Polyelectrolyte complex formation and stability when mixing polyanions and polycations in salted media: a model study...

Controlled drug delivery and gene transfection involve contact of artificial polyelectrolytic systems that can interact dramatically with biopolymers and cells when they are introduced in blood. Given the complexity of body aqueous media in terms of physical chemistry, a model approach was selected in attempt to understand the behavior of artificial polyelectrolytes introduced in body fluids. Selection in terms of molecular weight was highlighted in a previous paper. In the present study the formation and the stability of fractions obtained when a polycation is added to a polyanion according to a titrating process mimicking injection into blood was considered for different polycation/polyanion couples. Poly(amino serinate) and poly(L-lysine) were used as polybases, and poly(acrylic acid), poly(L-lysine citramide) and poly(L-lysine citramide imide) as polyacids. Four fractions corresponding to different positive/negative charge ratios were formed for each couple. At low polyion concentration (13 mg/L) and given salt concentration, the stability of the complex fractions depended on molecular weight and charge density of the polyions. The NaCl concentration required to destabilize the different interpolyelectrolyte complexes was found to decrease from the first fraction to the fourth one. Upon decreasing the salt concentration, macroscopic flocculation occurred in the case of PLL/PAA complex fractions only. For the other couples, dynamic light scattering showed that several hundreds nanometer sized particles were formed that were stable in a broad range of NaCl concentration, including the physiological 0.15 ionic strength. At higher polyion concentrations, stable solid precipitate was formed regardless of the system. The absence of flocculation in the case of highly diluted poly(L-lysine citramide) and poly(L-lysine citramide imide) polyanions in salted media is assigned to the presence of non-ionic hydroxyl and amide polar groups along the complexed chains. Data show that introducing non-ionic functions along the polyelectrolyte chains is a good means to keep interpolyelectrolyte complexes dispersed in salted media, a conclusion of interest in the field of condensation of genes by polycations.