Clustering of lecithin molecules in phosphatidylserine membranes induced by calcium ion binding to phosphatidylserine

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The effects of calcium ion on phosphatidyl L-serine (PS) have been studied with PS membranes containing a lecithin spin label (L ∗). The calcium ion makes the ESR spectra of the L ∗ in PS membranes broadened owing to the intermolecular spin-spin exchange interactions. The results indicate that the calcium ion binds to PS molecules to form rapidly rigid calcium ion-bound PS aggregates, the lecithin molecules being thereby separated from the host PS bilayers to form clusters. The magnesium ion is ineffective for the aggregation and exerts a quite different effect only at much higher concentrations.