Conversion of Light to Chemical Free Energy

Abstract Iron analysis and titration data show that each mole of ferredoxin from Clostridium pasteurianum reduces 2 moles of ferricyanide. The midpoint reduction potentials of the first and second reducing equivalents of this bacterial ferredoxin are found to be -0.367 and -0.398 volt, respectively. In aqueous solutions at pH 7.4 under helium, the oxidized ferredoxin is photoreduced by glutathione in the presence of hematoporphyrin. After the illumination, the reaction system slowly returns in the dark to its original condition in the absence of oxygen. Calculations based on the present data and the literature value of the midpoint potential of glutathione show that 3.2 kcal of light energy are stored per mole of electrons transferred from glutathione to ferredoxin in the final reaction mixture. The spectrometric and electron spin resonance data show that in this reaction the excited hematoporphyrin first captures an electron from glutathione to form a porphyrin radical, which then transfers its extra electron to ferredoxin.