Metal-Organic Frameworks as Biomimetic Catalysts

MOFzyme: Intrinsic protease-like activity of Cu-MOF

The construction of efficient enzyme mimetics for the hydrolysis of peptide bonds in proteins is challenging due to the high stability of peptide bonds and the importance of proteases in biology and industry. Metal-organic frameworks (MOFs) consisting of infinite crystalline lattices with metal clusters and organic linkers may provide opportunities for protease mimic which has remained unknown. Herein, we report that Cu₂(C₉H₃O₆)_4/3 MOF (which is well known as HKUST-1 and denoted as Cu-MOF here), possesses an intrinsic enzyme mimicking activity similar to that found in natural trypsin to bovine serum albumin (BSA) and casein. The Michaelis constant (K_m) of Cu-MOF is about 26,000-fold smaller than that of free trypsin indicating a much higher affinity of BSA for Cu-MOF surface. Cu-MOF also exhibited significantly higher catalytic efficiency than homogeneous artificial metalloprotease Cu(II) complexes and could be reused for ten times without losing in its activity. Moreover, Cu-MOF was successfully used to simulate trypsinization in cell culture since it dissociated cells in culture even without EDTA.

Nano-gold as artificial enzymes: hidden talents

Creating artificial enzymes that mimic the complexity and function of natural systems has been a great challenge for the past two decades. In this Progress Report, the focus is on recently discovered "hidden talents" of gold nanomaterials in artificial enzymes, including mimicking of nuclease, esterase, silicatein, glucose oxidase, peroxidase, catalase, and superoxide dismutase. These unexpected enzyme-like activities can be ascribed to nano-gold itself or the functional groups present on surrounding monolayer. Along with introducing the mechanisms of the various enzyme-like activities, the design and development of gold-based biomimetic catalysts, the search for efficient modulators, and their potential applications in bionics, biosensing, and biomedical sciences are highlighted. Eventually, it is expected that the rapidly growing interest in gold-based nanozymes will certainly fuel the excitement and stimulate research in this highly active field.

Study of heterogeneous catalysis by iron-squarate based 3D metal organic framework for the transformation of tetrazines to oxadiazole derivatives

We present here a simple, milder, and environmentally benign heterogeneous catalytic method for the transformation of tetrazines to oxadiazole derivatives at room temperature (25 °C) using our earlier synthesized iron-squarate based 3D metal organic framework, [Fe3(OH)3(C4O4)(C4O4)0.5]n (FeSq-MOF).

Guest molecule-responsive functional calcium phosphonate frameworks for tuned proton conductivity

We report the synthesis, structural characterization, and functionality (framework interconversions together with proton conductivity) of an open-framework hybrid that combines Ca(2+) ions and the rigid polyfunctional ligand 5-(dihydroxyphosphoryl)isophthalic acid (PiPhtA). Ca2[(HO3PC6H3COOH)2]2[(HO3PC6H3(COO)2H)(H2O)2]·5H2O (Ca-PiPhtA-I) is obtained by slow crystallization at ambient conditions from acidic (pH ≈ 3) aqueous solutions. It possesses a high water content (both Ca coordinated and in the lattice), and importantly, it exhibits water-filled 1D channels. At 75 °C, Ca-PiPhtA-I is partially dehydrated and exhibits a crystalline diffraction pattern that can be indexed in a monoclinic cell with parameters close to the pristine phase. Rietveld refinement was carried out for the sample heated at 75 °C, Ca-PiPhtA-II, using synchrotron powder X-ray diffraction data, which revealed the molecular formula Ca2[(HO3PC6H3COOH)2]2[(HO3PC6H3(COO)2H)(H2O)2]. All connectivity modes of the "parent" Ca-PiPhtA-I framework are retained in Ca-PiPhtA-II. Upon Ca-PiPhtA-I exposure to ammonia vapors (28% aqueous NH3) a new derivative is obtained (Ca-PiPhtA-NH3) containing 7 NH3 and 16 H2O molecules according to elemental and thermal analyses. Ca-PiPhtA-NH3 exhibits a complex X-ray diffraction pattern with peaks at 15.3 and 13.0 Å that suggest partial breaking and transformation of the parent pillared structure. Although detailed structural identification of Ca-PiPhtA-NH3 was not possible, due in part to nonequilibrium adsorption conditions and the lack of crystallinity, FT-IR spectra and DTA-TG analysis indicate profound structural changes compared to the pristine Ca-PiPhtA-I. At 98% RH and T = 24 °C, proton conductivity, σ, for Ca-PiPhtA-I is 5.7 × 10(-4) S·cm(-1). It increases to 1.3 × 10(-3) S·cm(-1) upon activation by preheating the sample at 40 °C for 2 h followed by water equilibration at room temperature under controlled conditions. Ca-PiPhtA-NH3 exhibits the highest proton conductivity, 6.6 × 10(-3) S·cm(-1), measured at 98% RH and T = 24 °C. Activation energies (Ea) for proton transfer in the above-mentioned frameworks range between 0.23 and 0.4 eV, typical of a Grothuss mechanism of proton conduction. These results underline the importance of internal H-bonding networks that, in turn, determine conductivity properties of hybrid materials. It is highlighted that new proton transfer pathways may be created by means of cavity "derivatization" with selected guest molecules resulting in improved proton conductivity.

Spin crossover-graphene nanocomposites: facile syntheses, characterization, and magnetic properties

[Fe(Htrz)₂(trz)](BF₄)–graphene spin-crossover nanocomposites have been successfully synthesized and characterized. [Fe(Htrz)₂(trz)](BF₄) nanoparticles (ca. 50 nm) distributed uniformly onto the surface of the graphene. Graphene as a substrate produced an effect on the spin crossover properties of [Fe(Htrz)₂(trz)](BF₄) nanoparticles.

Applications of metal–organic frameworks in heterogeneous supramolecular catalysis

The contributions of MOFs to the field of heterogeneous supramolecular catalysis are comprehensively reviewed with regard to active sites, selectivity, as well as host–guest chemistry.

Direct arylation of heterocycles through C–H bond cleavage using metal–organic-framework Cu2(OBA)2(BPY) as an efficient heterogeneous catalyst

Cu₂(OBA)₂(BPY) was showed to be an efficient heterogeneous catalyst for direct C-arylation of a variety of heterocycles by iodoarenes. The optimal conditions employed tBuOLi in dioxane at elevated temperature.

Rhodium-catalyzed ortho-cyanation of symmetrical azobenzenes with N-cyano-N-phenyl-p-toluenesulfonamide

A rhodium(iii)-catalyzed ortho-cyanation of symmetrical azobenzenes with NCTS via azo-group-directed C(sp²)–H bond activation is described.

Protein Immobilization in Metal–Organic Frameworks by Covalent Binding

Metal–organic frameworks (MOFs), possessing a well defined system of pores, demonstrate extensive potential serving as a platform in biological catalysis. Successful immobilization of enzymes in a MOF system retains the enzymatic activity, renders the active site more accessible to the substrate, and promises recyclability for reuse, and solvent adaptability in a broad range of working conditions. This highlight describes enzyme immobilization on MOFs via covalent binding and its significance.