Light-Induced Electron Spin-Polarized (ESP) EPR Signal of the P800⁺ Menaquinone^– Radical Pair State in Oriented Membranes of Heliobacterium modesticaldum: Role/Location of Menaquinone in the Homodimeric Type I Reaction Center

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 (FX–), were measured in the oriented membranes of Hbt. modesticaldum at cryogenic temperature. The spectral shape of transient electron spin-polarized signal of P800+FX– 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.