Reaction of Ai-menaquinone in Type I Reaction Center of Heliobacterium Modesticaldum at Cryogenic Temperature

摘要

Temperature dependence of electron transfer reaction via Ai-menaquinone in type I homodimeric reaction center of Heliobacterium modesticaldum (hRC) was observed by time-resolved electron paramagnetic resonance (EPR) spectroscopy. Flash excitation induced an electron spin polarization (ESP) signal due to P800+Fx-radical pair at 14 K, exhibiting an E/A (E, emission; A, absorption) spectral pattern. After the hRC core complex was pre-illuminated at 210 K for 1 hour and subsequently cooled to 5 K during illumination, another ESP signal due to P800+A1-, exhibiting an A/E pattern, was flash-induced at 14 K. The decay time was estimated to be 90 us at 5 K and 25 us at 20 K, respectively. The recombination of P800+A1-in hRC is significantly faster at all the temperatures, compared to that of P700+A1-in PS I, which occurs in around 100μs with little temperature dependence. The recombination time of P800+Fx-was a 4-11 ms at 5 K in contrast to the longer time than 100 ms in PSI. These results indicate that A1-menaquinone is involved in the rapid electron transfer between P700 and Fx in hRC, as phylloquinone in PSI, probably with unique energy gap or geometry.