INOR 530 |
| The catalytic cycles for H2 oxidation in [NiFe] and [FeFe] hydrogenases have been investigated through density functional theory (DFT) for a wide variety of redox and protonated structures of the active site models, (CO)(CN)2Fe(mu-SMe)2Ni(SMe)2 and L(CO)(CN)Fe((mu-SCH2)2X)(mu-CO)Fe(CO)(CN)(SMe), respectively. By combining a calibration curve for the calculated CO bond distances and frequencies and the measured IR stretching frequencies from related complexes with the DFT calculations on the [NiFe] model, the redox states and structures of the active site are predicted. Dihydrogen activation on the Fe(II)-Ni(III) species is more favorable than on the corresponding Ni(II) or Ni(I) species. Our final proposed structures are consistent with IR, EPR, and ENDOR measurements and the correlation coefficient between the measured CO frequency in the enzyme and the CO distance/frequency calculated for the model species is excellent. The unconstrained optimized geometries for high-spin Ni(II) species and for the Ni(III) species involved in the H2-cleavage reaction, especially the transition state, show remarkable structural resemblance to the active site in the enzyme crystal structure. For the [FeFe] modeling, full frequency calculations on models for the active site and for well-characterized complexes show that observed and catalytically active redox species in the enzyme must correspond to Fe(II)-Fe(II), Fe(II)-Fe(I), and Fe(I)-Fe(I). Furthermore, when X is NH rather than CH2, a single Fe and this N create a very favorable thermodynamic path for the heterolytic cleavage of H2. The H2-cleaved species shows an unusually short “dihydrogen bond”, Fe—H - - - - H—N. |
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Theoretical Inorganic Chemistry
8:30 AM-12:40 PM, Tuesday, 12 September 2006 Moscone Center -- Room 307, Oral
Division of Inorganic Chemistry |