Electron paramagnetic resonance active spin states

Photosystem II (PSII) contains a remarkably stable tyrosine radical D", located at tyr-160 in the D2 polypeptide, which exhibits a well-known electron paramagnetic resonance (EPR) spectrum, EPR Signal Ilg (1). The function of D" " in the mechanism of photosynthetic H2O oxidation remains unclear despite evidence that it can oxidize the Mn complex, the H20-oxidation catalyst in PSII, from the Sq state to the normally daik-stable Si state (2). Several studies indicate that die oxidation state of the Mn complex influences the electron spin-lattice relaxation rate of D" (3-5), perhaps via a weak dipolar coupling, as suggested by Evelo et al. (5). Hence, the relaxation properties of D may provide a probe for the topology of redox-active sites in the 02-evolving center (OEC) and of the magnetic properties of the Mn complex. 

The chapter Electron Spin Resonance in Catalysis by Lunsford was prompted by the extensive activity in this field since the publication of an article on a similar subject in Volume 12 of this serial publication. This chapter is limited to paramagnetic species that are reasonably well defined by means of their spectra. It contains applications of ESR technique to the study of adsorbed atoms and molecules, and also to the evaluation of surface effects. The application of ESR to the determination of the state of transition metal ions in catalytic reactions is also discussed. 

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