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

The vesicle - micelle transition of lipid bilayers induced by the addition of the nonionic detergent octyl-β-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 c_cmc = 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 X_b = K_cD,f where X_b is the degree of binding (effective surfactant-to-lipid ratio) and c_D,f is the equilibrium detergent concentration free in solution. The thermodynamic parameters were determined as K = 88 ± 3 M^-1 and ΔH_D° = 1.7 ± 0.4 kcal/mol for the transfer of OG from the aqueous phase to the membrane (15 mM ≤ c_D° ≤ 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 ΔH_mic = +1.85 ± 0.1 kcal/mol lipid. With continuous addition of lipid, bilayer micellization comes to a halt and is replaced by the reverse process. The demicellization process is exothermic with ΔH_demic = -1.84 ± 0.1 kcal/mol lipid. Micellization and demicellization are thus symmetrical processes. Once the free OG concentration falls below the critical limit of c_D* = 15.4 mM, the titration curves follow the predictions of the partition model. A quantitative model is proposed to describe the excess heat, ΔH̄_D^E, of the bilayer ⇄ micelle equilibrium. To a first approximation, ΔH̄_D^E, follows the symmetrical mixing behavior of two closely related nonpolar liquids. Phosphorus-31 nuclear magnetic resonance and right-angle light scattering were used to monitor the structural changes of the OG/POPC mixtures. They confirm a phase boundary between the bilayer phase and the bilayer/micelle coexistence phase at c_D* = 15.4 ± 0.3 mM and X_b = 1.36 ± 0.04. The phase boundary between the bilayer/micelle coexistence phase and the pure micellar phase is found for X_b ≥ 2.8. The calorimetric titrations can be used to construct a rather precise phase diagram of the POPC/OG system as the discontinuities of the titration curves define the critical concentrations of detergent and lipid at the phase boundaries.