Decomposition of mixed DMPC-aescin vesicles to bicelles is linked to the lipid's main phase transition: A direct evidence by using chain-deuterated lipid

Abstract

This work investigates the conversion of bicelles into larger sheets or closed vesicles upon dilution and temperature increase for a system composed of the phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and the saponin aescin. Due to its peculiar amphiphilic character, aescin is able to decompose DMPC bilayers into smaller, rim-stabilized bicelles. Aspects of the transition process are analyzed in an aescin content- and temperature-dependent manner by photon correlation spectroscopy (PCS), turbidimetry and small-angle neutron scattering (SANS). Both the conversion of bicelles into vesicles induced by temperature increase and the decomposition process upon cooling are presumably related to the main phase transition temperature Tm of DMPC. Therefore, not only conventional DMPC, but also chain-deuterated d54-DMPC was used due to its significantly lower Tm-value compared to the conventional DMPC. It will be demonstrated that the reconversion of vesicle structures (present at low aescin content) into bicelles shows a strong hysteresis effect whereas this is not observed for the reconversion at high aescin amounts, at which for high temperature still bicelle structures are present. The results indicate formation of a trapped state, correlated with the lipid's Tm and the decomposition of vesicles into bicelles is only possible if the lipid membrane entirely adopts the rigid phase state.