Effect of membrane thickness and unsaturation on dye efflux rates induced by [delta]-Lysin from phosphatidylcholine vesicles

An important region of the cellular membrane that appears to have major influence on membrane-peptide interactions is the hydrophobic acyl chain region. In this study two lipid series were used to determine whether specificity towards membranes is influenced by acyl chain length, or membrane thickness, or by the degree of acyl chain unsaturation. Additionally, this quantitative investigation also provided insight on the effects of free volume and bilayer elasticity. The interaction of the peptide d-lysin, a 26-residue peptide, to lipid bilayers was studied using stopped-flow fluorescence. Samples of carboxyfluorescein filled vesicles of various lipid concentrations were mixed with d-lysin resulting in dye efflux. The experimental curves of carboxyfluorescein efflux as a function of time were measured at lipid concentrations ranging from 12.5 µM to 200 µM. Constant t values were calculated to determine the weighted average of the relaxation times of each carboxyfluorescein release function. It was found the main cause for dye efflux was a thinner membrane. Also, bending of the membrane was found to be dependent on the thickness of the bilayer. The stretching ability of the membrane had less significance in peptide insertion. As a result of bending stiffness being a major factor for peptide insertion, it appears that d-lysin binds to the surface of the membrane due to the hydrophobic effect, and sink as a ‘wedge’ into the lipid bilayer