Theoretical characterization, of the reaction intermediates in a model of the nickel-iron hydrogenase of Desulfovibrio gigas

The catalytic cycle for H-2 oxidation in [NiFe] D. gigas hydrogenase has be en investigated through density functional theory (DFT) calculations on a w ide variety of redox and protonated structures of the active site model, (C O)(CN)(2)Fe(mu-SMe)(2)Ni(SMe)(2). DFT calculations on a series of known LFe (CO)(CN)(L')(n-) (L = Cp or Cp*, L' = CN, CO, CNCH3; n = 0, 1, 2) complexes are used to calibrate the calculated CO bond distances with the measured I R stretching frequency. By combining this calibration curve with the energy and CO bond distance of the DFT calculations on the active site model and the experimental IR frequencies on the enzyme, the redox states and structu res of active site species have been determined: Ni-B is a Ni(III)-Fe(II) s pecies, Ni-SI(a) is a Ni(LI)-Fe(II) species, Ni-SI(b) has a protonated term inal sulfur (Ni bound), Ni-R is a Ni(II)-Fe(II) dihydrogen complex with H-2 bound at Fe, and Ni-C is a Ni(III)-Fe(II) species with an Fe-H-Ni bridge. The latter species returns to W-SI through a Ni(I)-Fe(II) intermediate, whi ch is potentially observable. Protonation of the Ni bound terminal sulfur r esults in a folding of the Fe(mu-S)(2)Ni framework. Dihydrogen activation i s more exothermic on the Ni(III) species than on the corresponding Ni(II) o r Ni(I) species. Our final set of proposed structures are consistent with I R, EPRI ENDOR,and XAS measurements for these species, and the correlation c oefficient between the measured CO frequency in the enzyme and the CO dista nce calculated for the model species is 0.905.