Vesicle-micelle transformation of phosphatidylcholine/octyl-b-D-glucopyranoside mixtures as detected with titration calorimetry

The vesicle-micelle transition of lipid bilayers induced by the addition of the nonionic detergent octyl-â-d-glucopyranoside (OG) was studied with high-sensitivity titration calorimetry and spectroscopic methods. Sonified phospholipid vesicles composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) were titrated into OG solutions with concentrations between 15 and 22 mM. The critical OG concentration for bilayer micellization was determined as c*D) 15.4 ( 0.3 mM whereas the critical micellar concentration of pure OG was ccmc) 23.7 mM (in buffer). All calorimetric titrations were performed below the cmc of pure OG solutions. The titration curves can be explained by a superposition of three processes, namely (i) a partitioning of surfactant molecules into the lipid membrane, (ii) a membrane micellization, and (iii) a membrane re-formation. The basic process is a partitioning of OG molecules into the phospholipid vesicles which occurs during the whole titration experiment. It can be described according to Xb) KcD,f where Xb is the degree of binding (effective surfactant-to-lipid ratio) and cD,f is the equilibrium detergent concentration free in solution. The thermodynamic parameters were determined as K) 88 ( 3 M-1 and ¢H°D) 1.7 ( 0.4 kcal/mol for the transfer of OG from the aqueous phase to the membrane (15 mM e c°D e 22 mM). OG partitioning entails a bilayer micellization in the initial phase of the lipid-into-detergent titration experiment. Bilayer micellization is an endothermic process with ¢Hmic) +1.85 ( 0.1 kcal/mol lipid. With continuous addition of lipid