Lee, S.-Y.

Chem. - Asian J. 2018, 13, 334-341, 10.1002/asia.201701543

Carbonic anhydrase (CA) is a ubiquitous metalloenzyme with a Zn cofactor coordinated to trigonal histidine imidazole moieties in a tetrahedral geometry. Removal of the Zn cofactor in CA and subsequent binding of Ir afforded CA[Ir]. Under mild and neutral conditions (30 °C, pH 7), CA[Ir] exhibited water‐oxidizing activity with a turnover frequency (TOF) of 39.8 min−1, which is comparable to those of other Ir‐based molecular catalysts. Coordination of Ir to the apoprotein of CA is thermodynamically preferred and is associated with an exothermic energy change (ΔH) of −10.8 kcal mol−1, which implies that the CA apoprotein is stabilized by Ir binding. The catalytic oxygen‐evolving activity of CA[Ir] is displayed only if Ir is bound to CA, which functions as an effective biological scaffold that activates the Ir center for catalysis. The results of this study indicate that the histidine imidazoles at the CA active site could be exploited as beneficial biological ligands to provide unforeseen biochemical activity by coordination to a variety of transition‐metal ions.

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.

Fontecilla-Camps, J.C.

Eur. J. Inorg. Chem. 2013, 2013, 3596-3600, 10.1002/ejic.201300592

The crystal structure of bovine β‐lactoglobulin bound to a complex consisting of a (η5‐Cp*)Rh(2,2′‐dipyridylamine) head and a lauric acid derived hydrophobic tail has been solved at 1.85 Å resolution. Previous work has shown that this hybrid catalyzes the transfer hydrogenation of an aryl ketone in neat water with formate as hydrogen donor with enantiomeric excess (ee) of about 26 %. Calculations using the X‐ray model indicate that the complex head can adopt discrete conformations, which may explain the ee observed.