PROPERTIES OF THE S2 STATE ASSOCIATED WITH O2 EVOLUTION

Abstract

This chapter discusses the properties of the S2 state associated with O2 evolution. Photosynthetic water oxidation requires the generation of four oxidizing equivalents per molecule of O2 evolved. These oxidizing equivalents are produced by single charge separation events in photosystem II and are stored in an O2-evolving complex. A light-induced multiline electron paramagnetic resonance (EPR) signal in spinach chloroplasts arises from a paramagnetic species in the final oxidation product of the S2 state. The most obvious feature of the multiline S2 state EPR signal is the wealth of hyperfine structure. This hyperfine structure arises from a moderately strong coupling of the unpaired electrons to a large net nuclear spin. The splitting between the hyperfine lines in the low resolution spectrum of the S2 state is approximately 90 G. Manganese (Mn) is a component of the paramagnetic species in the S2 state that indicates that Mn participates in the oxidation state changes associated with S state advancement.