Reaction of A₁-menaquinone in Type I Reaction Center of Heliobacterium Modesticaldum at Cryogenic Temperature

Abstract

Temperature dependence of electron transfer reaction via A₁-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⁺F_X ⁻ 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 μs at 5 K and 25 μs 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 100 μs with little temperature dependence. The recombination time of P800⁺F_X ⁻ 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 F_X in hRC, as phylloquinone in PSI, probably with unique energy gap or geometry.