Abstract
The metabolism of many anaerobes relies on [NiFe]-hydrogenases, whose characterization when bound to substrates has proven non-trivial. Presented here is direct evidence for a hydride bridge in the active site of the 57Fe-labelled fully reduced Ni-R form of Desulfovibrio vulgaris Miyazaki F [NiFe]-hydrogenase. A unique ‘wagging' mode involving H− motion perpendicular to the Ni(μ-H)57Fe plane was studied using 57Fe-specific nuclear resonance vibrational spectroscopy and density functional theory (DFT) calculations. On Ni(μ-D)57Fe deuteride substitution, this wagging causes a characteristic perturbation of Fe–CO/CN bands. Spectra have been interpreted by comparison with Ni(μ-H/D)57Fe enzyme mimics [(dppe)Ni(μ-pdt)(μ-H/D)57Fe(CO)3]+ and DFT calculations, which collectively indicate a low-spin Ni(II)(μ-H)Fe(II) core for Ni-R, with H− binding Ni more tightly than Fe. The present methodology is also relevant to characterizing Fe–H moieties in other important natural and synthetic catalysts.
Highlights
The metabolism of many anaerobes relies on [NiFe]-hydrogenases, whose characterization when bound to substrates has proven non-trivial
The catalytic cycle is generally thought to involve three key redox states of the bimetallic centre: electron spin resonance (EPR)-silent Ni-SIa, EPRactive Ni-C and another electron spin (paramagnetic) resonance (EPR)-silent species known as Ni-R16–18
While much lower nuclear resonance vibrational spectroscopy (NRVS) intensity [10H/D] þ allowed for direct observation of nFe–H/D and nNi–H/D modes, the resolution of similar bands for [NiFe]-hydrogenase is beyond our current capabilities
Summary
The metabolism of many anaerobes relies on [NiFe]-hydrogenases, whose characterization when bound to substrates has proven non-trivial. Despite progress in characterizing hydrogenases by crystallography, spectroscopy, and theory[16], questions remain about the molecular and electronic structure of various intermediates and inhibited species. The structures proposed for these Ni-R subspecies (Fig. 1b–d) most commonly have a bridging hydride at the active site (Ni(m-H)Fe)[19,20,21,22], with some even featuring an additional (terminal) hydride at Ni (HNi(m-H)Fe)[23,24]. Another suggested form has an Fe-bound dihydrogen ligand (NiFe(Z2H2))[25,26]. Among proposals supporting the bridging hydride, there is further debate as to whether H À is bound more strongly to Fe or Ni (ref. 28) and whether Ni(II) is high[29] or low spin[30] or whether both spin configurations coexist in the bulk[31,32]
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