Effect of H-Bonding Compounds on the Adsorption of Ca2+ to Lipid Membranes in the Gel State

Abstract

The binding of Ca2+ to dipalmitoylphosphatidylcholine (DPPC) bilayers in the gel state is studied by determination of the spectroscopical features of merocyanine 540. The procedure is based on the measurement of the effect produced by the extent of the binding on the membrane surface. In the range between 0 and 5 mM Ca2+ a peak at 450 nm is obtained which disappears with the inclusion of cholesterol in a 1:1 ratio. The maximum of absorbance at 450 nm is obtained at around 2.5 mM, which is coincident with the critical concentration of Ca2+ promoting a drastic transition in the interbilayer distance as reported in the literature. Glycerol, sucrose, glucose, and ribose decrease the intensity of the peak corresponding to the number of OH groups present in each compound. For 0.5 mM Ca2+ , the 450 nm absorbance intensity shows a maximum at 30°C, which is absent in the presence of cholesterol. Based on the results in a previous paper (Bernik, D. L., and Disalvo, E. A., Biochim. Biophys. Acta 1146, 169, 1993) which demonstrated that the absorbance at 450 nm indicates the presence of Pβ phase in dimyristoylphatidylcholine (DMPC) bilayers, the magnitude of the 450 nm peak as a function of Ca2+ was taken as an indication that Ca2+ adsorption induces similar surface properties on DPPC membranes in the gel state. This action appears to be counteracted by hydrogen bonding compounds included in the membrane or in the aqueous phase.