Effect of the Membrane Boundary Potential on the Phase Transition of Dipalmitoylphosphatidylcholine Bilayer Liposomes

Abstract

Effect of the membrane boundary potential on the thermotropic fluid-solid phase transition of the uniformly sized single lamellar liposomes of diplamitoylphosphatidylcholine, DPPC, has been investigated by measuring the fluorescence intensity of N,N'-distearyloxacarbocyanine, DSOCC, embedded in the liposomes. The change in the electric field at the polar head group region of the lipid bilayers was induced by the addition of sodium tetraphenylborate, Na⁺TPB⁻ and/or tetraphenylphosphonium chloride, TPP⁺Cl⁻, in the liposome dispersions and it was confirmed to be established by the measurements of the electrochromic absorption change of DSOCC. An addition of Na ⁺TPB⁻ at the concentration of 0.1 mM resulted in the drop of the phase transition temperature of the lipid bilayers for about 7[ddot]C, while TPP⁺Cl⁻ gave a detectable but very small effect on the phase transition temperature. The relationships between the phase transition temperature and the amount of TPB⁻ absorbed in the bilayers and the electrochromic absorption change were obtained to discuss the effect of the electric field on the phase transition.