Effect of pressure on the properties of a surfactant micelle and a lipid membrane studied by the spin-probe method

Abstract

High-pressure e.s.r. spectra of a spin probe, 16-doxyl stearic acid methyl ester, in a surfactant micelle and in lipid bilayers have been measured up to ca. 300 atm. The spectra of the probe in a sodium dodecyl sulphate micelle altered little with pressure. The viscosity in the micelle, calculated from the rotational correlation time using the Stokes–Einstein equation, showed that it increases ca. 2.5 times under 3000 atm pressure. In an egg phosphatidylcholine bilayer, the effect of pressure was far larger than that in the micelle. The three-line spectrum under 1 atm became the rigid-limit one under 2900 atm, suggesting that the liquid-crystalline state changes into the gel state with an increase in pressure. The effect of pressure on the viscosities in the micelle and the lipid membrane has been discussed by comparison with those of water and dodecane. In a dilauroylphosphatidylcholine bilayer the spin probe as gradually segregated from the bilayer with increasing pressure, showing that phase separation takes place in the lipid bilayer with increasing pressure.