The effects of A23187 on the phospholipid phase transition of large unilammellar vesicles (LUVs) as detected by ultrasound spectroscopy

Abstract

The effect of the hydrophobic Ca 2+ ionophore, A23187, on the phospholipid dynamics of large unilamellar vesicle (LUVs: 4:1 (w/w) mixture of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylglycerol (DPPG) membranes, as a function of A23187 content, was investigated using techniques sensitive to the phospholipid phase transition. The ultrasonic absorption per wavelength, αλ, was determined with a double crystal acoustic interferometer, as a function of tempertaure and frequency for LUVs in the vicinity of their phospholipid phase transition. Differential scanning calorimetry (DSC) and electron spin resonance (ESR) were also employed to probe the thermodynamics and molecular environment of the hydrocarbon side chains. With increasing A23187 content, the phase transition temperature ( T m _of the LUV suspensions remained near 42.0°C, while the amplitude of αλ at the phase transition incresed dramatically. At T m the relaxation frequency, were αλ max occurs, decreased with A23187 content, suggesting that the relaxation rate of the event responsible for the absorption of ultrasound decreased. The ESR studies showed no change in the fluidity of the bilayer with the inclusion of 2 and 5 mol% A23187 in the C-12 region of the bilayer. Therefore, A23187 in LUV membranes slows the structural relaxation of the hydrocarbon side chains of the phospholipid bilayer at the phase transition.