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

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 C-16 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 mum. 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.