Chloroplast reactions with dipyridyl salts

Illuminated spinach chloroplasts reduce quarternary dipyridyl salts with oxidation-reduction potentials ranging from −342 to −656 mV. The accumulation of reduced dipyridyl was dependent upon the oxidation-reduction potential of each dipyridyl, ranging from 100% with the 1,1′-ethylene-2,2′-dipyridylium dibromide (E₀′ = −342 mV) to 3% with the 1,1′-trimethylene-2,2′-dipyridylium-4,4′-methyl dibromide (E₀′ = −656 mV). The data suggest that the oxidation-reduction potential of a natural photosynthetic reductant(s) may be near −521 mV. Photosynthetic phosphorylation occurs concurrent with reduction under both argon and air. In the presence of oxygen the reduced dipyridyls are oxidized and H₂O₂ is produced. The dipyridyls could be substituted for spinach ferredoxin in a dark reduction of cytochrome c in the presence of NADPH and NADP⁺ reductase (EC 1.6.99.4). The polagraphic half-wave oxidation-reduction potential of the dipyridyls is reported.