Synthesis, aggregation, and binding behavior of synthetic amphiphilic receptors

Amphiphilic bowl-shaped receptor molecules have been synthesized starting from diphenylglycoluril. Upon dispersion in water, these molecules self-assemble to form vesicles that bind neutral guests and alkali metal ions. In the case of bis(alkylester)-modified receptor compound 4, electron microscopy reveals that an increase in the size of the alkali metal ion (from Na(+) or K(+) to Rb(+) and to Cs(+)) leads to a change in the shape of the aggregates, viz. from vesicles to tubules. Monolayer experiments suggest that this behavior is due to a change in the conformation of this amphiphilic receptor. In water, molecules of 4 have an elongated conformation that changes to a sandwich-like one upon binding of alkali metal ions. Binding studies with vesicles from the bis-ammonium receptors 6 and 9 and the guest 4-(4-nitrophenylazo)resorcinol (Magneson) reveal that below the critical aggregation concentration (CAC) of the amphiphile 1:1 host-guest complexes are formed with high host-guest association constants. Above the CAC, a host-guest ratio of 2:1 was observed that indicates that only the cavities on the outside of the vesicle can be occupied. In the case of the naphthalene walled compound 8 changes in the vesicle structure are induced by the organic guest Magneson.