[FeFe]-Hydrogenases: maturation and reactivity of enzymatic systems and overview of biomimetic models

[FeFe]-hydrogenases recieved increasing interest in the last decades. This review summarises important findings regarding their enzymatic reactivity as well as inorganic models applied as electro- and photochemical catalysts.

Cysteine residue-bridged dinuclear Ni–Fe complexes related to [NiFe]-H2ases

Cysteine residue-containing [NiFe]-H2ase models 1–6 have been prepared for the first time and some of them were found to be catalysts for H2 production from HOAc under CV conditions.

(Spectro)electrochemical and Electrocatalytic Investigation of 1,1'-Dithiolatoferrocene-Hexacarbonyldiiron

Hexacarbonyldiiron bridged by a 1,1'-dithiolatoferrocene, [Fe(C₅H₄S)₂{Fe(CO)₃}₂] (1), was synthesized, and the electrochemistry showed reversible oxidation at the Fe(C₅H₄S)₂ site and quasi-reversible reduction at the hexacarbonyldiiron site. Spectroelectrochemical techniques showed reduction-induced ligand isomerization, where the thiolate ligand went from bridging to terminal and one carbon monoxide ligand moved to a quasi-bridging position; this mechanism was further supported by cyclic voltammetry simulation and density functional theory calculations. Complex 1 showed electrocatalytic activity toward hydrogen-evolving reaction.

Metal coordination of ferrocene–histidine conjugates

Synthesis and complete structural characterization of ferrocene–histidine dipeptides including detailed analysis of the ligand–metal complexation.

Biomimetic peptide-based models of [FeFe]-hydrogenases: utilization of phosphine-containing peptides

Two synthetic strategies for incorporating diiron analogues of [FeFe]-hydrogenases into short peptides via phosphine functional groups are described. First, utilizing the amine side chain of lysine as an anchor, phosphine carboxylic acids can be coupled via amide formation to resin-bound peptides. Second, artificial, phosphine-containing amino acids can be directly incorporated into peptides via solution phase peptide synthesis. The second approach is demonstrated using three amino acids each with a different phosphine substituent (diphenyl, diisopropyl, and diethyl phosphine). In total, five distinct monophosphine-substituted, diiron model complexes were prepared by reaction of the phosphine-peptides with diiron hexacarbonyl precursors, either (μ-pdt)Fe2(CO)6 or (μ-bdt)Fe2(CO)6 (pdt = propane-1,3-dithiolate, bdt = benzene-1,2-dithiolate). Formation of the complexes was confirmed by UV/Vis, FTIR and (31)P NMR spectroscopy. Electrocatalysis by these complexes is reported in the presence of acetic acid in mixed aqueous-organic solutions. Addition of water results in enhancement of the catalytic rates.

Proteins as templates for complex synthetic metalloclusters: towards biologically programmed heterogeneous catalysis

Despite nature's prevalent use of metals as prosthetics to adapt or enhance the behaviour of proteins, our ability to programme such architectural organization remains underdeveloped. Multi-metal clusters buried in proteins underpin the most remarkable chemical transformations in nature, but we are not yet in a position to fully mimic or exploit such systems. With the advent of copious, relevant structural information, judicious mechanistic studies and the use of accessible computational methods in protein design coupled with new synthetic methods for building biomacromolecules, we can envisage a 'new dawn' that will allow us to build de novo metalloenzymes that move beyond mono-metal centres. In particular, we highlight the need for systems that approach the multi-centred clusters that have evolved to couple electron shuttling with catalysis. Such hybrids may be viewed as exciting mid-points between homogeneous and heterogeneous catalysts which also exploit the primary benefits of biocatalysis.

Synthesis of a Novel Cysteine-Incorporated Anthraquinone Derivative and Its Structural Properties

A novel cysteine-incorporated anthraquinone derivative was synthesized, and its molecular structure was determined by X-ray crystal analysis. Each mercapto group was located separately and did not form a disulfide bond, and hydrogen bondings and π-π interaction were observed from the packing structure.

[FeFe]-hydrogenase models assembled into vesicular structures

Compartmentalization is a major prerequisite for the origin of life on earth according to Wächtershäuser "Iron-Sulfur-World". The hypothesis is mainly based on an autocatalytic inorganic energy reproducing redox system consisting of iron and sulfur as requirement for the subsequent synthesis of complex organic structures. Here, we modified [FeFe]-hydrogenase models by means of covalent coupling to either oleic acid or the amphiphilic block copolymer polybutadiene-polyethyleneoxide (PB-PEO) and incorporated those into the membranes of vesicles composed of phospholipids (liposomes) or the unmodified amphiphilic polymer (polymersomes). We employed a [2Fe-2S] cluster as a hydrogenase model, since these structures are known to be suitable catalysts for the generation of H2 in the presence of weak acids. Successful incorporation was confirmed by spectrophotometric iron quantification and the vesicles formed were characterized by size determination (photon correlation spectroscopy (PCS)), and zeta potential as well as by cryo-transmission electron microscopy (Cryo-TEM). The modified models could be incorporated into liposomes or polymersomes up to molar proportions of 3.15% and 28%, respectively. Due to the immobilization in vesicular bilayers the [FeFe]-hydrogenase models can even exhibit catalytic action under the particular conditions of the intravesicular microenvironment. Our results suggest that the vesicular systems described may be applied as a nanoreactor for the reduction of encapsulated substances by generating hydrogen and thus as a minimal cell model.

Towards [NiFe]-hydrogenase biomimetic models that couple H2 binding with functionally relevant intramolecular electron transfers: a quantum chemical study

[FeFe]- and [NiFe]-hydrogenases are dihydrogen-evolving metalloenzymes that share striking structural and functional similarities, despite being phylogenetically unrelated. Most notably, they are able to combine substrate binding and redox functionalities, which has important bearings on their efficiency. Model complexes of [FeFe]-hydrogenases that are able to couple H2 binding with a substrate-dependent intramolecular electron transfer promoting dihydrogen activation were recently shown to reproduce the complex redox chemistry of the all-iron enzyme. Notably, coupling of H2 binding and intramolecular redox events was proposed to have a key role also in [NiFe]-hydrogenases, but this feature is not reproduced in currently available nickel-iron biomimetic compounds. In the present study, we exploit dedicated density functional theory approaches to show that H2 binding and activation on a NiFe core can be favored by the installment of conveniently substituted isocyanoferrocenes, thanks to their ability to undergo intramolecular reduction upon substrate binding. Our results support the concept that a unified view on hydrogenase chemistry is a key element to direct future efforts in the modeling of microbial H2 metabolism.

H2 binding and splitting on a new-generation [FeFe]-hydrogenase model featuring a redox-active decamethylferrocenyl phosphine ligand: a theoretical investigation

[FeFe]-hydrogenases are dihydrogen-evolving metalloenzymes that are able to combine substrate binding and redox functionalities, a feature that has important bearing on their efficiency. New-generation bioinspired systems such as Fe(2)[(SCH(2))(2)NBn](CO)(3)(Cp*Fe(C(5)Me(4)CH(2)PEt(2)))(dppv) were shown to mimic H(2) oxidation and splitting processes performed by the [FeFe]-hydrogenase/ferredoxin system, and key mechanistic aspects of such reaction are theoretically investigated in the present contribution. We found that H(2) binding and heterolytic cleavage take place concomitantly on DFT models of the synthetic catalyst, due to a substrate-dependent intramolecular redox process that promotes dihydrogen activation. Therefore, formation of an iron-dihydrogen complex as a reaction intermediate is excluded in the biomimetic system, at variance with the case of the enzyme. H(2) uptake at the synthetic system also requires an energetically disfavored isomerization of the amine group acting as a base during splitting. A possible strategy to stabilize the conformation competent for H(2) binding is proposed, along with an analysis of the reactivity of a triiron complex in which di(thiomethyl)amine--the chelating group naturally occurring in [FeFe]-hydrogenases--substitutes the benzyl-containing dithiolate ligand.

Construction of heterometallic clusters in a small peptide scaffold as [NiFe]-hydrogenase models: development of a synthetic methodology

[NiFe]-hydrogenases are enzymes that catalyze the reversible interconversion of protons and hydrogen at a heterobimetallic site containing Ni and Fe. This organometallic site has served as an inspiration for the synthesis of a number of biomimetic complexes, but, unfortunately, most close structural mimics have shown little to no reactivity with either of the substrates for hydrogenases. This suggests that interactions between the metallo-active site and the protein scaffold are crucial in tuning reactivity. As a first step toward development of peptide-based models, in this paper we demonstrate a synthetic strategy for construction of peptide coordinated, cysteinyl thiolate bridged Ni-M complexes in which M is a hetero-organometallic fragment. We utilize the seven amino acid peptide ACDLPCG as a scaffold for construction of these peptide-coordinated metallocenters. This peptide binds Ni in an N(2)S(2) environment consisting of the amino terminus, an amide nitrogen, and the two cysteinyl thiolates. We show that these thiolates serve as reactive sites for formation of heterometallic complexes in which they serve as bridging ligands. The method is general, and a number of heterometallic fragments including Ru(η(6)-arene)(2+), M(CO)(4)(piperidine) for M = Mo and W, and Fe(2)(CO)(6) were successfully incorporated, and the resulting metallopeptides characterized via a range of spectroscopic techniques. This methodology serves as the first step to construction of hydrogenase peptidomimetics that incorporate defined outer coordination sphere interactions intended to tune reactivity.

A theoretical study on the enhancement of functionally relevant electron transfers in biomimetic models of [FeFe]-hydrogenases

Recent advances aimed at modeling the chemistry of the active site of [FeFe]-hydrogenases (the H-cluster, composed by a catalytic Fe(2)S(2) subcluster and an Fe(4)S(4) portion) have led to the synthesis of binuclear coordination compounds containing a noninnocent organophosphine ligand [2,3-bis(diphenylphosphino)maleic anhydride, bma] that is able to undergo monoelectron reduction, analogously to the tetranuclear Fe(4)S(4) subcluster portion of the H-cluster. However, such a synthetic model was shown to feature negligible electronic communication between the noninnocent ligand and the remaining portion of the cluster, at variance with the enzyme active site. Here, we report a theoretical investigation that shows why the electron transfer observed in the enzyme upon protonation of the catalytic Fe(2)S(2) subsite cannot take place in the bma-containing cluster. In addition, we show that targeted modifications of the bma ligand are sufficient to restore the electronic communication within the model, such that electron density can be more easily withdrawn from the noninnocent ligand, as a result of protonation of the iron centers. Similar results were also obtained with a ligand derived from cobaltocene. The relevance of our findings is discussed from the perspective of biomimetic reproduction of proton reduction to yield molecular hydrogen.

Favorable Protonation of the (μ-edt)[Fe(2)(PMe(3))(4)(CO)(2)(H-terminal)](+) Hydrogenase Model Complex Over Its Bridging μ-H Counterpart: A Spectroscopic and DFT Study

The mechanism of hydrogen production in [FeFe] hydrogenase remains elusive. However, a species featuring a terminal hydride bound to the distal Fe is thought to be the key intermediate leading to hydrogen production. In this study, density functional theory (DFT) calculations on the terminal (H-term) and bridging (μ-H) hydride isomers of (μ-edt)-[Fe(2)(PMe(3))(4)(CO)(2)H](+) are presented in order to understand the factors affecting their propensity for protonation. Relative to H-term, μ-H is 12.7 kcal/mol more stable, which contributes to its decreased reactivity towards an acid. Potential energy surface (PES) calculations for the reaction of the H-term isomer with 4-nitropyridinium, a proton source, further reveal a lower activation energy barrier (14.5 kcal/mol) for H-term than for μ-H (29 kcal/mol). Besides these energetic considerations, the H-term isomer displays a key molecular orbital (MO <139>) that has a relatively strong hydride (1s) contribution (23%), which is not present in the μ-H isomer. This indicates a potential orbital control of the reaction of the hydride complexes with acid. The lower activation energy barrier and this key MO together control the overall catalytic activity of (μ-edt)[Fe(2)(PMe(3))(4)(CO)(2)(H-term)](+). Lastly, Raman and IR spectroscopy were performed in order to probe the ν(Fe-H) stretching mode of the two isomers and their deuterated counterparts. A ν(Fe-H) stretching mode was observed for the μ-H complex at 1220 cm(-1). However, the corresponding mode is not observed for the less stable H-term isomer.