Acid- and calcium-induced structural changes in phosphatidylethanolamine membranes stabilized by cholesteryl hemisuccinate.

The membrane stabilization effect of cholesteryl hemisuccinate (CHEMS) and the sensitivity of the CHEMS-phosphatidylethanolamine membranes to protons and calcium ions were studied by differential scanning calorimetry, freeze-fracture electron microscopy, and 31P NMR. (1) At neutral pH, the addition of 8 mol % CHEMS to transesterified egg phosphatidylethanolamine (TPE) raised the lamellar-hexagonal transition temperature of TPE by 11 degrees C. Stable bilayer vesicles were formed when the incorporated CHEMS exceeded 20 mol %. (2) At a pH below 5.5, the protonation of CHEMS enhanced the formation of the hexagonal phase (HII) of TPE. At 25 mol % CHEMS the bilayer-hexagonal transition temperature was lowered by 30 degrees C at pH 4.5. (3) The endothermic acid-induced hexagonal hexagonal transition of TPE-CHEMS was suppressed at 35 mol % CHEMS. However, 31P NMR and electron microscopy indicated that a lamellar-hexagonal transition still occurred at this composition. (4) The main transition of TPE was not affected by the protonation of the incorporated CHEMS, indicating that no macroscopic phase separation occurred in TPE-CHEMS mixtures at low pH. (5) In contrast to the HII-promoting effect of H+, the neutralization of the negative charge on TPE-CHEMS by Ca2+ resulted in aggregates that remained in the lamellar structure even at the hexagonal transition temperature of TPE. It is suggested that calcium might form a complex between CHEMS in apposed bilayers. These results are related to the possible biological function of acidic cholesterol esters in biomembranes.