Effects of copper and zinc ions on photosystem II studied by EPR spectroscopy

The effect of Zn2+ or Cu2+ ions on Mn-depleted photosystem II (PS II) has b een investigated using EPR spectroscopy. In Zn2+-treated and Cu2+-treated P S II, chemical reduction with sodium dithionite gives rise to a signal attr ibuted to the plastosemiquinone, Q(A)(.-), the usual interaction with the n on-heme iron being lost. The signal was identified by Q-band EPR spectrosco py which partially resolves the typical g-anisotropy of the semiquinone ani on radical, Illumination at 200 K of the unreduced samples gives rise to a single organic free radical in Cu2+-treated PS II, and this is assigned to a monomeric chlorophyll cation radical, Chl a(.+), based on its H-1-ENDOR s pectrum. The Zn2+-treated PS II under the same conditions gives rise to two radical signals present in equal amounts and attributed to the Chi a(.+) a nd the Q(A)(.-) formed by light-induced charge separation. When the Cu2+-tr eated PS II is reduced by sodium ascorbate, at greater than or equal to 77 K electron donation eliminates the donor-side radical leaving the Q(A)(.-) EPR signal. The data are explained as follows: (1) Cu2+ and Zn2+ have simil ar effects on PS II (although higher concentrations of Zn2+ are required) c ausing the displacement of the non-heme Fe2+. (2) In both cases chlorophyll is the electron donor at 200 K. It is proposed that the lack of a light-in duced Q(A)(.-) Signal in the unreduced Cu2+-treated sample is due to Cu2+ a cting as an electron acceptor from Q(A)(.-) at low temperature, forming the Cu+ state and leaving the electron donor radical Chl a(.+) detectable by E PR. (3) The Cu2+ im PS II is chemically reducible by ascorbate prior to ill umination, and the metal can therefore no longer act as an electron accepto r; thus Q(A)(-) is generated by illumination in such samples. (4) With dith ionite, both the Cu2+ and the quinone are reduced resulting in the presence of Q(A)(.-) in the dark. The suggested high redox potential of Cu2+ when i n the Fe2+ site in PS II is in contrast to the situation in the bacterial r eaction center where it has been shown in earlier work that the Cu2+ is unr educed by dithionite. It cannot be ruled out however that Q(A)-Cu2+ is form ed and a magnetic interaction is responsible for the lack of the Q(A)(-) Si gnal when no exogenous reductant is present, With this alternative possibil ity, the effects of reductants would be explained as the loss of Cu2+ (due to formation of Cu+) leading to loss of the Cu2+ from the Fe2+ site due to the binding equilibrium. The quite different binding and redox behavior of the metal in the iron site in PS II compared to that of the bacterial react ion center is presumably a further reflection of the differences in the coo rdination of the iron in the two systems.