New insights on the primary electron-acceptor complex of photosystem II.

Absorbance changes in the 520nm region have been interpreted as indicating an electrochromic response of pigment molecules to the electric field formed across the membrane by the primary charge separation of the two chloroplast photoreactions, Photosystem I and I1 (Junge & Witt, 1968; Wolff et al., 1969). We have measured the flash-induced 518nm absorbance change in chloroplasts as a function of defined oxidation-reduction potential in order to characterize more fully the contributing components. In experiments at pH7.6, three components were observed in the potential range from +500mV to -350mV. A high-potential component (Component 1: Em at pH7.6 = +385mV) was associated with Photosystem I on the basis of its response to far-red flash activation and to the inhibitors of Photosystem 11, 3-(3,4-dichlorophenyl)-1,1dimethylurea and o-phenanthroline. This component is most likely associated with pigment P700, the Photosystem-I reaction-centre chlorophyll. Two additional components with more electronegative midpoint potentials were also observed. One (Component 2) had Em=+35mV, whereas the other (Component 3) had Em = -250mV. Components 2 and 3 were found to be associated with the Photosystem-I1 light reaction by using the above-described procedures to distinguish the two chloroplast photoreactions. In agreement with previous results, which indicated an equal contribution of Photosystems I and I1 to the electric potential difference across the membrane (Schliephake et al., 1968; Amesz & de Grooth, 1975), Components 2 and 3 from Photosystem I1 contributed an equal absorbance change to that of Component 1 from Photosystem I. In addition, the relative contribution of Components 2 and 3 was about equal. The midpoint oxidation-reduction potentials of Components 2 and 3 were found to be dependent on pH and showed a -60mV per pH unit dependence in the pH range from 6.5 to 8.5. In contrast, the E,,, of Component 1 was independent of pH in this pH range.