Electron donation to photosystem I was studied in highly resolved particles from spinach. Divalent cations increased the efficiency of electron donation from spinach plastocyanin to P700+ through a decrease in the apparent Km for plastocyanin. Cytochrome f was not an efficient electron donor for P700+ in the presence or absence of divalent cations. Cytochrome f photooxidation could be observed in the presence of both plastocyanin and divalent cations.The efficiencies of electron donors from eukaryotic and prokaryotic algae to P700+ were also examined. Divalent cations enhanced the effectiveness of electron donors from eukaryotic organisms, while inhibiting electron donors from prokaryotic organisms. The prokaryotic electron donors were also much more efficient donors than were the electron donors from eukaryotic organisms. A correlation between the Km for the electron donor and its isoelectric point suggests that the net charge on the donor protein is a major determinant of the efficiency for electron donation. The data presented raise interesting questions with respect to the evolution of electron donation to photosystem I and the possibility of an additional electron carrier between plastocyanin and P700+.
Contact-shifted resonances have been detected in the pmr spectra of both oxidized and reduced forms of spinach and parsley ferredoxins. These resonances are assigned to the fl-CH2 protons of four cysteine residues that are thought to bind the iron-sulfur redox center to the polypeptide chain. Temperature dependences of contact shifts reveal that the two iron atoms are antiferromagnetically coupled in both redox forms of each of these proteins. Thermal population of magnetic states gives rise to the contact shifts observed in the formally diamagnetic oxidized forms of these ferredoxins and accounts for the failure of contact shifts in the reduced forms exhibit to a Curie Law temperature dependence. It appears that the unpaired electron of reduced spinach and parsley ferredoxin is unequally distributed over the two iron centers. Valence states for the iron pairs of high-spin
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