seelig_titration calorimetry Lidia K

seelig_titration calorimetry Lidia K - 5224 J. Phys. Chem....

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Vesicle - Micelle Transformation of Phosphatidylcholine/Octyl- ± - D -glucopyranoside Mixtures As Detected with Titration Calorimetry ² Markus R. Wenk and Joachim Seelig* Department of Biophysical Chemistry, Biocenter of the Uni V ersity of Basel, Klingelbergstr. 70, CH-4056 Basel, Switzerland Recei V ed: March 3, 1997; In Final Form: April 16, 1997 X The vesicle - micelle transition of lipid bilayers induced by the addition of the nonionic detergent octyl- ± - 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 ) Kc D,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 ( 3M - 1 and Δ 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 Δ H 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 Δ H 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, Δ H D E , of the bilayer a micelle equilibrium. To a first approximation, Δ H 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 g 2.8. The calorimetric titrations can be used to construct a rather
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This note was uploaded on 09/19/2011 for the course CHM 6304 taught by Professor Richards during the Fall '08 term at University of Florida.

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seelig_titration calorimetry Lidia K - 5224 J. Phys. Chem....

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