A new infrared spectroscopoic marker for cochleate phases in phosphatidylserine-containing model membranes

Fourier transform-infrared (IR) spectroscopic and electron microscopic studies are reported for 1,2-dimyristoylphosphatidylserine (DMPS) and for DMPS/1,2-dimyristoylphosphatidylcholine mixtures in the presence and absence of Ca2+ ion. The frequency of the methyl symmetric deformation mode near 1,378 cm-1, previously assumed insensitive to changes in lipid morphology, has been found to respond to cochleate phase formation by undergoing an approximately 8 cm-1 increase. The new IR spectroscopic marker at 1,386 cm-1 has been used to identify and verify structures suggested from the phase diagram of J. R. Silvius and J. Gagné (1984. Biochemistry. 23:3241-3247) for this system. In addition, the ability of Mg2+ ion to induce cochleate formation has been demonstrated. Higher Mg2+ than Ca2+ levels are required for this process. Finally, IR spectroscopy has been used to monitor dehydration of the lipid surface through changes in the asymmetric PO2- stretching mode. Dehydration precedes cochleate phase formation (i.e., occurs at a lower Ca2+/phosphatidylserine level).