Pulsed and parallel-polarization EPR characterization of the photosystem II oxygen-evolving complex

Photosystem II uses visible light to drive the oxidation of water, resultin g in bioactivated electrons and protons, with the production of molecular o xygen as a byproduct. This water-splitting reaction is carried out by a man ganese cluster/tyrosine radical ensemble, the oxygen-evolving complex. Alth ough conventional continuous-wave, perpendicular-polarization electron para magnetic resonance (EPR) spectroscopy has significantly advanced our knowle dge of the structure and function of the oxygen-evolving complex, significa nt additional information can be obtained with the application of additiona l EPR methodologies. Specifically, parallel-polarization EPR spectroscopy c an be used to obtain highly resolved EPR spectra of integer spin Mn species , and pulsed EPR spectroscopy with electron spin echo-based sequences, such as electron spin echo envelope modulation and electron spin echo-electron nuclear double resonance, can be used to measure weak interactions obscured in continuous-wave spectroscopy by inhomogeneous broadening.