Salmain, M.

Appl. Organomet. Chem. 2013, 27, 6-12, 10.1002/aoc.2929

Several ruthenium and rhodium complexes including 2,2′‐dipyridylamine ligands substituted at the central N atom by an alkyl chain terminated by a maleimide functional group were tested along with a newly synthesized Rh(III) complex of unsubstituted 2,2′‐dipyridylamine as catalysts in the transfer hydrogenation of aryl ketones in neat water with formate as hydrogen donor. All of them except one led to the secondary alcohol products with conversion rates depending on the metal complex. Site‐specific anchoring of the N‐maleimide complexes to the single free cysteine residue of the cysteine endoproteinase papain endowed this protein with transfer hydrogenase properties towards 2,2,2‐trifluoroacetophenone. Quantitative conversions were reached with the Rh‐based biocatalysts, while modest enantioselectivities were obtained in certain reactional conditions.

Fujieda, N.; Itoh, S.

Chem. - Asian J. 2012, 7, 1203-1207, 10.1002/asia.201101014

Teaching metalloenzymes new tricks: An artificial type III dicopper oxidase has been developed using a hydrolytic enzyme, metallo‐β‐lactamase, as a metal‐binding platform. The triple mutant D88G/S185H/P224G redesigned by computer‐assisted structural analysis showed spectroscopic features similar to those of type III copper proteins and exhibited a high catalytic activity in the oxidation of catechols under aerobic conditions.

Onoda, A.; Salmain, M.

Bioorganometallic Chemistry: Applications in Drug Discovery, Biocatalysis, and Imaging 2014, 305-338, 10.1002/9783527673438.ch10

Enzymes are the catalysts of the living world. Nature has tailored proteins to catalyze an incredibly wide range of reactions with exquisite selectivity and efficiency under very mild conditions of temperature, pH, pressure, and so on. Protein engineering combined with molecular modeling techniques affords tailor‐made biocatalysts for the industrial production of chiral synthons. Nonetheless, endowing a given protein scaffold with a totally new activity remains a challenging task for the biochemist. Among the current strategies to impart proteins with unnatural activity, those dealing with the construction of artificial metalloenzymes are particularly promising. By definition, artificial metalloenzymes are hybrid catalysts resulting from the incorporation of a transition metal species within a biomacromolecular scaffold. The rationale behind this concept is to combine the wide catalytic scope of transition metal complexes with the high activity and selectivity of biocatalysts. In most of the hybrid catalysts reported so far, the roles devoted to both partners are clearly separated: the metal complex being responsible for reactivity, while the protein environment is used to induce selectivity in the chemical process. In that, artificial metalloenzymes truly resemble enzymes whose efficiency relies on both the active site and the second sphere of coordination (also called the outer coordination sphere). In this chapter, we intend to give an overview of the various anchoring strategies reported over the last decade for the controlled, site‐selective attachment of nonnative metal cofactors within protein matrices together with the activity/selectivity displayed by these hybrid enzymes.

Salmain, M.

Dalton Trans. 2014, 43, 5482-5489, 10.1039/c3dt53253d

Protein hybrids resulting from the supramolecular anchoring to bovine β-lactoglobulin of fatty acid-derived Rh(iii) diimine complexes catalysed the asymmetric transfer hydrogenation of trifluoroacetophenone with up to 32% ee.

Bäckvall, J.E.; Diéguez, M.; Pàmies, O.

Adv. Synth. Catal. 2015, 357, 1567-1586, 10.1002/adsc.201500290

Artificial metalloenzymes combine the excellent selective recognition/binding properties of enzymes with transition metal catalysts, and therefore many asymmetric transformations can benefit from these entities. The search for new successful strategies in the construction of metal‐enzyme hybrid catalysts has therefore become a very active area of research. This review discusses all the developed strategies and the latest advances in the synthesis and application in asymmetric catalysis of artificial metalloenzymes with future directions for their design, synthesis and application (Sections 2–4). Finally, advice is presented (to the non‐specialist) on how to prepare and use artificial metalloenzymes (Section 5).

Fujieda, N.; Itoh, S.

J. Am. Chem. Soc. 2017, 139, 5149-5155, 10.1021/jacs.7b00675

Thermally stable TM1459 cupin superfamily protein from Thermotoga maritima was repurposed as an osmium (Os) peroxygenase by metal-substitution strategy employing the metal-binding promiscuity. This novel artificial metalloenzyme bears a datively bound Os ion supported by the 4-histidine motif. The well-defined Os center is responsible for not only the catalytic activity but also the thermodynamic stability of the protein folding, leading to the robust biocatalyst (Tm ≈ 120 °C). The spectroscopic analysis and atomic resolution X-ray crystal structures of Os-bound TM1459 revealed two types of donor sets to Os center with octahedral coordination geometry. One includes trans-dioxide, OH, and mer-three histidine imidazoles (O3N3 donor set), whereas another one has four histidine imidazoles plus OH and water molecule in a cis position (O2N4 donor set). The Os-bound TM1459 having the latter donor set (O2N4 donor set) was evaluated as a peroxygenase, which was able to catalyze cis-dihydroxylation of several alkenes efficiently. With the low catalyst loading (0.01% mol), up to 9100 turnover number was achieved for the dihydroxylation of 2-methoxy-6-vinyl-naphthalene (50 mM) using an equivalent of H2O2 as oxidant at 70 °C for 12 h. When octene isomers were dihydroxylated in a preparative scale for 5 h (2% mol cat.), the terminal alkene octene isomers was converted to the corresponding diols in a higher yield as compared with the internal alkenes. The result indicates that the protein scaffold can control the regioselectivity by the steric hindrance. This protein scaffold enhances the efficiency of the reaction by suppressing disproportionation of H2O2 on Os reaction center. Moreover, upon a simple site-directed mutagenesis, the catalytic activity was enhanced by about 3-fold, indicating that Os-TM1459 is evolvable nascent osmium peroxygenase.

Salmain, M.

Org. Biomol. Chem. 2011, 9, 5720, 10.1039/c1ob05482a

Organometallic complexes of the general formula [(η6-arene)Ru(N⁁N)Cl]+ and [(η5-Cp*)Rh(N⁁N)Cl]+ where N⁁N is a 2,2′-dipyridylamine (DPA) derivative carrying a thiol-targeted maleimide group, 2,2′-bispyridyl (bpy), 1,10-phenanthroline (phen) or ethylenediamine (en) and arene is benzene, 2-chloro-N-[2-(phenyl)ethyl]acetamide or p-cymene were identified as catalysts for the stereoselective reduction of the enzyme cofactors NAD(P)+ into NAD(P)H with formate as a hydride donor. A thorough comparison of their effectiveness towards NAD+ (expressed as TOF) revealed that the RhIII complexes were much more potent catalysts than the RuII complexes. Within the RuII complex series, both the N⁁N and arene ligands forming the coordination sphere had a noticeable influence on the activity of the complexes. Covalent anchoring of the maleimide-functionalized RuII and RhIII complexes to the cysteine endoproteinase papain yielded hybrid metalloproteins, some of them displaying formate dehydrogenase activity with potentially interesting kinetic parameters.

Fujieda, N.; Itoh, S.

Angew. Chem. 2020, 132, 7791-7794, 10.1002/ange.202000129

Cupin superfamily proteins (TM1459) work as a macromolecular ligand framework with a double-stranded β-barrel structure ligating to a Cu ion through histidine side chains. Variegating the first coordination sphere of TM1459 revealed that H52A and H54A/H58A mutants effectively catalyzed the diastereo- and enantioselective Michael addition reaction of nitroalkanes to an α,β-unsaturated ketone. Moreover, calculated substrate docking signified C106N and F104W single-point mutations, which inverted the diastereoselectivity of H52A and further improved the stereoselectivity of H54A/H58A, respectively.

Salmain, M.

Chem. Commun. 2012, 48, 11984, 10.1039/c2cc36980j

Artificial metalloproteins resulting from the embedding of half-sandwich Ru(II)/Rh(III) fatty acid derivatives within β-lactoglobulin catalysed the asymmetric transfer hydrogenation of trifluoroacetophenone with modest to good conversions and fair ee's.

Mangiatordi, G.F.; Salmain, M.

J. Mol. Catal. B: Enzym. 2015, 122, 314-322, 10.1016/j.molcatb.2015.10.007

Two half-sandwich d6-rhodium(III) complexes of the general formula [(η5-Cp*)Rh(N^N)Cl]Cl where N^N is a phenanthroline or a bispyridine methane derivative carrying a thiol-targeting maleimide or chloroacetamide function were synthesized and characterized. Both complexes were able to catalyse the transfer hydrogenation of 2,2,2-trifluoroacetophenone in aqueous medium using formate or phosphite as hydrogen donor. Covalent anchoring of these complexes to the cysteine endoproteinase papain yielded hybrid metalloproteins with transfer hydrogenase properties. Under optimized conditions of pH, hydrogen donor concentration and catalyst load, conversion of substrate was nearly quantitative within 24 h at 40 °C and the (S)-enantiomer was obtained preferably albeit with a modest enantiomeric excess of 7–10%. Covalent docking simulations complemented the experimental findings suggesting a molecular rationale for the observed low enantioselectivity. The harmonious use of experimental and theoretical approaches represents an unprecedented starting point for driving the rational design of artificial metalloenzymes built up from papain with higher catalytic efficiency.

Salmain, M.

Eur. J. Inorg. Chem. 2018, 2018, 1383-1393, 10.1002/ejic.201701359

Biohybrid catalysts resulting from the dative anchoring of half‐sandwich organometallic complexes [M(arene)(H2O)x(Cl)y]n+ (M = RuII, arene = η6‐benzene, p‐cymene or mesitylene; M = IrIII, RhIII, arene = η5‐Cp*; x = 1–3, y = 0–2, n = 0–2) to bovine beta‐lactoglobulin (βLG) or hen egg white lysozyme showed unprecedented behaviour. These constructs were shown to catalyse the asymmetric transfer hydrogenation of aryl ketones in water with sodium formate as hydrogen donor at a much faster rate than the complexes alone. Full conversion of the benchmark substrate 2,2,2‐trifluoroacetophenone was reached with an ee of 86 % for the most selective biohybrid. Surprisingly, even the crude biohybrid gave a good ee despite the presence of non‐protein‐bound metal species in the reaction medium. Other aryl ketones were reduced in the same way, and the highest ee was obtained for ortho‐substituted acetophenone derivatives. Furthermore, treatment of βLG with dimethyl pyrocarbonate resulted in a noticeable decrease of the activity and selectivity of the biohybrid, indicating that the sole accessible histidine residue (His146) was probably involved in the coordination and activation of Ru(benzene). This work underscores that protein scaffolds are efficient chiral ligands for asymmetric catalysis. The use of sodium formate instead of dihydrogen makes this approach safe, inexpensive and environmentally friendly.

Salmain, M.; Thorimbert, S.

Eur. J. Inorg. Chem. 2017, 2017, 3622-3634, 10.1002/ejic.201700365

Several prochiral NCN‐pincer complexes of palladium(II), with hemilabile ligands and a long aliphatic chain, were synthesized and characterized spectroscopically. In some of the complexes, the presence of two different substituents on the N donor atoms made them stereogenic, so that they were isolated as a mixture of diastereoisomers, which could be differentiated by 1H NMR spectroscopy. Binding of some of these complexes to bovine β‐lactoglobin by insertion within its inner cavity was theoretically investigated by molecular‐docking simulations and was experimentally confirmed by CD spectroscopy. Adjunction of H‐bond donor substituents on the ligand framework gave more‐stable supramolecular protein–complex assemblies. These constructs were shown to catalyze aldol condensation reactions in aqueous media, affording, in some cases, the less‐favorable cis product. Since the corresponding complexes exclusively gave the trans product in the absence of β‐lactoglobulin, this unusual diastereoselectivity was ensued by the second sphere of coordination brought by the protein host.

Salmain, M.

Dalton Trans. 2010, 39, 5605, 10.1039/c001630f

Covalent embedding of a (η6-arene) ruthenium(II) complex into the protein papain gives rise to a metalloenzyme displaying a catalytic efficiency for a Lewis acid-mediated catalysed Diels–Alder reaction enhanced by two orders of magnitude in water.