Effect of ${\mathrm{Gd}}^{3+}$ on the colloidal stability of liposomes

Lanthanide ions such as ${\mathrm{La}}^{3+}$ and ${\mathrm{Gd}}^{3+}$ are well known to have large effects on the structure of phospholipid membranes. Unilamellar vesicles of dipalmitoylphosphatidylcholine (DPPC) were prepared by sonication method and confirmed by transmission electron microscopy. The effects of concentration of gadolinium ions ${\mathrm{Gd}}^{3+}$ on DPPC unilamellar vesicles in aqueous media were studied by different techniques. As physical techniques, photon correlation spectroscopy, electrophoretic mobility, and differential scanning calorimetry were used. The theoretical predictions of the colloidal stability of liposomes were followed using the Derjaguin-Landau-Verwey-Overbeek theory. Changes in the size of liposomes and high polydispersities values were observed as ${\mathrm{Gd}}^{3+}$ concentration increases, suggesting that this cation induces the aggregation of vesicles. Electrophoretic mobility measurements on unilamellar vesicles as a function of ${\mathrm{Gd}}^{3+}$ ion concentration show that the vesicles adsorb ${\mathrm{Gd}}^{3+}$ ions. Above ${\mathrm{Gd}}^{3+}$ concentrations of $0.1\mathrm{mol}{\mathrm{dm}}^{-3}$, the $\zeta$ potential and light scattering measurements indicate the beginning of aggregation process. For comparison with similar phospholipids, the zeta potential of phosphatidylcholine interacting with ${\mathrm{Gd}}^{3+}$ was measured, showing an analogous behavior. Differential scanning calorimetry has been used to determine the effect of ${\mathrm{Gd}}^{3+}$ on the transition temperature $\left(T_c\right)$ and on the enthalpy $\left(\mathrm{\Delta }{H}_c\right)$ associated with the process.

Figure 1

$\zeta$ potential of DPPC liposomes as a function of the ${\mathrm{Gd}}^{+3}$ concentrations at $25°\mathrm{C}$.

Figure 2

$\zeta$ potential of DPPC $(•)$ and EYPC $(∎)$ liposomes as a function of the ${\mathrm{Gd}}^{+3}$ concentrations at $25°\mathrm{C}$.

Figure 3

Diameters of the DPPC liposomes for different ${\mathrm{Gd}}^{+3}$ concentrations measured by DLS at $25°\mathrm{C}$.

Figure 4

Transmission electron micrograph of spontaneous DPPC liposomes at presence of (a) $0.05\mathrm{mol}{\mathrm{dm}}^{-3}$ of ${\mathrm{Gd}}^{+3}$ and (b) $0.7\mathrm{mol}{\mathrm{dm}}^{-3}$ ${\mathrm{Gd}}^{+3}$, respectively.

Figure 5

Polydispersities of the DPPC liposomes for different ${\mathrm{Gd}}^{+3}$ concentrations measured by DLS at $25°\mathrm{C}$. The inset represents the size distribution, calculated by CONTIN methods, in absence of ${\mathrm{Gd}}^{+3}$ and at $0.7\mathrm{mol}{\mathrm{dm}}^{-3}$ of ${\mathrm{Gd}}^{+3}$.

Figure 6

Diameters of the DPPC liposomes for different phospholipid concentrations measured by DLS at $25°\mathrm{C}$. $(∎)$ $1.36\times {10}^{-3}\mathrm{mol}{\mathrm{dm}}^{-3}$ and $(•)$ $13.6\times {10}^{-3}\mathrm{mol}{\mathrm{dm}}^{-3}$.

Figure 7

DLVO potentials of EYPC liposomes as a function of the distance between two liposomes for different ${\mathrm{Gd}}^{3+}$ concentrations, ${10}^{-3}$, 0.01, 0.05, and $0.10\mathrm{mol}{\mathrm{dm}}^{-3}$.

Figure 8

The DSC endotherms of DPPC liposomes with varying amounts of Gadolinium: heating (a), cooling (b).

Figure 9

Variation of the transition temperature $\left(T_c\right)$ as a function of ${\mathrm{Gd}}^{3+}$ concentration, by heating and cooling. The inset shows the variation of the enthalpy for the gel to liquid crystalline transition $\left(\mathrm{\Delta }{H}_c\right)$ of DPPC liposomes as a function of ${\mathrm{Gd}}^{3+}$ concentration, by heating and cooling.