We report redox potentials (E_m) for one-electron reduction for all chlorophylls in the two electron-transfer branches of water-oxidizing enzyme photosystem II (PSII), photosystem I (PSI), and purple bacterial photosynthetic reaction centers (PbRC). In PSI, E_m values for the accessory chlorophylls were similar in both electron-transfer branches. In PbRC, the corresponding E_m value was 170 mV less negative in the active L-branch (B_L) than in the inactive M-branch (B_M), favoring B_L˙⁻ formation. This contrasted with the corresponding chlorophylls, Chl_D1 and Chl_D2, in PSII, where E_m(Chl_D1) was 120 mV more negative than E_m(Chl_D2), implying that to rationalize electron transfer in the D1-branch, Chl_D1 would need to serve as the primary electron donor. Residues that contributed to E_m(Chl_D1) < E_m(Chl_D2) simultaneously played a key role in (i) releasing protons from the substrate water molecules and (ii) contributing to the larger cationic population on the chlorophyll closest to the Mn₄CaO₅ cluster (P_D1), favoring electron transfer from water molecules. These features seem to be the nature of PSII, which needs to possess the proton-exit pathway to use a protonated electron source—water molecules.

中文翻译：

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对光驱动的能量转换酶中两个电子转移途径的有力见解†

我们报告水氧化酶光系统II（PSII），光系统I（PSI）和紫色细菌光合作用反应中心（PbRC）的两个电子转移分支中所有叶绿素的单电子还原的氧化还原电位（E _m）。在PSI中，在两个电子转移分支中，辅助叶绿素的E _m值相似。在PBRC，相应È_米值为170毫伏少在活性左旋支（B负_大号）中比在非活性M-分支（B_中号），有利于乙_大号˙ ^-形成。这与相应的叶绿素Chl _D1和Chl _D2形成对比在PSII中，E _m（Chl _D1）比E _m（Chl _D2）负120 mV ，这意味着要使D1分支中的电子转移合理化，Chl _D1将需要充当主要的电子供体。导致E _m（Chl _D1）< E _m（Chl _D2）的残基同时在（i）从底物水分子释放质子和（ii）促成最接近Mn的叶绿素上的较大阳离子种群上起关键作用。₄ CaO ₅团簇（_PD1），有利于水分子中的电子转移。这些特征似乎是PSII的本质，PSII需要具备质子退出途径才能使用质子化的电子源（水分子）。