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

Abstract

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 nm. 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.