Anion binding to lipid bilayers: determination using fluorescent membrane probe by direct quenching or by competitive displacement approaches.

An approach is described that enables anion binding to liposomal membranes to be assessed from the resulting quenching of fluorescent lipid probes included in the membranes. Lipid derivatives such as anthrylvinyl-labeled phosphatidylcholine (ApPC) and methyl 4-pyrenylbutyrate (MPB) were used because they bear nonpolar fluorophores that localize in the bilayer close to polar heads. Association constants (K(a)) of iodide binding to bilayers of different composition were determined on the basis of direct quenching experiments. For anions that are non-quenchers or weak quenchers (thiocyanate, perchlorate and trichloroacetate), K(a) values were obtained from the data of competitive displacement of iodide by these anions. This approach increases possibilities of fluorescence studies of ion-membrane interactions.