Anion binding to lipid bilayers: A study with the help of fluorescent lipid probes

Anion quenching of fluorescent lipid probes included in the liposomal membrane was applied to study the binding of anions to it. As probes were used lipid derivatives bearing nonpolar fluorophore located in bilayer close to polar heads (anthrylvinyl-labeled phosphatidylcholine, ApPC; methyl 4-pyrenylbytyrate, MPB) or in polar zone (rhodamine 19 oleyl ester, OR19). The iodide binding to bilayers of different composition was studied. Isotherm of the quencher (iodide) adsorption on membrane was build on the basis of direct quenching experiments. For anions that are non-quenchers or weak quenchers (thiocyanate, percholorate and trichloroacetate), binding parameters were obtained from the data of competitive displacement of iodide by these anions. Association constants of anion binding to bilayer (K-a) were determined for the stoichiometry of 1 ion / 1 lipid, and for independent anion binding as well. It appeared inaccessible to determine the anion / binding site stoichiometry neither from the Guy Chapman model, nor from the data of direct measurements. At physiological concentration of salt that does not bind noticeably to membrane (150 mM NaCl), anion binding can the satisfactory described by the Langmuir isotherm. This approach increases possibilities of fluorescence studies of ion-membrane interactions.