Redox titration of the primary electron acceptor of Photosystem I in spinach chloroplasts

The midpoint potential of the primary electron acceptor of Photosystem I in spinach chloroplasts was titrated using the photooxidation of P₇₀₀ at −196 °C as an index of the amount of primary acceptor present in the oxidized state. The redox potential of the chloroplast suspension was established by the reducing power of hydrogen gas (mediated by clostridial hydrogenase and 1,1′-trimethylene-2,2′-dipyridylium dibromide) at specific pH values at 25 °C. Samples were frozen to −196 °C and the extent of the photooxidation of P₇₀₀ was determined from light-minus-dark difference spectra. This titration indicated a midpoint potential of −0.53 V for the primary electron acceptor of Photosystem I.