Reduction-responsive vesicles composed of dimethylaminopropyl octadecanamide and dithiodipropionic acid

Reduction-responsive vesicle was prepared by salt-bridging N-3-(dimethylamino)propyl]-octadecanamide (DMAPODA, a cationic amphiphile) using 3,3′-dithiodipropionic acid (DTPA, a disulfide diacid compound). According to the transmission electron micrograph and the fluorescence quenching degree (53.2%), it could be said that vesicles were formed when the DMAPODA to DTPA molar ratio was 2:2. The DMAPODA/DTPA associate was considered to be a building block for vesicle formation because DTPA could electrostatically associate with DMAPODA and help the cationic amphiphile assemble into the vesicle. On a differential scanning calorimetric thermogram, the DMAPODA/DTPA vesicle showed two endothermic peaks at 50.6°C and 63.2°C. The peak found at the lower temperature was possibly due to the solid gel-to-liquid crystal phase transition of the vesicular membrane and the peak found at the higher temperature was considered to be due to the melting of DMAPODA, indicating that unassociated DMAPODA coexisted with DMAPODA/DTPA vesicles. The release of calcein enveloped in the vesicle was promoted by DL-dithiothreitol, possibly because DTPA can be broken by the reducing agent to form mercaptopropionic acids and the vesicle could be disintegrated and/or the vesicular membrane would become defective.