Ammonium and imidazolium-based amphiphilic tetramethoxy resorcinarenes: Adsorption, micellization, and protein binding

To date, the knowledge about the properties of calixarene-based surfactants have been limited although it is hypothesized that the calixarene framework may endow calixarene-based surfactants with unique properties. In the present study, the adsorption and micellization behaviors of amphiphilic calix[4]resorcinarenes and their interactions with proteins were studied by various techniques, including surface tension measurement and fluorescence, UV–vis absorption, and 1H NMR spectroscopy; the amphiphilic calix[4]resorcinarenes were obtained by modifying tetramethoxyresorcinarenes with ammonium and imidazolium groups on the upper rim and alkyl groups on the opposite rim. We found that a tetramethoxyresorcinarene with a very short alkyl chain exhibited good surface activity and the critical micelle concentration (cmc) was much lower than that for the conventional surfactant having a long alkyl chain. Moreover, the cmc and saturated adsorption values (Γmax) decreased but the surface tension (γ) at the cmc (γcmc) and single molecular cross-sectional area (Amin) increased with increasing alkyl chain length. In the presence of proteins, the surface tension remained unchanged at less than stoichiometric tetramethoxyresorcinarene concentrations; however, at higher tetramethoxyresorcinarene concentrations, the surface tension began to decrease drastically, much faster than the case without protein. In addition, the binding of the tetramethoxyresorcinarene to the protein increased the protein activity, and the degree of enhancement in the protein activity was closely related to the protein structure. The molecular simulations further explained the binding modes of the tetramethoxyresorcinarenes to the protein.