Function/location of menaquinone (MQ) was studied in the photosynthetic reaction center of Heliobacterium (Hbt.) modesticaldum (hRC), which is one of the most primitive homodimeric type I RCs. The spin-polarized electron paramagnetic resonance signals of light-induced radical pair species, which are made of oxidized electron donor bacteriochlorophyll g (P800⁺) and reduced menaquinone (MQ^–) or iron–sulfur cluster (F_X^–), were measured in the oriented membranes of Hbt. modesticaldum at cryogenic temperature. The spectral shape of transient electron spin-polarized signal of P800⁺F_X^– radical pair state varied little with respect to the direction of the external magnetic field. It suggested a dominant contribution of the spin evolution on the precursor primary radical pair P800⁺A₀^– state with the larger isotropic magnetic exchange interaction J than the anisotropic dipole interaction D. The pure P800⁺MQ^– signal was simulated by subtracting the effects of spin evolution during the electron-transfer process. It was concluded that the J value of the P800⁺MQ^– radical pair is negative with an amplitude almost comparable to |D|. It is in contrast to a positive and small J value of the P700⁺PhyQ^– state in photosystem I (PS I). The results indicate similar but somewhat different locations/binding sites of quinones between hRC and PS I.

中文翻译：

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P800 ⁺ Menaquinone ^–自由基对模式的HeliobacteriumModesticaldum膜中的光诱导电子自旋极化（ESP）EPR信号：Menaquinone在同型I型反应中心中的作用/位置

在最原始的同型二聚体I型RCs之一的Heliobacter（modemicalticalum）（hRC）的光合反应中心研究了甲萘醌（MQ）的功能/位置。测量了由氧化电子给体细菌叶绿素g（P800 ⁺）和还原性甲萘醌（MQ ^–）或铁硫簇（F _X ^–）组成的光诱导自由基对物种的自旋极化电子顺磁共振信号。Hbt的定向膜。低温下的modesticaldum。P800 ⁺ F _X ^–的瞬态电子自旋极化信号的频谱形状^–自由基对状态相对于外部磁场的方向变化很小。它建议自旋演化对前体初级自由基对的主要贡献P800 ⁺甲₀ ^-状态具有较大的各向同性磁交换相互作用Ĵ比各向异性偶极相互作用d。通过减去电子转移过程中自旋演化的影响来模拟纯P800 ⁺ MQ ^–信号。结论是，P800 ⁺ MQ ^–自由基对的J值是负的，其幅度几乎可与|| | |相比较。d|。与光系统I（PS I）中P700 ⁺ PhyQ ^–状态的正J值和较小的J值相反。结果表明，hRC和PS I之间醌的位置相似/但有所不同。