Bimodal effect of amphiphilic biocide concentrations on fluidity of lipid membranes

Using the spin label method (ESR) it has been shown that biologically active, amphiphilic compounds (quaternary ammonium salts (AS) containing polar heads with single and double positive charge caused, at low concentrations, decrease fluidity of liposome membranes formed with egg yolk lecithin (EYL). At higher concentrations an increase in fluidity was observed. With compounds having a single positive charge minimum fluidity of membrane structure occurs in the range of 1 to 3%, with compounds containing double positive charge-in the range of 4-6%. That effect does not depend on polar head size and length of alkyl chains of the AS used. Analysis of the electrostatic interaction between positive charges and dipole system suggest that at low ion concentrations the binding energy of the system increases, while it decreases at high concentrations. For the model presented, maximum of binding energy of the system occurs at 3% of positive monovalent ions and at 6% of positive divalent ions admixed.