The g = 4.1 EPR signal of the S2 state of the photosynthetic oxygen-evolving complex arises from a multinuclear manganese cluster

Abstract

The process of water oxidation and dioxygen evolution by the photosystem II (PSII) component of plant photosynthesis is cyclic, with intermediate states of the oxygen-evolving complex (OEC) designated S{sub 0} through S{sub 4}. Two electron paramagnetic resonance (EPR) signals have been assigned to the S{sub 2} state of the complex. A multiline' EPR signal centered at the g = 2 region of the spectrum shows 16 or more partially resolved Mn hyperfine transitions and arises from a cluster with a minimum of two exchange-coupled mixed-valence Mn atoms. The other S{sub 2} EPR signal occurs in the g = 4.1 region of the spectrum. The lack of resolved Mn hyperfine couplings has prevented conclusive assignment of the g = 4.1 EPR signal to a Mn center. However, a shift of the Mn X-ray K edge to higher energies is correlated with the appearance of the g = 4.1 signal in PSII membranes illuminated at 140 K. A considerable body of experimental work, including measurements of the temperature dependence of the EPR signals and observations of the interconversion between the multiline and the g = 4.1 signals, has given rise to two different models involving S = 3/2 Mn origins for themore » g = 4.1 signal. In this communication, the authors present direct spectral evidence of a multinuclear Mn origin for the S{sub 2} g = 4.1 signal.« less