Vesicle-cholesterol interactions: Effects of added cholesterol on gel-to-liquid crystal transitions in a phospholipid membrane and five dialkyl-based vesicles as monitored using DSC

Abstract

Differential scanning calorimetry (DSC) results are reported characterising the effects of added cholesterol (CHOL) on vesicle bilayer systems formed in aqueous systems by three sodium dialkylphosphates, (RO)₂PO₂⁻–Na⁺ where R = dodecyl (DDP), tetradecyl (DTP) and octadecyl (DOP), (ii) two dialkyldimethylammonium bromides R₂N⁺Me₂Br⁻ where R = hexadecyl (DHAB) and octadecyl (DOAB) and (iii) dimyristoylphosphatidylcholine (DMPC). The results are examined in terms of the effects of added CHOL on the melting temperature T_m and enthalpy change for the gel to liquid crystal transition together with the extent of co-operative structural changes on melting as measured by patch numbers. A key factor is the extent to which added CHOL changes the enthalpies of gel and liquid crystal phases. For DOAB, DDP, DTP and DHAB bilayers added CHOL lowers the enthalpies of both liquid crystal and gel phases, the impact on the liquid-crystal state being more dramatic. Added CHOL has no effect on the gel state for DOP but raises the enthalpy of the liquid-crystal state. In the case of DMPC added CHOL increases the enthalpy of the gel state with concomitant decrease in patch number indicating that melting of the bilayer involves more re-organisation than in the absence of CHOL. For all bilayer systems, the effects of CHOL on the enthalpies is more dramatic than the effects on the melting temperature T_m, pointing to the importance of compensation between enthalpy and entropy changes on melting. The impact of added CHOL can be understood in terms of the relative effects on the enthalpies of liquid crystal and gel states of the bilayers. Also important are the effects of CHOL on patch numbers, the latter indicating the extent of co-operative melting in the bilayers.