Abstract Photosystem I fragments were prepared by digitonin treatment of spinach chloroplasts. The mid­point potential of cytochrome b6 in the fragments is close to 0 V, showing a one electron transition. No cytochrome b559 was detectable, neither in difference absorption spectra nor in light-induced absorbance changes. In the absence of added cofactors only cytochrome b6 photoreduction can be observed. This photoreduction is stimulated by ferredoxin. Ferredoxin-NADP+ reductase appears not to be involved in cytochrome b6 reduction. Photooxidation of cytochrome b6 is dependent on plastocyanin addition and inhibited by DBMIB, a plastoquinone antagonist. Addition of plastocyanin restores cytochrome f photooxidation as well, reacting quite specifically in about equimolar concentrations to bound cytochrome f. The stimulation of cytochrome f oxidation is abolished by an antibody prepared against plastocyanin, indicating a surface location of plastocyanin in digitonin treated membranes. Biphasic kinetics of dark-reduction of cytochrome f by ascorbate indicate that part of this cyto­chrome f is relatively inaccessible in the membrane. After preillumination a monophasic reduction is observed and the slowly oxidized component is absent. Illumination in the presence of plastocyanin causes a fast and complete reduction of cytochrome f, suggesting equilibration of cyto­chrome f with added plastocyanin, residing in the membrane surface. It appears that actinic light causes conformation and/or structural changes in the membrane of these digitonin fragments, influencing cytochrome f accessibility.

Böhme, H. (1976). Photoreactions of Cytochrome b6 and Cytochrome f in Chloroplast Photosystem I Fragments. Zeitschrift Für Naturforschung C, 31(1–2), 68–77.