Phase behavior of synthetic amphiphile vesicles investigated by calorimetry and fluorescence methods

The understanding of biological membranes may be improved by investigating physical properties of vesicles from natural or synthetic amphiphiles. The application of vesicles as mimetic agents depends on the knowledgment of th eir structure and properties, Vesicles having different curvature and size may be obtained using different preparation protocols. We have used differe ntial scanning calorimetry (DSC) and steady-state fluorescence to investiga te the gel to liquid-crystal phase transition of vesicles prepared by sonic ation (SUV) and non-sonication (GUV) of the synthetic dioctadecyldimethylam monium bromide (DODAB) in aqueous solution. DSC thermograms for a non-sonic ated dispersion show a well-defined pre- and main transition corresponding to two narrow peaks at 36 and 45 degrees C in the first upscan, while in a second upscan, only the main peak was observed. The sharpness of the peaks indicate a cooperative phase behavior for GUV. For a sonicated DODAB disper sion, the first upscan shows a third peak at 40.3 degrees C, whereas for th e second upscan the peaks are not well-defined, indicating a less cooperati ve phase behavior. Alternatively, the fluorescence quantum yield (Phi(f)) a nd the anisotropy (r) of trans, trans, trans-1-[4-(3-carboxypropyl)-phenyl] -6-[4-butylphenyl]-1,3,5-hexatriene (4H3A) and the ratio I-1/I-3 of the fir st to the third vibronic peaks of the pyrene emission spectrum as function of temperature are used as well to describe the phase behavior of DODAB son icated and non-sonicated dispersions. It is in good agreement with the DSC results that the cooperativity of the thermotropic process is diminished un der sonication of the DODAB dispersion, meaning that sonication changes fro m homogeneous to heterogeneous populations of the amphiphile aggregates, Th e pre- and main transitions obtained from these techniques are in fairly go od accord with results from the literature. (C) 1999 Elsevier Science B.V. All rights reserved.