Interaction between Ca²⁺ and dipalmitoylphosphatidylcholine membranes. II. Fluorescence anisotropy study

The effect of Ca²⁺ on the molecular mobility in dipalmitoylphosphatidylcholine membranes was studied by steady-state and time-resolved measurements of fluorescence anisotropy. The fluorescence anisotropy decay of 1,6-diphenyl-1,3,5-hexatriene in the hydrocarbon region indicated that the free volume of molecular rotation became more restricted when the Ca²⁺ concentration was increased. The decrease of the molecular mobility was observed from 1 mM Ca²⁺, at which the number of bound Ca²⁺ is much less than that of the total lipid molecules. A distinct difference between Ca²⁺ and Mg²⁺ effects suggested that the change in various membrane properties was induced by the binding of these ions. From these results we propose a long-range attractive interaction between bound Ca²⁺ and the polar head groups of distant phosphatidylcholine molecules.