Abstract: The interaction between melamine and 1,12-diaminododecanediorotate (DDO), a bolaamphiphile bearing molecular recognition sites for amine functional groups, was transformed into visible color changes using polymerized 2,4-tricosadiynoic acid (TCDA) vesicles. TCDA was found to be more sensitive than other polydiacetylene vesicles with various alkane chain-lengths in reporting the molecular recognition event. Colorimetric changes occurred as a result of modifications in the conjugated backbone of TCDA triggered by changes in the chemical environment associated with the recognition process. To better understand this mechanism, the influence of melamine on the thermotropic behavior of polydiacetylene vesicles and the thermodynamics were investigated via differential scanning calorimetry (DSC). It was found that the phase transitions of both TCDA and DDO/TCDA vesicles were shifted to higher temperatures when melamine was present, and that the magnitude of this shift increased as the melamine levels were raised. Color changes associated with the molecular recognition process were only observed with the unaided eye when melamine was introduced at levels well in excess of those theoretically required. Using sucrose and urea as model kosmotropic and chaotropic compounds, respectively, the effect of such solutes on the phase transition of polydiacetylene vesicles and equilibrium energetics of the molecular recognition were examined. Results indicate that the excess melamine likely behaves as a chaotrope in the colloid. In combination with the multipoint hydrogen bond formation between melamine and DDO, these effects are responsible for the blue-red color transition of the DDO/TCDA vesicles. Identification of this chaotrope-assisted color visualization mechanism for PDA vesicles provides new insights into the Hofmeister series effect on the conformation change of conjugated polymers.

Key words: Molecular recognition, Melamine, Chaotrope, Polydiacetylene vesicle, Differential scanning calorimetry