Calix[6]tren and Copper(II): A Third Generation of Funnel Complexes on the Way to Redox Calix-Zymes

• Published in: Proceedings of the National Academy of Sciences - PNAS 2005-05, Vol.102 (19), p.6831-6836
• Main Authors: Izzet, Guillaume, Douziech, Bénédicte, Prangé, Thierry, Tomas, Alain, Jabin, Ivan, Le Mest, Yves, Reinaud, Olivia, Halpern, Jack
• Format: Article
• Language: English
• Subjects: Adducts; Binding Sites; Bridged-Ring Compounds - chemistry; Calixarenes; Calixarenes - chemistry; Chemical compounds; Coordination numbers; Copper; Copper - chemistry; Crystallography, X-Ray; Dimethylformamide - chemistry; Electrochemistry; Electrodes; Electron Spin Resonance Spectroscopy; Electron Transport; Electrons; Enzymes; Ethylenediamines - chemistry; Ligands; Macromolecular Substances - chemistry; Models, Molecular; Molecular structure; Molecules; Oxidation-Reduction; Oxygen - chemistry; Phenols - chemistry; Physical Sciences; Protein Binding; Solvents; Spectrophotometry; Spectroscopy; Temperature; Thermodynamics; X-Ray Diffraction
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0500240102

Mono-copper enzymes play an important role in biology and their functionality is based on Cu(II)/Cu(I) redox processes. Modeling a mono-nuclear site remains a challenge for a better understanding of its intrinsic reactivity. The first member of a third generation of calixarene-based mono-copper "funnel" complexes is described. The ligand is a calix[6]arene capped by a tren unit, hence presenting a N4coordination site confined in a cavity. Its Cu(II) complexes were characterized by electronic and EPR spectroscopies. The x-ray structure of one of them shows a five-coordinated metal ion in a slightly distorted trigonal bipyramidal geometry thanks to its coordination to a guest ligand L (ethanol). The latter sits in the heart of the hydrophobic calixarene cone that mimics the active site chamber and the hydrophobic access channel of enzymes. Competitive binding experiments showed a preference order dimethylformamide > ethanol > MeCN for L binding at the single exchangeable metal site. Cyclic voltammetry studies showed irreversible redox processes in CH2Cl2when L is an oxygen donor caused by the redox-driven ejection of the guest at the Cu(I) level. In the presence of MeCN, a pseudoreversible process was obtained, owing to a fast equilibrium between a four- and a five-coordinate Cu(I) species. Finally, a redox-driven ligand interchange of dimethylformamide for MeCN at the Cu(I) state allowed the trapping of the thermodynamically less stable Cu(II)-MeCN adduct. Hence, this work represents an important step toward the elaboration of a functional supramolecular model for redox mono-copper enzymes, named redox calix-zymes.