245 GHz high-field EPR study of tyrosine-D degrees and tyrosine-Z degrees in mutants of photosystem II

A 245 GHz 8.7 T high-field EPR study of tyrosine-D (TyrD degrees) and tyrosine-Z (TyrZ degrees) radicals of photosystem II (PSII) from Synechocystis PCC 6803 was carried out. Identical principal g values for the wild-type Synechocystis and spinach TyrD degrees showed that the two radicals were in similiar electrostatic environments. By contrast, the principal g values of the TyrD degrees in the D2-His189Gln mutant of Synechocystis were different from those of the wild-type and spinach radicals and were similar to those of the tyrosyl radical in ribonucleotide reductase. These comparisons indicate that the D2-His189Gln mutant TyrD degrees is not hydrogen-bonded or is only weakly so. The HF-EPR spectrum of TyrZ degrees was obtained from the D2-Tyr160Phe mutant that lacks TyrD degrees. The principal g values were nearly identical to those of the wild-type TyrD degrees. The low-field edge of the TyrZ degrees spectrum was much broader than at the other two principal g values and was also much broader than the TyrD degrees spectrum. From the identical g values and previous work on tyrosyl radical g values [Un S., Atta M., Fontecave, M., & Rutherford, A. W. (1995) J. Am. Chem. Sec. 117, 10713-10719], it was concluded that TyrZ degrees, like TyrD degrees, is hydrogen-bonded. The broadness of the g(x) component was interpreted as a distribution in strength of the hydrogen-bonding due to disorder in the protein environment about TyrZ degrees.