The solution behavior and physicochemical characteristics of polymer–colloid complexes based on cationic imidazolium amphiphile with a dodecyl tail (IA-12) and polyacrylic acid (PAA) or DNA decamer (oligonucleotide) were evaluated using tensiometry, conductometry, dynamic and electrophoretic light scattering and fluorescent spectroscopy and microscopy. It has been established that PAA addition to the surfactant system resulted in a ca. 200-fold decrease in the aggregation threshold of IA-12, with the hydrodynamic diameter of complexes ranging within 100–150 nm. Electrostatic forces are assumed to be the main driving force in the formation of IA-12/PAA complexes. Factors influencing the efficacy of the complexation of IA-12 with oligonucleotide were determined. The nonconventional mode of binding with the involvement of hydrophobic interactions and the intercalation mechanism is probably responsible for the IA-12/oligonucleotide complexation, and a minor contribution of electrostatic forces occurred. The latter was supported by zeta potential measurements and the gel electrophoresis technique, which demonstrated the low degree of charge neutralization of the complexes. Importantly, cellular uptake of the IA-12/oligonucleotide complex was confirmed by fluorescence microscopy and flow cytometry data on the example of M-HeLa cells. While single IA-12 samples exhibit roughly similar cytotoxicity, IA-12–oligonucleotide complexes show a selective effect toward M-HeLa cells (IC50 1.1 µM) compared to Chang liver cells (IC50 23.1 µM).

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Self-organization of hydroxyethylated gemini surfactants with different spacer fragments 16-s-16­(OH) (s = 4, 6, 10, and 12) was studied in single solutions and in binary surfactant-oligonucleotide systems. Despite the fact that aggregation activity and solubilization capacity of aggregates decrease with an increase in spacer length, gemini with the longer spacer demonstrate superior binding capacity toward oligonucleotide as compared to single head surfactant and gemini analogs with shorter spacers. The detailed study testified that gemini with longer spacers are characterized by a looser packing mode, a moist and more polar interior, and tend to show polymorphism. These features in combination with favorable geometry factor providing suitable orientation of components are probably responsible for the beneficial lipoplex formation in the case of longer spacers. The effectiveness of oligonucleotide-surfactant complexation changes in the same order as transfection efficacy mediated by these gemini reported earlier [Zakharova, L. et al. Colloids Surf. B, 2016, 140, 269–277], which indicates that physicochemical aspects probably play a key role upon the design of nonviral vectors and may be used for prediction of transfection efficacy mediated by amphiphilic agents.

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