Monovalent nickel in hydrogenase from Chromatium vinosum

Redox titrations with hydrogenase from Chromatium vinosum show that its nickel ion can exist in 3, possibly 4, different redox states: the 3+, 2+, 1+ and possibly a zero valent state. The 1+ state is unstable: oxidation to Ni(II) occurs unless H2 gas is present. The Ni(I) coordination, but not that of Ni(III), is highly light sensitive. A photoreaction occurs on illumination. It is irreversible below 77 K, but reversible at 200 K. The rate of this photodissociation reaction in 2H2O is nearly 6-times slower than in H2O, indicating the breakage of a nickel-hydrogen bond. This forms the first evidence for an H atom in the direct coordination sphere of Ni in hydrogenase and for the involvement of this metal in the reaction with hydrogen.