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

Publisher Website
Google Scholar
Abstract
The membrane stabilization effect of cholesteryl hemisuccinate (CHEMS) and the sensitivity of the CHEMS-phosphatidylethanolamine membranes to H1 and Ca2+ were studied by differential scanning calorimetry, freeze-fracture electron microscopy, and 31P NMR. At neutral pH, the addition of 8 mol% CHEMS to transesterified egg phosphatidylethanolamine (TPE) raised the lamellar-hexagonal transition temperature of TPE by 11.degree. C. Stable bilayer vesicles were formed when the incorporated CHEMS exceeded 20 mol%. 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.degree. C at pH 4.5. The endothermic acid-induced hexagonal transition of TPE-CHEMS was suppressed at 35 mol% CHEMS. 31P NMR and EM indicated that a lamellar-hexagonal transition still occurred at this composition. 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. In contrast to the HII-promoting effect of H+, the neutralization of the negative charge of TPE-CHEMS by Ca2+ resulted in aggregates that remained in the lamellar structure even at the hexagonal transition temperature of TPE. Ca might form a complex between CHEMS in apposed bilayers. These results are related to the possible biological function of acidic cholesterol esters in biomembranes.

This publication has 12 references indexed in Scilit: