Wavelength dependence of the photo-induced conversion of the Ni-C to the Ni-L redox state in the [NiFe] hydrogenase of Desulfovibrio vulgaris Miyazaki F

The conversion process of the Ni-C to the Ni-L redox state of the [NiFe] center in Desulfovibrio vulgaris Miyazaki F hydrogenase is investigated by EPR spectroscopy following laser excitation at distinct wavelengths. The Ni-L state is characterized by a rhombic g tensor with principal values of 2.30, 2.12 and 2.05. The action spectrum associated with the Ni-C to Ni-L photoconversion spans the complete visible range and local maxima are present at 590 and 700 nm and a shoulder around 850 nm. In the UV/VIS spectrum of the Ni-C state, bands of low intensity can also be observed at 590 and 700 urn. It is proposed that these bands are associated with the [NiFe] center and correspond to electronic transitions that trigger the conversion process. On the basis of the small molar absorption coefficients (1100 M-1 cm(-1) and 1400 M-1 cm(-1)), these transitions contain at most a small amount of ligand-to-metal charge transfer character. A light-induced back conversion, starting from the Ni-L redox state, has not been observed. Based on comparison with data available from X-ray crystallography and from ENDOR and pulse EPR spectroscopy of the Ni-C and Ni-L states, we propose that the Ni-C to Ni-L conversion process is associated with a dissociation of the Ni-H bond and translocation of the released proton to the sulfur of one of the terminal cysteines.