Abstract
X-ray crystallographic studies [Ogata et al., J. Am. Chem. Soc. 124 (2002) 11628-11635] have shown that carbon monoxide binds to the nickel ion at the active site of the [NiFe] hydrogenase from Desulfovibriovulgaris Miyazaki F and inhibits its catalytic function. In the present work spectroscopic aspects of the CO inhibition for this bacterial organism are reported for the first time and enable a direct comparison with the existing crystallographic data. The binding affinity of each specific redox state for CO is probed by FTIR spectro-electrochemistry. It is shown that only the physiological state Ni-SI(a) reacts with CO. The CO-inhibited product state is EPR-silent (Ni2+) and exists in two forms, Ni-SCO and Ni-SCO(red). At very negative potentials, the exogenous CO is electrochemically detached from the active site and the active Ni-R states are obtained. At temperatures below 100 K, photodissociation of the extrinsic CO from the Ni-SCO state results in Ni-SI(a) that is identified to be the only light-induced state. In the dark, rebinding of CO takes place; the recombination rate constants are of biexponential character and the activation barrier is determined to be approximately 9 kJ mol(-1). In addition, formation of a paramagnetic CO-inhibited state (Ni-CO) was observed that results from the interaction of carbon monoxide with the Ni-L state. It is proposed that the nickel in Ni-CO is in a formal monovalent state (Ni1+).