Formation of charge separated state P +Q −A and triplet state 3P at low temperature in Rhodobacter sphaeroides (R-26) reaction centers in which bacteriopheophytin a is replaced by plant pheophytin a

Abstract

Low temperature optical and photochemical properties of Rhodobacter sphaeroides (R-26) reaction centers, in which bacteriopheophytin a has been replaced by plant pheophytin a, are reported. Modified reaction centers preserve the ability for photoinduced electron transfer from the primary electron donor P to the primary quinone acceptor Q A at 80K. The triplet state ESR signal of modified reaction centers with prereduced Q A at 10K shows an electron spin polarization pattern and ZFS parameters analogous to those for the triplet state 3P in non-treated reaction centers. It was found that at low temperature both P +Q − A and 3P states are formed via a precursor radical pair P +I − in which I is the introduced plant pheophytin molecule. This shows that acceptor systems of bacterial and plant (photosystem II) reaction centers are mutually replacable in structural and functional aspects.