Applications of pulsed ELDOR-detected NMR measurements to studies of photosystem II: Magnetic characterization of Y-D tyrosine radical and Mn2+ bound to the high-affinity site

The pulsed electron-electron double resonance (ELDOR)-detected nuclear magnetic resonance (NMR) technique has been applied to the investigation of the hyperfine structure of the oxidized radical Of Y-D tyrosine (Y-D*) and Mn2+ ion bound to the high-affinity site in photosystem II. The resulting ELDOR spectrum of Y-D* was found to correspond with the pulsed electron-nuclear double resonance (ENDOR) spectrum except for a slightly larger linewidth (by a factor of 1.7). The spectrum showed marked anisotropy and revealed three peaks which can be assigned to matrix protons, C-3 and -5 protons and one of the beta-protons in the tyrosine molecule. The results demonstrate that the pulsed ELDOR-detected NMR technique is applicable to the study of organic radicals in biological systems. The Mn2+ ion bound to the high-affinity site in photosystem II yielded well-resolved ELDOR signals spreading over +/-1000 MHz. The magnetic properties of the Mn2+ were characterized on the basis of the calculation of the ENDOR transitions and the anisotropy of the ELDOR spectrum.