PSII) reaction center (RC) preparations is pheophytin a (Pheo a) within the D1 protein (Pheo(D1)), while Pheo(D2) (within the D2 protein) is photochemically inactive. The Pheo site energies, however, have remained elusive, due to inherent spectral congestion. While most researchers over the past two decades placed the Q(y)-states of Pheo(D1) and Pheo(D2) bands near 678-684 and 668-672 nm, respectively, recent modeling [Raszewski et al. Biophys. J. 2005, 88, 986 - 998; Cox et al. J. Phys. Chem. B 2009, 113, 12364 - 12374] of the electronic structure of the PSII RC reversed the assignment of the active and inactive Pheos, suggesting that the mean site energy of Pheo(D1) is near 672 nm, whereas Pheo(D2) (677.5 nm) and Chl(D1) (680 nm) have the lowest energies (i.e., the Pheo(D2)-dominated exciton is the lowest excited state). In contrast, chemical pigment exchange experiments on isolated RCs suggested that both pheophytins have their Q(y) absorption maxima at 676-680 nm [Germano et al. Biochemistry 2001, 40, 11472 - 11482; Germano et al. Biophys. J. 2004, 86, 1664 - 1672]. To provide more insight into the site energies of both Pheo(D1) and Pheo(D2) (including the corresponding Q(x) transitions, which are often claimed to be degenerate at 543 nm) and to attest that the above two assignments are most likely incorrect, we studied a large number of isolated RC preparations from spinach and wild-type Chlamydomonas reinhardtii (at different levels of intactness) as well as the Chlamydomonas reinhardtii mutant (D2-L209H), in which the active branch Pheo(D1) is genetically replaced with chlorophyll a (Chl a). We show that the Q(x)-/Q(y)-region site energies of Pheo(D1) and Pheo(D2) are ~545/680 nm and ~541.5/670 nm, respectively, in good agreement with our previous assignment [Jankowiak et al. J. Phys. Chem. B 2002, 106, 8803 - 8814]. The latter values should be used to model excitonic structure and excitation energy transfer dynamics of the PSII RCs.
人们普遍认为，各种光系统 II (PSII) 反应中心 (RC) 制剂中的主要电子受体是 D1 蛋白 (Pheo(D1)) 中的叶绿素 a (Pheo a)，而 Pheo(D2)（位于 D2 蛋白中）则是光化学失活的。然而，由于固有的光谱拥挤，Pheo 位点能量仍然难以捉摸。虽然在过去的二十年中，大多数研究人员将 Pheo(D1) 和 Pheo(D2) 带的 Q(y)-态分别置于 678-684nm 和 668-672nm 附近，但最近对 PSII RC 电子结构的建模[Raszewski 等人。生物物理杂志。2005 年，88,986-998;考克斯等人。物理化学杂志 B 2009,113,12364-12374] 逆转了活性和失活 Pheo 的分配，表明 Pheo(D1) 的平均位点能量接近 672nm，而 Pheo(D2)（677.5nm）和 Chl(D1)（680nm）具有最低的能量（即，Pheo(D2) 主导的激子是最低激发态）。相比之下，对分离的 RC 进行的化学色素交换实验表明，两种叶绿素都在 676-680nm 处具有其 Q(y)吸收最大值[Germano 等人。生物化学 2001 年，40,11472-11482；Germano 等人。生物物理杂志 2004 年，86,1664-1672]。为了更深入地了解 Pheo(D1) 和 Pheo(D2) 的位点能量（包括通常声称在 543nm 处简并的相应 Q(x)跃迁），并证明上述两个分配最有可能是不正确的，我们研究了来自菠菜和野生型莱茵衣藻的大量分离 RC 制剂（在不同的完整程度下）以及莱茵衣藻突变体（D2-L209H），其中活性分支 Pheo(D1) 通过基因被叶绿素 a（Chl a）取代。我们表明 Pheo(D1) 和 Pheo(D2) 的 Q(x)-/Q(y)-区域位点能量分别为545/680nm 和541.5/670nm，与我们之前的分配结果一致[Jankowiak 等人。物理化学杂志 B 2002,106,8803-8814]。后者的值应用于模拟 PSII RC 的激子结构和激发能量转移动力学。
