Pulsed EPR Study of Spin-Coupled Radical Pairs in Photosynthetic Reaction Centers: Measurement of the Distance Between and in Photosystem I and between and in Bacterial Reaction Centers

Abstract

The spin-polarized radical pairs in protonated and deuterated Zn-substituted reaction centers from two different mutants of the photosynthetic bacteria Rhodobacter sphaeroides and in plant photosystem I from Synechococcus elongatus are investigated by pulsed EPR spectroscopy. Spin-polarized radical pairs give rise to a characteristic out-of-phase electron spin echo. This echo shows a deep envelope modulation with a frequency governed by the spin−spin interaction. The known distance dependence of the magnetic dipolar interaction allows the determination of the distance between the cofactors carrying the unpaired electron spins. For the bacterial reaction centers this distance is known for the electronic ground state from crystal structures and is compared here with the distance of the radical pair spins, i.e. the charge-separated state. In photosystem I the location of the acceptor A1 is not known yet. A distance of 25.4 ± 0.3 Å between and is obtained here and gives new structural information on photosystem I.