Search

The biosynthesis of the active site of the [FeFe]-hydrogenases (HydA1), the H-cluster, is of interest because these enzymes are highly efficient catalysts for the oxidation and production of H2. The biosynthesis of the [2Fe]H subcluster of the H-cluster proceeds from simple precursors, which are processed by three maturases: HydG, HydE, and HydF. Previous studies established that HydG produces an Fe(CO)2(CN) adduct of cysteine, which is the substrate for HydE. In this work, we show that by using the synthetic cluster [Fe2(μ-SH)2(CN)2(CO)4]2- active HydA1 can be biosynthesized without maturases HydG and HydE.

The H-cluster of [Fe-Fe] hydrogenase consists of a [4Fe]H subcluster linked by the sulfur of a cysteine residue to an organometallic [2Fe]H subcluster that utilizes terminal CO and CN ligands to each Fe along with a bridging CO and a bridging SCH2NHCH2S azadithiolate (adt) to catalyze proton reduction or hydrogen oxidation. Three Fe-S "maturase" proteins, HydE, HydF, and HydG, are responsible for the biosynthesis of the [2Fe]H subcluster and its incorporation into the hydrogenase enzyme to form this catalytically active H-cluster. We have proposed that HydG is a bifunctional enzyme that uses S-adenosylmethione (SAM) bound to a [4Fe-4S] cluster to lyse tyrosine via a transient 5'-deoxyadenosyl radical to produce CO and CN ligands to a unique cysteine-chelated Fe(II) that is linked to a second [4Fe-4S] cluster via the cysteine sulfur. In this "synthon model", after two cycles of tyrosine lysis, the product of HydG is completed: a [Fe(CN)(CO)2(cysteinate)]- organometallic unit that is vectored directly into the synthesis of the [2Fe]H sub-cluster. However our HydG-centric synthon model is not universally accepted, so further validation is important. In this Frontiers article, we discuss recent results using a synthetic "Syn-B" complex that donates [Fe(CN)(CO)2(cysteinate)]- units that match our proposed HydG product. Can Syn-B activate hydrogenase in the absence of HydG and its tyrosine substrate? If so, since Syn-B can be synthesized with specific magnetic nuclear isotopes and with chemical substitutions, its use could allow its enzymatic conversions on the route to the H-cluster to be monitored and modeled in fresh detail.

[FeFe]-hydrogenases use a unique organometallic complex, termed the H cluster, to reversibly convert H2 into protons and low-potential electrons. It can be best described as a [Fe4S4] cluster coupled to a unique [2Fe]H center where the reaction actually takes place. The latter corresponds to two iron atoms, each of which is bound by one CN- ligand and one CO ligand. The two iron atoms are connected by a unique azadithiolate molecule (-S-CH2-NH-CH2-S-) and an additional bridging CO. This [2Fe]H center is built stepwise thanks to the well-orchestrated action of maturating enzymes that belong to the Hyd machinery. Among them, HydG converts l-tyrosine into CO and CN- to produce a unique l-cysteine-Fe(CO)2CN species termed complex-B. Very recently, HydE was shown to perform radical-based chemistry using synthetic complex-B as a substrate. Here we report the high-resolution crystal structure that establishes the identity of the complex-B-bound HydE. By triggering the reaction prior to crystallization, we trapped a new five-coordinate Fe species, supporting the proposal that HydE performs complex modifications of complex-B to produce a monomeric "SFe(CO)2CN" precursor to the [2Fe]H center. Substrate access, product release, and intermediate transfer are also discussed.

Numerous redox transformations that are essential to life are catalyzed by metalloenzymes that feature Earth-abundant metals. In contrast, platinum-group metals have been the cornerstone of many industrial catalytic reactions for decades, providing high activity, thermal stability, and tolerance to chemical poisons. We assert that nature's blueprint provides the fundamental principles for vastly expanding the use of abundant metals in catalysis. We highlight the key physical properties of abundant metals that distinguish them from precious metals, and we look to nature to understand how the inherent attributes of abundant metals can be embraced to produce highly efficient catalysts for reactions crucial to the sustainable production and transformation of fuels and chemicals.

The H-cluster of [FeFe]-hydrogenase consists of a [4Fe-4S]H-subcluster linked by a cysteinyl bridge to a unique organometallic [2Fe]H-subcluster assigned as the site of interconversion between protons and molecular hydrogen. This [2Fe]H-subcluster is assembled by a set of Fe-S maturase enzymes HydG, HydE and HydF. Here we show that the HydG product [FeII(Cys)(CO)2(CN)] synthon is the substrate of the radical SAM enzyme HydE, with the generated 5'-deoxyadenosyl radical attacking the cysteine S to form a C5'-S bond concomitant with reduction of the central low-spin Fe(II) to the Fe(I) oxidation state. This leads to the cleavage of the cysteine C3-S bond, producing a mononuclear [FeI(CO)2(CN)S] species that serves as the precursor to the dinuclear Fe(I)Fe(I) center of the [2Fe]H-subcluster. This work unveils the role played by HydE in the enzymatic assembly of the H-cluster and expands the scope of radical SAM enzyme chemistry.

The enzyme [FeFe]-hydrogenase (HydA1) contains a unique 6-iron cofactor, the H-cluster, that has unusual ligands to an Fe-Fe binuclear subcluster: CN-, CO, and an azadithiolate (adt) ligand that provides 2 S bridges between the 2 Fe atoms. In cells, the H-cluster is assembled by a collection of 3 maturases: HydE and HydF, whose roles aren't fully understood, and HydG, which has been shown to construct a [Fe(Cys)(CO)2(CN)] organometallic precursor to the binuclear cluster. Here, we report the in vitro assembly of the H-cluster in the absence of HydG, which is functionally replaced by adding a synthetic [Fe(Cys)(CO)2(CN)] carrier in the maturation reaction. The synthetic carrier and the HydG-generated analog exhibit similar infrared spectra. The carrier allows HydG-free maturation to HydA1, whose activity matches that of the native enzyme. Maturation with 13CN-containing carrier affords 13CN-labeled enzyme as verified by electron paramagnetic resonance (EPR)/electron nuclear double-resonance spectra. This synthetic surrogate approach complements existing biochemical strategies and greatly facilitates the understanding of pathways involved in the assembly of the H-cluster. As an immediate demonstration, we clarify that Cys is not the source of the carbon and nitrogen atoms in the adt ligand using pulse EPR to target the magnetic couplings introduced via a 13C3,15N-Cys-labeled synthetic carrier. Parallel mass-spectrometry experiments show that the Cys backbone is converted to pyruvate, consistent with a cysteine role in donating S in forming the adt bridge. This mechanistic scenario is confirmed via maturation with a seleno-Cys carrier to form HydA1-Se, where the incorporation of Se was characterized by extended X-ray absorption fine structure spectroscopy.

A cluster‐ligand is disclosed in the form of [Ru3(CN)3(CO)9]3− ([1]3−). Produced by simple reaction of [Ru3(CO)12] with cyanide, [1]3− serves as a precursor to a series of μ‐CN cages. When treated with [Ru3(CO)12], it readily forms the prism [Ru6(μ‐CN)3(CO)18]3−. With 1.5 equiv. of [Cu(MeCN)4]+ [1]3− reacts to give the expanded prism {Cu3[Ru3(μ‐CN)3(CO)9]2}3−, which features three two‐coordinate Cu(I) centers. Sources of Ni2+ and Fe2+ bind two equivalents of [1]3−, giving the double cages {M[Ru3(μ‐CN)3(CO)9]2}4− (M=Ni, Fe) wherein the central metal is octahedral. The 1 : 1 reaction using [Fe(H2O)6]2+ gave the interpenetrated super‐tetrahedrane {Fe4(μ4‐O)[Ru3(μ‐CN)3(CO)9]4}4−. [ABSTRACT FROM AUTHOR]

The degree of substitution that entails the use of chelating ligands is studied which stabilizes the complexes of the type [Fe-Fe]-hydrogenases. It is suggested that the dithiolato complexes with unsymmetrical coordination environments has led to a better understanding of the influence of the dithiolate on the stereochemistry of these complexes.

The acid-base reactivity of MS2(dmpe)2, where M = Mo and W and dmpe = Me2PCH2CH2PMe2 is examined. Consistent with its ready protonation, complex M=Mo is methylated, and the use of excess MeOTf gives [MoS(OTf)(dmpe)2](super +) and Me2S in a rare example of double alkylation at a sulfido ligand.

The first systematic study on the use of tricyanoborates as ligands and also the use of the organoboran tricyanides as precursors to coordination polymers are described. The treatment of [PhB(CN)(sub 3)](super -) with [Cu(PCy3)(sub 2)(NCMe)(sub x)]PF6 has given the one-dimensional polymer [PhB(CN)(sub 3)Cu(PCy3)(Cub 2)], wherein two pf the three BCN substituents are coordinated.

The structural chemistry of cation-containing defect cyanometalate boxes is elucidated. A promising motif for cation receptors is represented by the defect box as these cations are more solvent-accessible and kinetically labile.

Research has been conducted on mixed ligand species [Fe (sub)2 -(S (sub)2 C (sub)3 H (sub)6)(CN)(CO) (sub)4 (PMe (sub)3)] (super)- which catalyzes proton electroreduction to dihydrogen. The description of this compound and its derivatives and analogues as they relate to the proton reduction process, analyzed theoretically and electrochemically, is presented.

Cubic cage compounds composed of Co-CN-Ru linkages have been prepared which illustrate the following features: (i) new motifs for alkali metal ion complexation (i.e., cationic receptors for cations), (ii) a new family of triaza-metalloligands, and (iii) a double box-like cage. The cages are synthesized by the condensation of [[CpCo[(CN).sub.3]].sup.-] and [[Cp*Ru[(NCMe).sub.3]].sup.+] (cyclopentadienyl, Cp; pentamethylcyclopentadienyl, Cp*) the presence of [Cs.sup.+]. The species [{Cs[subset]([[CpCo[(CN).sub.3]].sub.4][[Cp*Ru].sub.4]).sup.+] and {Cs[subset][{[[CpCo[(CN).sub.3]].sub.4][[Cp*Ru].sub.3][Cp*Rh]).sup.2+] illustrate the box-completion reaction Cs [subset] [Co.sub.4][Ru.sub.3] + M (M = Cp*[Rh.sup.2+], Cp*[Ru.sup.+]). With the naked ion precursors [[Na[(NCMe).sub.6]].sup.+] and [[Fe[(NCMe).sub.6]].sup.2+], the box-completion reactions afforded [{Na[{Cs[subset][[CpCo[(CN).sub.3]].sub.4][[Cp*Ru].sub.3]]}.sub.2]}.sup.+] and [{Fe[{Cs[subset] [[CpCo[(CN).sub.3]].sub.4][[Cp*Ru].sub.3]]}.sub.2]}.sup.2+]. These cages provide the first examples, to our knowledge, of the double-box motif.

Research has been conducted on the carbon monoxide, cyanide and ferrous derivatives. Results indicate that the reaction of these derivatives has led to the isolation of the new products characterized via the use of single-crystal X-ray diffraction.

A range of derivatives of the type [Fe2(SR)(Sub 2)(CN)(Sub x)(CO)(Sub 4)] (Super 2-) can be prepared by CN-for -CO substitution reactions. These syntheses occur for alkyl- and arylthiolato derivatives and for alkyldithiolates. The products are stable and have been fully characterized by X-ray diffraction for three derivatives.

A DFT study into the N-H...S hydrogen bond in (TACN)2Fe2S6 and in model systems, was undertaken.

Studies have shown that H(sub 2)S and organic thiols strongly influence the binding of unsaturated substrates to tetrathiorhenate centers. It is stated that in the presence of H(sub 2)S or thiols, one can readily observe the adducts ReS(S(sub 2)C(sub 2)H(sub 3)R)(SH)(SR')(super -) and shown that the addition of thiols to [ReS(sub 2)(S(sub 2)C(sub 2)Rn)](super -) represents an unsual route to SH-containing dithiolene complexes.