Amino acid terminated polydiacetylene lipid microstructures: Morphology and chromatic transition

Novel lipid microstructures with various morphologies have been synthesized using a series of amino acid terminated diacetylenic lipids, and the chromatic transitions of the polymerized microstructures have been characterized by W-vis and FTIR spectroscopies. The observed morphologies for the microstructures include tubule, helix, ribbon, sheet, braided fiber, and planar platelet, formation of which has shown strong dependency on the chemical structure of the headgroup. All amino acid lipids studied in this work form microstructures that allow polymerization by UV irradiation to yield a blue appearance. Molecular chirality, electrostatic interactions, and hydrogen-bonding interactions in the headgroup region determine the formation of microstructures with twisted features, while nonchiral molecules do not form curved structures. The polymerized lipid microstructures exhibit similar colorimetric properties as observed for related bilayer vesicles, undergoing a blue-to-red color transition in response to thermal and pH changes. Microstructures with hydrophobic headgroup lipids are more sensitive to pH change than those with hydrophilic headgroups. For hydrophilic headgroup lipids, microstructures are more pH-resistant relative to their vesicle counterparts. FTIR studies suggest that thermal and pH-induced chromism of PDA microstructures proceed by different pathways. A mechanism is proposed that links function and change of hydrogen-bonding interactions to the observed chromatic behaviors of the PDA microstructures. In thermochromism, hydrogen-bonding interactions lock in the lipid headgroups so that the temperature-induced gauche-trans conformational transition of the side chains imposes strain on the assembly. In pH-induced chromism, surface ionization and breakdown of hydrogen-bonding interactions lead to reorganization of the headgroups that affects the electron delocalization along the conjugated backbone.