Coordination Sphere Flexibility of Active-Site Models for Fe-only Hydrogenase: Studies in Intra- And Intermolecular Diatomic Ligand Exchange

J Am Chem Soc. 2001 Apr 11;123(14):3268-78. doi: 10.1021/ja003147z.

Abstract

A series of dinuclear complexes, (mu-SRS)Fe(2)(CO)(6) (R = -CH(2)CH(2)-, -CH(2)CH(2)CH(2)-, -CH(2)-C(6)H(4)-CH(2)-; edt, pdt, and o-xyldt, respectively) has been examined for specific characteristics that might relate to structural similarity with the active site of Fe-only hydrogenases. Variable-temperature proton NMR studies display the fluxionality of the iron-dithiocyclohexane unit in (mu-pdt)Fe(2)(CO)(6) while in the (mu-o-xyldt)Fe(2)(CO)(6) compound, the bridge is fixed. Temperature-dependent (13)C NMR spectral studies establish intramolecular CO site exchange localized on discrete Fe(CO)(3) units in all complexes, which is influenced by steric effects of the mu-SRS unit. Kinetic studies of intermolecular CO/CN(-) ligand-exchange reactions establish associative or I(a) mechanisms in sequential steps to form the dicyano dianion, (mu-SRS)[Fe(CO)(2)(CN)](2)(=) with 100% selectivity. Theoretical calculations (DFT) of transition states in the intramolecular site-exchange processes lead to a rationale for the interesting cooperativity in the CN(-)/CO intermolecular ligand-exchange process. The hinge motion of the three light atom S-to-S bridge is related to a possible heterolytic H(2) activation/production process in the enzyme.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Binding Sites
  • Carbon Isotopes
  • Crystallography, X-Ray
  • Hydrogenase / chemistry*
  • Hydrogenase / metabolism
  • Iron-Sulfur Proteins / chemistry*
  • Iron-Sulfur Proteins / metabolism
  • Kinetics
  • Ligands
  • Models, Chemical*
  • Nuclear Magnetic Resonance, Biomolecular / methods
  • Protein Conformation
  • Spectrophotometry, Infrared
  • Spectrophotometry, Ultraviolet
  • Temperature
  • Thermodynamics

Substances

  • Carbon Isotopes
  • Iron-Sulfur Proteins
  • Ligands
  • iron hydrogenase
  • Hydrogenase