Biomimetic 1-Aminocyclopropane-1-Carboxylic Acid Oxidase Ethylene Production by MIL-100(Fe)-Based Materials

A novel core@shell hybrid material based on biocompatible hydroxyapatite nanoparticles (HA) and the well-known MIL-100(Fe) (Fe₃O(H₂O)₂F(BTC)₂·nH₂O, BTC: 1,3,5-benzenetricarboxylate) has been prepared following a layer-by-layer strategy. The core@shell nature of the studied system has been confirmed by infrared, X-ray powder diffraction, N₂ adsorption, transmission electron microscopy imaging, and EDS analyses revealing the homogeneous deposition of MIL-100(Fe) on HA, leading to HA@MIL-100(Fe) rod-shaped nanoparticles with a 7 nm shell thickness. Moreover, both MIL-100(Fe) and HA@MIL-100(Fe) have demonstrated to act as efficient heterogeneous catalysts toward the biomimetic oxidation of 1-aminocyclopropane-1-carboxylic acid into ethylene gas, a stimulator that regulates fruit ripening. Indeed, the hybrid material maintains the catalytic properties of pristine MIL-100(Fe) reaching 40% of conversion after only 20 min. Finally, the chemical stability of the catalyst in water has also been monitored for 21 days by inductively coupled plasma-mass spectrometry confirming that only ca. 3% of Ca is leached.

A structural and functional model for the 1-aminocyclopropane-1-carboxylic acid oxidase

The hitherto most realistic low-molecular-weight analogue for the 1-aminocyclopropane-1-carboxylic acid oxidase (ACCO) is reported. The ACCOs 2-His-1-carboxylate iron(II) active site was mimicked by a TpFe moiety, to which the natural substrate ACC could be bound. The resulting complex [Tp(Me,Ph) FeACC] (1), according to X-ray diffraction analysis performed for the nickel analogue, represents an excellent structural model, featuring ACC coordinated in a bidentate fashion-as proposed for the enzymatic substrate complex-as well as a vacant coordination site that forms the basis for the first successful replication also of the ACCO function: 1 is the first known ACC complex that reacts with O2 to produce ethylene. As a FeOOH species had been suggested as intermediate in the catalytic cycle, H2 O2 was tested as the oxidant, too, and indeed evolution of ethylene proceeded even more rapidly to give 65 % yield.