Electro-Mechanical Coupling in Charged Liposome Suspensions

Abstract

Like-charged liposomes in aqueous suspension interact by mutual electrostatic repulsions. When the mean distance between liposomes is comparable to, or less than, several Debye screening lengths, the suspension self-organizes, develops a colloid osmotic pressure, and exerts an expansive force on the container walls. The result is an electrostatically stabilized gel whose rigidity is related to the liposome charge, size, concentration, and solution ionic strength. It is in principle possible to measure the liposome charge by measurement of the bulk elastic constants of the gel. Here we use an oscillating laser-tweezer technique, described previously, to measure the shear modulus (G') of the gel in deionized water as a function of the intrinsic liposome charge at various POPG:POPC stoichiometries. The shear modulus is proportional to the second derivative of the interaction potential energy with respect to the direction of shear. The interaction potential can be modeled as a screened Coulomb potential in the Debye-Huckel approximation with an effective liposome charge Z∗ (Alexander et al., 1984). The experimental G' as a function of liposome intrinsic charge (Zo) is maximal at Zo=20,000-30,000, while the theory gives a G' maximum at Z∗=300-400. The relation of Zo to Z∗ yields information on interfacial phenomena at the liposome surface via Poisson-Boltzmann analysis. The magnitudes of the experimental G' values are 3-5x higher than those calculated by the Z∗ Debye-Huckel theory, which ignores liposome size. A more realistic theory using the Derjaguin potential for sphere-sphere repulsions yields G' closer to the experimental values.