Are The Molecular Electronics Of The Reaction Centres Of Bacteria And Photosystem Two Comparable

Although the reaction centres of photos stem two (PS2) and purple photosynthetic bacteria dif&r in that only the former can oxidise H 0, both show remarkable similarities in their primary electronic events. The initial light induced charge separation occurs in about 3 ps and creates the radical pair P A-, where in both systems P seems to be a special pair of chlorophyll molecules and A a pheophytin molecule. Recombination occurs via singlet and triplet excitation of P with lifetimes in the ns and ,us time domains respectively. Comparison of the two systems has been possible because of the recent success in isolating the PS2 reaction from higher plants. This complex, however, differs from the isolated bacterial reaction centre in that it does not bind QA and QB, the secondary quinone electron acceptors, and is unstable to light when 0 is present. Removal of 0, stabilizes the reaction cent& and allows a wide range of studies to be conducted. Flash absorption stuqies indicate that 0, quenches the triplet on P (i.e. P680 ) and that the mechanism of photodamage involves singlet oxygen.