Influence of anions and cations on the dipole potential of phosphatidylcholine vesicles: A basis for the Hofmeister effect

Anions and cations have long been recognized to be capable of modifying the functioning of various membrane-related physiological processes. Here, a f luorescent ratio method using the styrylpyridinium dyes, RH421 and di-8-ANE PPS, was applied to determine the effect of a range of anions and cations o n the intramembrane dipole potential of dimyristoylphosphatidylcholine vesi cles. It was found that certain anions cause a decrease in the dipole poten tial. This could be explained by binding within the membrane, in support of a hypothesis originally put forward by A. L. Hodgkin and P. Horowicz [1960 , J. Physiol. (Lond.) 153:404-412.] The effectiveness of the anions in redu cing the dipole potential was found to be ClO4- > SCN- > I- > NO3- > Br- > Cl- > F- > SO42-. This order could be modeled by a partitioning of ions bet ween the membrane and the aqueous phase, which is controlled predominantly by the Gibbs free energy of hydration. Cations were also found to be capabl e of reducing the dipole potential, although much less efficiently than can anions. The effects of the cations was found to be trivalent > divalent > monovalent. The cation effects were attributed to binding to a specific pol ar site on the surface of the membrane. The results presented provide a mol ecular basis for the interpretation of the Hofmeister effect of lyotropic a nions on ion transport proteins.