Synergetic effects of Ca 2+ and Cu 2+ on phase transition in phosphatidylserine membranes

Abstract

In complexes of divalent metals with large exchange rate constant ( K H 2O ) of the coordinated H 2O, such as Ca 2+ and Cu 2+, the cubic structure in the ligand field is usually unstable and conformation changes are easily induced. We observed the molecular motion of phosphatidylserine (PS) in an amphipathic solvent (water / methanol / chloroform) by 1H-NMR and ESR using Ca 2+ and / or Cu 2+, which has a similar K H 2O to that of Ca 2+. We found that Ca 2+ did not hinder the molecular movements of PS. However, Cu 2+ reduced the movements of both headgroups and the double bonds in the fatty acids of PS. By addition of both Ca 2+ and Cu 2+, phase transition to a soft solid phase in the PS membrane was observed at room temperature. The results indicate that the headgroups are clustered in two-dimensional network with each ligand field displaced from the aqueous phase to the water / oil interface. The structure changes of the polar headgroups after the binding of divalent cations are considered to trigger the phase transition of this acidic phospholipid membrane.