Osmotic swelling of unilamellar vesicles by the stopped-flow light scattering method. Influence of vesicle size, solute, temperature, cholesterol and three α,ω-dihydroxycarotenoids

Abstract

α,ω-Dihydroxycarotenoids are postulated to play the rôle of membrane reinforcers in certain procaryotes, like cholesterol in eucaryotes. In order to evaluate the rigidifying effect of these polyterpenoids on lipid bilayers, osmotic swelling of unilamellar vesicles has been followed by measuring the light scattering intensity changes in a stopped-flow apparatus. A model based upon the Rayleigh-Gans theory was developped. It shows that the variation of light scattering intensity ( ΔI I 0 ) is proportional to that of vesicle radius ( ΔR R 0 ) for a given R 0 (initial vesicle radius). An emperical relationship between −ΔI I 0 and Z (dissymmetry measured by light scattering) was established: −ΔI I 0 is proportional to ( Z − 1). Therefore, the value −ΔI (I 0 · (Z − 1)) is independent of vesicle radius and can be used for the evaluation of bilayer rigidity. The water permeability was measured and it was shown that it is the limiting factor of the kinetics of swelling. When incorporated into dimyristoylphophatidylcholine vesicles, cholesterol and α,ω-dihydroxycarotenoids lower the water permeability and the value of −ΔI (I 0 · (Z − 1)) . So, at least on model systems, α,ω-dihydroxycarotenoids exert on a phospholipid bilayer a reinforcement effect similar to cholesterol.