Molecular Recognition of Melamine by Vesicles Spontaneously Formed from Orotic Acid Derived Bolaamphiphiles

Abstract

Molecular recognition by means of multiple hydrogen bonds is of great importance in biological functions. In this paper, an orotic acid derived bolaamphiphile 1,12-diaminododecane diorotate (DDO) with molecular recognition function moieties was designed. Both self-aggregation behavior and molecular recognition with melamine were extensively examined. This bolaamphiphile itself can form vesicles easily in aqueous solutions at 25 °C. Steady-state fluorescence was used to characterize the detailed molecular recognition process. The fluorescence of melamine was quenched more effectively by the spontaneously formed vesicles than by the monomers of the surfactant. Two mechanisms were involved in the fluorescence quench process. At lower concentration, the fluorescence of melamine was found to be quenched by static complex formation. While at higher concentration, both static and dynamic quenching mechanisms coexisted in interaction process. Thermodynamic parameters measured by isothermal titration calorimetry showed that the free energy (ΔG) is negative, indicating that binding of DDO molecules with melamine is favorable energetically. Hydrogen-bonded interactions contribute comparatively a lot for the DDO monomer binding with melamine; at the higher concentration above its critical aggregation concentration, the dissociation of the aggregates take place and lead to an entropically driven molecular recognition process. As complicated binding sites can be constructed through self-assembly at the vesicle interface rather than simple molecular modules, this bolaamphiphile with the molecular recognition functional group may make it possible to generate well-defined recognition sites to mimic biomolecular receptors. Moreover, the present research will give a guide to design chemosensors for melamine detection based on molecular recognition.