Aggregation of a crown ether-based copper amphiphile as a mimic for the superstructure of hemocyanin

Abstract

Inspired by the alkali metal ion-controlled self-assembly of the oxygen-transporting dinuclear copper protein hemocyanin, a novel crown ether amphiphile (1) has been designed. A diaza-18-crown-6 moiety (diaza-18-crown-6 = 1,10-diaza-4,7,13,16-tetraoxacyclooctadecane) is appended with an aliphatic C₁₆ chain and with a PY2 ligand (PY2 = bis[2-(2-pyridyl)ethyl]amine), resulting in an amphiphile that has independent ligand sets for the coordination of copper ions and alkali metal ions. The copper complex of this amphiphile, [Cu^II(1)](ClO₄)₂, forms monolayers at the air–water surface. In the presence of certain alkali metal salts, the molecules in the monolayers stretch out and form sandwich complexes (2 ∶ 1 amphiphile : metal ion stoichiometry). This is in contrast to the 1 ∶ 1 complexes which are obtained with amphiphiles that lack the Cu–PY2 part. Electron microscopy experiments reveal that hollow tubules are generated when [Cu^II(1)](ClO₄)₂ is dispersed in water. The diameters of these tubes range from 45–55 nm, while their length can extend up to 5 μm. The presence of an alkali metal salt leads to vesicular structures with diameters ranging from 180–210 nm. Based on powder X-ray diffraction results, the amphiphile [Cu^II(1)](ClO₄)₂ is packed in strongly intercalated bilayers in both tubes and vesicles with a layer thickness of 4.8 nm. Cyclic voltammetry shows that the midpoint potential of aqueous dispersions of [Cu^II(1)](ClO₄)₂, E_1/2 = −0.08 V, undergoes an anodic shift to +0.083 V in the presence of K⁺ ions. An in situ preparation of [Cu^I(1)](ClO₄) did not show binding of molecular oxygen, whereas [Cu^II(1)](ClO₄)₂ reacted with H₂O₂, leading to degradation of ligand 1.