Intravesicular and intervesicular interaction by orthogonal multivalent host–guest and metal–ligand complexation

Abstract

Host vesicles composed of amphiphilic β-cyclodextrin CD1 recognize metal-coordination complexes of the adamantyl-functionalized ethylenediamine ligand L via hydrophobic inclusion in the β-cyclodextrin cavities at the vesicle surface. In the case of Cu(II) and L, the resulting coordination complex was exclusively CuL₂, and the interaction with the host vesicles was intravesicular, unless the concentration of metal complex and vesicles was high (>0.1 mM). In the case of Ni(II) and L, a mixture was formed consisting of mainly NiL and NiL₂, the interaction with the host vesicles was effectively intervesicular, and addition of the guest–metal complex resulted in aggregation of the vesicles into dense, multilamellar clusters even in dilute solution [1 μM Ni(II)]. The metal–L complex could be eliminated by a strong chelator such as EDTA, and the intervesicular interaction could be suppressed by a competitor such as unmodified β-cyclodextrin. The result from this investigation is that the strongest metal-coordination complex [Cu(II) with L] binds exclusively intravesicularly, whereas the weakest metal-coordination complex [Ni(II) with L] binds predominantly intervesicularly and is the strongest interfacial binder. These experimental observations are confirmed by a thermodynamic model that describes multivalent orthogonal interactions at interfaces.