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

Anion quenching of fluorescent lipid probes included in the liposomal membr ane was applied to study the binding of anions to it. As probes were used l ipid derivatives bearing nonpolar fluorophore located in bilayer close to p olar heads (anthrylvinyl-labeled phosphatidylcholine, ApPC; methyl 4-pyreny lbytyrate, MPB) or in polar zone (rhodamine 19 oleyl ester, OR19). The iodi de binding to bilayers of different composition was studied. Isotherm of th e 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 anion s. 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 st oichiometry neither from the Guy Chapman model, nor from the data of direct measurements. At physiological concentration of salt that does not bind no ticeably to membrane (150 mM NaCl), anion binding can the satisfactory desc ribed by the Langmuir isotherm. This approach increases possibilities of fl uorescence studies of ion-membrane interactions.