Monovalent nickel in hydrogenase from Chromatium vinosum

Redox titrations with hydrogenase from Chromatium vinosumshow 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 H 2gas 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 2H 2O is nearly 6-times slower than in H 2O, 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.