Electronic structure of the tyrosine D radical and the water-splitting complex from pulsed ENDOR spectroscopy on photosystem II single crystals

Abstract

Pulsed electron nuclear double resonance (ENDOR) spectroscopy at Q- and W-band frequencies was applied to single crystals of photosystem II from Th. elongatus. W-Band ¹H-ENDOR on the dark-stable radical state Y^˙_D of the redox-active tyrosine residue Y_D yields a complete mapping of the electronic structure of this amino acid radical in terms of an assignment of all hyperfine coupling tensors of the protons covalently bound to the side chain. This study can serve as a model case for the potential of high-field/high-frequency ENDOR on protein single crystals for obtaining highly resolved electronic structure information. Q-band ⁵⁵Mn-ENDOR was applied to the S₂oxidation state of the water-splitting complex in photosystem II single crystals. Irrespective of the difficulties arising from the extremely broad electron paramagnetic resonance (EPR) spectroscopy (≈200 mT) and ENDOR (≈100 MHz) spectra a tentative assignment of the Mn ion in the formal oxidation state III to a Mn position in the structural model of PSII is possible on the basis of the ENDOR data.