Spin-Polarized Radical Pair in Photosystem I Reaction Center That Contains Different Quinones and Fluorenones as the Secondary Electron Acceptor

Abstract

The spin-polarized radical pair of the primary donor chlorophyll P700+ and the secondary acceptor Q- was studied by the electron spin echo envelope modulation (ESEEM) technique in the spinach photosystem I reaction center in which artificial quinones and fluorenones were reconstituted as Q. From the dipole interaction between the spins on P700+ and Q-, the distance between P700 and anilinochloronaphthoquinone or hydroxyanthraquinone was estimated to be 25.6 ± 0.3 Å or 25.7 ± 0.3 Å, respectively, suggesting their proper binding at the original phylloquinone binding site. Distances of 26.0 Å with larger heterogeneity were estimated for (NO2)3- and (NO2)4-fluorenone compounds that have only one carbonyl group, suggesting their multiple orientations in the binding pocket. The lifetimes of the radical pairs estimated by the time dependency of the two-pulse ESE signals were almost temperature independent (30−80 μs at 4−80 K). In the three-pulse ESE measurement, the lifetimes were lengthened by 10−103 times and showed strong temperature dependencies.