DODAB-ODN lipoplex structure is highly dependent on ODN concentration: A multitechnique experimental study

This study explores the structural properties of lipid vesicles composed of dioctadecyldimethylammonium bromide (DODAB) and varying concentrations of the oligonucleotide 5'-AAAAAAAAAA-3' (ODN). The vesicles, in both gel and fluid phases, were characterized using dynamic light scattering (DLS), Zeta potential, small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), and fluorescence with the fluorescent probe Laurdan embedded into DODAB vesicles. The DODAB-ODN interaction induces concentration-dependent structural changes on the lipoplex. At relatively low ODN concentrations ([ODN]/[DODAB] < 0.050), the dispersion remains stable, with positively charged vesicles, though with an increase in vesicle size and sample turbidity, and SAXS indicates coalescence of vesicles with multilamellar structure formation. DSC revealed the coexistence of ODN-free and ODN-rich regions, with the latter showing increased thermal stability. Laurdan fluorescence indicates that the probe does not significantly partition into the lipoplex ODN-rich regions. At intermediate concentrations (0.050 < [ODN]/[DODAB] < 0.075), colloidal stability is lost, and pure DODAB domains are no longer detected. At relatively high ODN concentrations (0.075 ≤ [ODN]/[DODAB] ≤ 0.200), the dispersion is stable, showing a negative vesicle surface potential, and the presence of small multilamellar vesicles. Laurdan lifetimes are significantly increased, suggesting that ODN induces lipid packing or dehydration at the surface of the vesicles in both the gel and fluid phases. These findings enhance our understanding of the structure of lipid-ODN aggregates, paving the way for their application in gene therapy.