Molecular
cobalt-dmg (dmg = dimethylglyoxime) complexes are an
important class of electrocatalysts used heavily in mechanistic model
studies of the hydrogen evolution reaction (HER). Schrauzer’s
early isolation of a phosphine-stabilized “[H-CoIII(dmgH)2P(nBu)3]” complex
has long provided circumstantial support for the plausible intermediacy
of Co(III)-H species in HER by cobaloximes in solution. Our investigation
of this complex has led to a reassignment of its structure as [CoII(dmgH)2P(nBu)3], a
complex that contains no hydride ligand and dimerizes to form an unsupported
Co–Co bond in the solid state. A paramagnetic S = 3/2 impurity that forms during the synthesis of [CoII(dmgH)2P(nBu)3] when exposed
to adventitious oxygen has also been characterized. This impurity
features a 1H NMR resonance at −5.06 ppm that was
recently but erroneously attributed to the hydride resonance of “[H-CoIII(dmgH)2P(nBu)3]”.
We draw attention to this reassignment because of its relevance to
cobaloxime hydrides and HER catalysis and because Schrauzer’s
“hydridocobaloxime” is often cited as the primary example
of a bona fide hydride that can be isolated and characterized
on this widely studied HER platform.
The active site of Hyd-1, an oxygen-tolerant membrane-bound [NiFe]-hydrogenase from Escherichia coli, contains four highly conserved residues that form a "canopy" above the bimetallic center, closest to the site at which exogenous agents CO and O 2 interact, substrate H 2 binds, and a hydrido intermediate is stabilized. Genetic modification of the Hyd-1 canopy has allowed the first systematic and detailed kinetic and structural investigation of the influence of the immediate outer coordination shell on H 2 activation. The central canopy residue, arginine 509, suspends a guanidine/guanidinium side chain at close range above the open coordination site lying between the Ni and Fe atoms (N−metal distance of 4.4 Å): its replacement with lysine lowers the H 2 oxidation rate by nearly 2 orders of magnitude and markedly decreases the H 2 /D 2 kinetic isotope effect. Importantly, this collapse in rate constant can now be ascribed to a very unfavorable activation entropy (easily overriding the more favorable activation enthalpy of the R509K variant). The second most important canopy residue for H 2 oxidation is aspartate 118, which forms a salt bridge to the arginine 509 headgroup: its mutation to alanine greatly decreases the H 2 oxidation efficiency, observed as a 10-fold increase in the potential-dependent Michaelis constant. Mutations of aspartate 574 (also salt-bridged to R509) to asparagine and proline 508 to alanine have much smaller effects on kinetic properties. None of the mutations significantly increase sensitivity to CO, but neutralizing the expected negative charges from D118 and D574 decreases O 2 tolerance by stabilizing the oxidized resting Ni III −OH state ("Ni-B"). An extensive model of the catalytic importance of residues close to the active site now emerges, whereby a conserved gas channel culminates in the arginine headgroup suspended above the Ni and Fe.
Four novel dinuclear Ag(i) and Au(i) NHC complexes bearing two 2,2-acetate-bridged bisimidazolylidene ligands (R = Me and iPr) of zwitterionic and metallacyclic forms are reported.
Methanobactins
(Mbns) are ribosomally produced, post-translationally
modified peptidic natural products that bind copper with high affinity.
Methanotrophic bacteria use Mbns to acquire copper needed for enzymatic
methane oxidation. Despite the presence of Mbn operons in a range
of methanotroph and other bacterial genomes, few Mbns have been isolated
and structurally characterized. Here we report the isolation of a
novel Mbn from the methanotroph Methylosinus (Ms.) sp. LW3. Mass spectrometric and nuclear magnetic resonance
spectroscopic data indicate that this Mbn, the largest characterized
to date, consists of a 13-amino acid backbone modified to include
pyrazinedione/oxazolone rings and neighboring thioamide groups derived
from cysteine residues. The pyrazinedione ring is more stable to acid
hydrolysis than the oxazolone ring and likely protects the Mbn from
degradation. The structure corresponds exactly to that predicted on
the basis of the Ms. sp. LW3 Mbn operon content,
providing support for the proposed role of an uncharacterized biosynthetic
enzyme, MbnF, and expanding the diversity of known Mbns.
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