Counterion release in overcharging of polyion-liposome complexes

We present a set of electrical conductivity measurements of a mesoscopic equilibrium cluster phase in the aggregation process of charged particles induced by oppositely charged polyions. These measurements supply strong experimental evidence that correlated adsorption of polyions is driven by the counterion release. This phenomenon, known to occur in DNA-liposome mixtures in lamellar phase, i.e., when liposomes fuse together to form a sandwichlike structure encompassing DNA chains, was not previously observed in aqueous suspension of clusters of intact liposomes stuck together by polyions to form reversible aggregates. A simple statistical model of the lateral correlation of polyions at the particle surface justifies quantitatively the observed behavior of the counterion release, as shown by electrical conductivity measurements.

Figure 1

The low-frequency electrical conductivity $\sigma$ of polyion-induced liposome aggregates as a function of the Na-PAA polyion concentration. Polyion in saline form is employed, so that the counterion is ${\mathrm{Na}}^{+}$. Experiment was carried out at four different liposome concentrations: (a) $0.82\mathrm{mg}∕\mathrm{mL}$; (b) $1.64\mathrm{mg}∕\mathrm{mL}$; (c) $2.87\mathrm{mg}∕\mathrm{mL}$; (d) $4.12\mathrm{mg}∕\mathrm{mL}$. The conductivities of the polyion solution in the absence of liposomes (●) are also shown for comparison. The arrows mark the isoelectric condition.

Figure 2

The low-frequency electrical conductivity $\sigma$ of polyion-induced liposome aggregates as a function of the H-PAA polyion concentration. Polyion in acidic form is employed, so that the counterion is ${\mathrm{H}}^{+}$. Experiment was carried out at three different liposome concentrations: (a) $0.838\mathrm{mg}∕\mathrm{mL}$; (b) $1.67\mathrm{mg}∕\mathrm{mL}$; (c) $2.86\mathrm{mg}∕\mathrm{mL}$. The conductivities of the polyion solution in the absence of liposomes (●) are also shown for comparison. The arrows mark the isoelectric condition.

Figure 3

(a) The electrical conductivity $\sigma$ (∎) compared with the values of the H-PAA polyion solution (●); (b) the electrophoretic mobility $u$; (c) the average hydrodynamic diameter $2R$ of the polyion-induced liposome aggregates as a function of the polyion to lipid molar ratio $\xi =\left[\mathrm{H}\text{−}\mathrm{PA}\right]∕\left[\mathrm{DOTAP}\right]$. The liposome concentration is $1.67\mathrm{mg}∕\mathrm{mL}$ and the polyion is in acidic form. The arrows mark the isoelectric condition.

Figure 4

(Color) The increment in the electrical conductivity $\Delta \sigma$ associated with the counterion release during the polyion-induced liposome complexation, for different lipid concentrations: (∎) $0.82\mathrm{mg}∕\mathrm{mL}$; (▴) $1.64\mathrm{mg}∕\mathrm{mL}$; (◆) $2.87\mathrm{mg}∕\mathrm{mL}$; (●) $4.12\mathrm{mg}∕\mathrm{mL}$. The full lines are the values calculated according to the correlated adsorption model. The arrows mark the concentrations at which isoelectric condition occurs.

Figure 5

The electrical conductivity $\sigma$ of liposome suspension as a function of the simple salt (NaCl) concentration. The lipid concentration is $0.8\mathrm{mg}∕\mathrm{mL}$. The arrow marks the isoelectric point. The upper and lower panels show the constancy of the aggregate size (absence of “reentrant” condensation) and the constancy of the electrophoretic mobility $u$ (absence of “charge inversion”).