Solubilisation of model membrane by DDAO surfactant - partitioning, permeabilisation and liposome-micelle transition

Solubilisation of model membranes of dioleoylphosphatidylcholine (DOPC) and DOPC-cholesterol (CHOL) induced by surfactant N,N-dimethyl-1-dodecanamine-N-oxide (DDAO) was studied. At the maintained pH similar to 7.5, the DDAO molecules are in their neutral state with respect to the pK similar to 5. Pore formation in lipid bilayer was studied by fluorescence probe leakage method. The changes in the size of lipid aggregates upon increasing DDAO concentration were followed turbidimetrically. Effective ratio R-e at different steps of the solubilisation process was determined. The molar partition coefficient of DDAO in case of the DOPC membrane is K-p = 2262 +/- 379, for DOPC-CHOL membrane K-p = 2092 +/- 594. Within the experimental error, the partition coefficient, as well as effective ratios R-e, are not considerably influenced when one third of DOPC molecules is substituted with CHOL (DOPC:CHOL = 2:1). Constituents of buffer (50 mmol/dm(3) PBS, 150 mmol/dm(3)NaCl) caused aggregation of DOPC and DOPC-CHOL unilamellar liposomes at zero and low DDAO concentration, as was shown by SANS, turbidimetry and DIC microscopy. After solubilisalion of bilayer structures by surfactant, mixed DOPC-DDAO and DOPC-CHOL-DDAO micelles with the shape of cylinders with elliptical cross section were detected.