Comparative study of the photochemistry of chloroplast membranes and photosystem II particles

A comparative study of the photoreducing potentials of spinach thylakoid membranes and spinach photosystem II particles has been made. Hexachloroplatinate ions have been used as electron acceptors in a Hill-like assay for oxygen evolution measurements with both thylakoid membranes and photosystem II particles. However, unlike other Hill acceptors, such as ferricyanide, hexachloroplatinate can be fully reduced to metallic platinum that is catalytically active for hydrogen evolution. This is experimentally confirmed in the ability of chloroplast membranes to photoprecipitate platinum and photoproduce molecular hydrogen. Although similar experiments with photosystem II particles resulted in hexachloroplatinate-supported oxygen evolution, hydrogen evolution was not observed. Moreover, photosystem II particles coupled to ferredoxin and hydrogenase resulted in neither hydrogen nor oxygen evolution—a distinct contrast to the results obtained with chloroplast membranes.