Effect of Ethanol on Lipid Bilayer Relaxation Kinetics

Lipid bilayer relaxation induced by the ethanol addition was studied with a fluorescence stopped flow and steady state techniques. Extruded liposomes were labeled with 2 mol% of PE-NBD and PC-NBD-C 12 , respectively. Fluorophores were covalently attached to the lipid molecules at various positions, PE-NBD at the headgroup region and PC-NBD in the alcyl chain region. Their quantum yields are sensitive to the local polarity changes what makes the dyes excellent reporters of the lipid bilayer hydration and conformational changes. As a result the kinetic traces of the membrane response to the ethanol presence were obtained. The detected relaxation proces cannot be associated with the ethanol diffusion through the lipid bilayer what has been shown by the fluorescence steady state measurements. Although the overall signal decreases simultaneously upon treatment with ethanol, the kinetics acquired in the time scale of tens of seconds demonstrated that the emission of the dyes rises. The process observed consists of two subsequent relaxation events which may result from the long-range, cooperative processes taking place in the lipid bilayer. Data analysis based on the relaxation theory revealed that the signal changes can be approximated with a single exponential function and that one of the processes is concentration dependent.