The Photoactivated Production of Molecular Hydrogen Over Prolonged Periods Catalysed by Platinum or Hydrogenase

We have studied the ability of platinum (Adam’s catalyst, PtO 2 ) to replace the enzyme hydrogenase in photoactivated H 2 evolving systems. (1) Both platinum and hydrogenase evolved H 2 at similar rates (7—9 μ mol H 2 /h/mg chlorophyll) on illumination of isolated chloroplasts with water as the electron source and methyl viologen as the electron carrier. In the absence of a catalyst or in the presence of an electron transport inhibitor there was no H 2 production. (2) Methyl viologen may also be reduced by irradiation with visible light in the presence of proflavine and a suitable electron donor. Under continuous illumination platinum catalysed the evolution of substantial quantities of H 2 from this system for up to 24 h with ethylenediaminetetraacetic acid (EDTA), cysteine, glutathione, mercaptoethanol or dithio- threitol (DTT) as electron sources. With EDTA plus glutathione platinum catalysed H 2 production for over 50 h with a yield of 19% based on the amount of electron donor present. The turnovers of methyl viologen and proflavine were high, being 54 and 1350 respectively. The photodestruction of proflavine was shown to be the limiting factor of the longevity of H 2 production as on addition of fresh proflavine to the latter system, H 2 evolution continued for a further 50 h (27% yield). The replacement of proflavine and of methyl viologen with ferredoxin, iron-sulphur analogues and cytochrome was also investigated.