OH to serve as a pendant base for heterolytic splitting of H 2 , similarly to the
azadithiolate in the [FeFe]-H 2 ase. As with the diiron hydrogenase, a single cysteine
is the sole covalent binding unit of the iron center to the protein, although van der
Waals interactions to the overall cofactor are extensive. The current consensus
mechanism is given in Fig. 7 [65, 81–85].
Further details from x-ray crystallography addressed the active site/substrate
interaction, finding that methenyl-tetrahydromethanopterin binds in a p-fashion and
leaves the active site geometry essentially unchanged [65]. The closest approach of
the substrate to the iron is the hydride acceptor site, the methenyl carbon C-14, at
3.73 A. When bound in the active site, the aromatic heterocycle of the substrate is
no longer planar, having a twist of 30° from aromaticity, presumably via
hydrophobic interactions that activate the carbon for accepting the hydride.
4.3 Active Site Binding and Spectral Responses
Whereas protein crystallography satisfactorily solved most questions of the structure of the static [Fe]-H 2 ase enzyme, the plethora of characterizations, spectroscopies observing transients on a fast time scale, and computations gave the needed
fundamental support for the steps in H 2 -uptake and splitting process given in the
current state of understanding in Fig. 7 [65, 81–85].
The central metal complex gives tempting synthetic targets to synthetic chemists,
however their/our plan to replace the 4 billion years of evolution that gives ideal
electronic conditions for low barrier mechanistic pathways, usually meets with
failure. Nevertheless, the assumed mechanistic pathway, made plausible or fleshed
out by spectroscopic clues as to transient intermediates, can give hope that
replacement of Nature’s design of outer-sphere interactions with unusual synthetic
components might illicit the desired catalytic properties. A discussion of some
spectral properties follows.
Consistent with the snapshot from protein crystallography, the two CO molecules bound to the iron center display nearly identical intensities of the symmetric
and asymmetric infrared bands indicating that the ligands are adjacent to one
another, coordinated at nearly 90° [59]. The infrared bands are found to reversibly
shift to lower frequencies when the pH was lowered, suggesting that an acidic
group, with a pKa near 5, is in close proximity to the iron center. Furthermore,
extrinsic CO was found to bind reversibly to iron, in a mirror plane bisecting the
enzyme’s intrinsically bound CO groups—i.e., the open site on iron. Similar to both
the [NiFe]- and [FeFe]-hydrogenases, the intrinsically bound CO’s do not exchange
with extrinsic CO. Cyanide was found to reversibly bind to the active site and its
binding is competitive with the extrinsic CO.
Binding of oxidized substrate, methenyl-tetrahydromethanopterin, CH =
H 4 MPT
+
, or reduced substrate, methylene-tetrahydromethanopterin, CH 2 =
H 4 MPT, led to slightly higher infrared stretching frequencies for the intrinsic CO
Organometallic Chemistry Control of Hydrogenases
285
azadithiolate in the [FeFe]-H 2 ase. As with the diiron hydrogenase, a single cysteine
is the sole covalent binding unit of the iron center to the protein, although van der
Waals interactions to the overall cofactor are extensive. The current consensus
mechanism is given in Fig. 7 [65, 81–85].
Further details from x-ray crystallography addressed the active site/substrate
interaction, finding that methenyl-tetrahydromethanopterin binds in a p-fashion and
leaves the active site geometry essentially unchanged [65]. The closest approach of
the substrate to the iron is the hydride acceptor site, the methenyl carbon C-14, at
3.73 A. When bound in the active site, the aromatic heterocycle of the substrate is
no longer planar, having a twist of 30° from aromaticity, presumably via
hydrophobic interactions that activate the carbon for accepting the hydride.
4.3 Active Site Binding and Spectral Responses
Whereas protein crystallography satisfactorily solved most questions of the structure of the static [Fe]-H 2 ase enzyme, the plethora of characterizations, spectroscopies observing transients on a fast time scale, and computations gave the needed
fundamental support for the steps in H 2 -uptake and splitting process given in the
current state of understanding in Fig. 7 [65, 81–85].
The central metal complex gives tempting synthetic targets to synthetic chemists,
however their/our plan to replace the 4 billion years of evolution that gives ideal
electronic conditions for low barrier mechanistic pathways, usually meets with
failure. Nevertheless, the assumed mechanistic pathway, made plausible or fleshed
out by spectroscopic clues as to transient intermediates, can give hope that
replacement of Nature’s design of outer-sphere interactions with unusual synthetic
components might illicit the desired catalytic properties. A discussion of some
spectral properties follows.
Consistent with the snapshot from protein crystallography, the two CO molecules bound to the iron center display nearly identical intensities of the symmetric
and asymmetric infrared bands indicating that the ligands are adjacent to one
another, coordinated at nearly 90° [59]. The infrared bands are found to reversibly
shift to lower frequencies when the pH was lowered, suggesting that an acidic
group, with a pKa near 5, is in close proximity to the iron center. Furthermore,
extrinsic CO was found to bind reversibly to iron, in a mirror plane bisecting the
enzyme’s intrinsically bound CO groups—i.e., the open site on iron. Similar to both
the [NiFe]- and [FeFe]-hydrogenases, the intrinsically bound CO’s do not exchange
with extrinsic CO. Cyanide was found to reversibly bind to the active site and its
binding is competitive with the extrinsic CO.
Binding of oxidized substrate, methenyl-tetrahydromethanopterin, CH =
H 4 MPT
+
, or reduced substrate, methylene-tetrahydromethanopterin, CH 2 =
H 4 MPT, led to slightly higher infrared stretching frequencies for the intrinsic CO
Organometallic Chemistry Control of Hydrogenases
285