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.

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 T m of DMPC. Therefore, not only conventional DMPC, but also chain-deuterated d54-DMPC was used due to its significantly lower T m -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 T m and the decomposition of vesicles into bicelles is only possible if the lipid membrane entirely adopts the rigid phase state.   • A mixture of DMPC and the saponin β -aescin forms bicelles from 10 mol% aescin. • Dilution or temperature increase triggers growth of mixed DMPC-aescin aggregates. • The T-induced transition is fully reversible independent on the aescin content. • Decomposition of DMPC vesicles back to bicelles shows a pronounced hysteresis effect. • Usage of chain-deuterated DMPC directly evidences correlation with lipid's Tm. [ABSTRACT FROM AUTHOR]