LYSOLIPID EXCHANGE WITH LIPID VESICLE MEMBRANES

While the aqueous solubility for bilayer phospholipids is less than 10 (-10) M-keeping lipid membranes at essentially constant mass, single c hain surfactants can have a significant aqueous solubility. Thus, in s urfactant solutions, both monomer and micelles can interact with a lip id bilayer, and the mass and composition of the bilayer can be changed in seconds. These changes in composition are expected to have direct consequences on bilayer structure and material properties. We have fou nd that the exchange of surfactants like lysolecithin can be described in terms of a kinetic model in which monomer and micelles are transpo rted to the membrane from bulk solution. Molecular transport is consid ered at the membrane interfaces and across the midplane between the tw o monolayers of the bilayer. Using micropipet manipulation, single ves icles were transferred into lysolecithin solutions, and the measuremen t of vesicle area change gave a direct measure of lysolecithin uptake. Transfer back to lysolecithin-free media resulted in desorption. The rates of uptake and desorption could therefore be measured at controll ed levels of membrane stress. With increasing lysolecithin concentrati on in the bulk phase, the amount of lysolecithin in the membrane reach ed saturation at similar to 3 mol% for concentrations below the critic al micelle concentration (CMC) and at >30 mol% for concentrations abov e the CMC. When convective transport was used to deliver lysolecithin, uptake occurred via a double exponential: initial uptake into the out er monolayer was fast (similar to 0.2 sec(-1)); transfer across the bi layer midplane was much slower (0.0019 sec(-1)).