Fundamental study on the transport dynamics of small molecules across liposome bilayers

Abstract

One of the main functions of the cellular membranes is to control the transport of various molecules into and out of cells. Especially, the ions having hydrophobic parts can adsorb on and transport through the lipid bilayer without the aid of protein ion channels. We have studied the adsorption and transport dynamics of the hydrophobic cationic dyes (Malachite Green: MG) in the anionic liposome bilayer made of unsaturated (DOPG: Dioleoyl-phosphatidylglycerol) and saturated (DSPG: Distearoyl-phosphatidylglycerol) lipids by using a nonlinear optical technique called second harmonic generation (SHG). Because the SH field is proportional to the difference between the surface density of dye molecules on the outer and inner surfaces of the liposome bilayer, we could obtain the nondestructive and in-situ information about the adsorption and transport of dyes in the liposome bilayers by monitoring the time dependence of the SH signal. Although the main driving force for the adsorption of dyes is the electrostatic interaction, it could be also affected by the non-electrostatic hydrophobic interaction between dyes and the tail parts of the lipid. The transport rate of dyes across the liposome bilayer significantly depends on the ordering of the hydrophobic tail parts of the lipid bilayer. By using parameters such as the dye concentration, temperature, structure of lipids (using unsaturated DOPG or saturated DSPG lipids), or salt (NaCl) concentration, we could change the transport rate of dyes. The transport rate was an exponentially increasing function of the dye concentration, and it significantly increases with the temperature increase in the unsaturated liposome made of DOPG lipids. The dependence of the transport rate on the temperature in the dilute limit of the dye concentration could be explained by the free volume model, which was confirmed by the experiments on the saturated liposome made of DSPG lipids. In the DSPG liposome, dyes could not transport across the...