The oxidized form of the redox-active Y-Z tyrosyl residue involved in photosynthetic oxygen evolution has been generated and trapped in Mn-depleted Photosystem II core complexes from a D2-Y160F mutant strain of Synechocystis 6803. This system eliminates interference from P-700+ and Y-D(.) and allowed characterization of Y-Z(.) by using a combination of specific H-2-labeling and electron magnetic-resonance techniques that included CW-EPR, frequency-modulated and transient detected ENDOR, and H-2-ESEEM. Using these complementary techniques, we have carried out a detailed evaluation of the hyperfine structure of Y-Z(.) and obtained the dipolar interactions to weakly coupled nuclei, the strongly anisotropic tensors of the ring-hydrogens, and the more isotropic interactions to the beta-methylene site. No (H2O)-H-2 exchangeable features could be detected by CW-ENDOR, implying that Y-Z is not involved in a well-ordered hydrogen bond in its radical form. From the hyperfine coupling tensors the spin-density distribution of Y-Z(.) was derived with the following values : C-1 (0.37), C-2,C-6 (-0 07), C-3,C-5 (0.26), and C-4-O (0.25). These values are similar to those reported for other tyrosyl radicals, both hydrogen bonded and non-hydrogen bonded. : C-1 (0.37), C-2,C-6 (-0 07), C-3,C-5 (0.26), and C-4-O (0.25). These values are similar to those reported for other tyrosyl radicals, both hydrogen bonded and non-hydrogen bonded.