Ab initio investigation of the structural and electronic differences between active-site models of [NiFe] and [NiFeSe] hydrogenases

The structural and electronic differences between coordination compounds wh ich model [NiFe] and [NiFeSe] hydrogenase active sites were investigated us ing quantum chemical methods. A previous study showed that the structural a nd electronic properties of these model compounds are largely consistent wi th available experimental data in the case of [NiFe] hydrogenases (L. De Gi oia, P. Fantucci, B. Guigliarelli, P. Bertrand, submitted). The extension o f this study to models of the Ni-B, Ni-SI, Ni-C, and Ni-R states of [NiFeSe ] hydrogenases show that (i) a hydrogen atom can bridge the two metal cente rs in the Ni-C and Ni-R states of [NiFeSe] hydrogenases and (ii) the struct ure and electronic features of [NiFeSe] models are in good agreement with a vailable experimental data, except for the Ni-B model for which the analysi s of the spin-density distribution suggests that a bridging oxygen species is still present. In addition, (iii) the similarity between the structural and electronic properties of the [NiFe] and [NiFeSe] models suggests that t he activity differences observed between the two kinds of hydrogenases cann ot be attributed to the S --> Se substitution alone, (iv) analysis of front ier orbitals strongly suggests that dihydrogen interacts with the Ni center in the initial step of its activation, and (v) the optimized models of the Ni-R state of [NiFe] and [NiFeSe] enzymes are characterized by an unusual Ni-H-S bridge and Lie about 25 kcal/mol lower in energy when compared to th e unbridged structure. (C) 1999 John Wiley & Sons, Inc. Int J Quant Chem 73 : 187-195, 1999.