CU(II)-INHIBITORY EFFECT ON PHOTOSYSTEM-II FROM HIGHER-PLANTS - A PICOSECOND TIME-RESOLVED FLUORESCENCE STUDY

The influence of Cu(II) inhibition on the primary reactions of photosy stem II (PSII) electron transport was studied by picosecond time-resol ved fluorescence on isolated PSII membranes. The fluorescence decay fr om Cu(II)-inhibited PSII centers showed a dominant amplitude of a fast phase (100-300 ps) similar to PSII centers in the uninhibited ''open state'' and minor contributions of components around 600 ps and 2.6 ns . These data indicate efficient primary charge separation in PSII memb ranes incubated with Cu(II). The quantum yield of primary reactions in the inhibited PSII centers was similar to that of ''open'' PSII cente rs. Kinetic analysis of the decay curves in the framework of the excit on/radical pair equilibrium model showed no significant changes in the rate constants associated with the charge separation/recombination eq uilibrium. However, in closed centers (Q(A) reduced), a decrease in th e rate constant k(23), associated with the back-reaction of a relaxed radical pair, by a factor of 4 was calculated. The free energy losses upon primary charge separation (Delta G(1)) and during subsequent radi cal pair relaxation (Delta G(2)) were also determined in Cu(II)-inhibi ted centers and were compared with uninhibited centers. No changes in the Delta G(1) values and a significant decrease in the Delta G(2) val ues were found as compared with those of control PSII centers in the ' 'closed'' state. These data indicate that Cu(II) does not affect prima ry radical pair formation, but strongly affects the formation of a rel axed radical pair, by neutralizing the negative charge on Q(A)(-) and eliminating the repulsive interaction between Pheo(-) and Q(A)(-) and/ or by modifying the general dielectric properties of the protein regio n, surrounding these cofactors. Moreover, a close attractive interacti on between Pheo(-), Q(A)(-), and Cu2+ be proposed. Our results are in good agreement with very recent EPR results indicating an additional e ffect of Cu2+ on the acceptor side [Jegerschold et al. (1995) Biochemi stry 34, 12747-12758].