Temperature dependence of the interaction of cholate and deoxycholate with fluid model membranes and their solubilization into mixed micelles

The interaction of bile salts with model membranes composed of soybean phosphatidylcholine (SPC) and synthetic 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) was investigated using high sensitivity isothermal titration calorimetry (ITC). The partitioning and incorporation of the bile salts sodium cholate (NaC) and sodium deoxycholate (NaDC) from an aqueous phase (pure water or 0.1 M NaCl) into fluid bilayer vesicles was studied as a function of temperature and ionic strength. The thermodynamic parameters of partitioning were determined with a model taking electrostatic interactions into account. In addition, the solubilization of SPC and POPC vesicles with NaC and NaDC as a function of temperature was also studied by ITC and the phase diagrams for the vesicle to micelle transition at two different temperatures were established. Unsaturated phospholipids require higher amounts of detergent to be transformed into micelles compared to saturated phospholipids. In addition, the width of the coexistence region of mixed micelles and mixed vesicles is larger for phosphatidylcholines with unsaturated chains. A comparison of NaDC with NaC shows the higher solubilization effectiveness of NaDC in agreement with its lower cmc. Furthermore, increasing the ionic strength decreases the amount of bile salt necessary for the formation of mixed micelles, which is also expected from the decrease of the cmc with ionic strength due to the shielding of the charges of the bile salts.     

Micellization of bile salts in a formamide solution: A gas liquid chromatography study

Sodium cholate and sodium deoxycholate dissolved in formamide were applied as stationary phases in gas chromatography. The critical micelle concentration of sodium cholate and deoxycholate in formamide was determined by surface tension measurements. The relation of retention times vs. concentration of bile salts was investigated for isomers of monoterpenes and xylenes. The enthalpy of binding of selected compounds with sodium cholate and sodium deoxycholate monomers and micelles was determined.     

Micellization of bile salts in aqueous medium: a fluorescence study

Owing to the physiological importance of the micellization process of bile salts, the critical micelle concentration (CMC) becomes a fundamental parameter in the evaluation of their biological activities. The present study suggests fluorescence probing, using 1,6-diphenylhexatriene (DPH), as a simple, convenient, sensitive and economic method for monitoring the micellization process of bile salts in aqueous medium. Three independent parameters: fluorescence intensity, anisotropy and lifetime of DPH have been employed successfully for determining the CMC of two bile salts, sodium deoxycholate (NaDC) and sodium cholate (NaC), in aqueous medium. The CMC values reported by all the above three parameters of DPH are found to be same and it is 16 mM for NaC and 6 mM for NaDC at 25 degrees C in unbuffered solution. The effect of temperature and ionic strength on the micellization process has also been investigated employing DPH as a fluorescent probe. Increasing temperature leads to the formation of fluffier micelles with less rigid interior for both NaC and NaDC. The micelle core of NaC is less perturbed by the presence of NaCl whereas in case of NaDC, the aggregates provide DPH a more nonpolar and rigid environment in presence of NaCl than that in absence of salt.     

Micellar Aggregation Behavior and Electrochemically Reversible Solubilization of a Redox-Active Nonionic Surfactant

For the purpose of studying the potential of a novel nonionic switchable surfactant, 11-ferrocenylundecyl polyoxyethylene ether (FPEG), applied to surfactant-enhanced remediation (SER), the surface properties and micelle solubilization behavior of FPEG were investigated with different inorganic salts. With the addition of inorganic salts (NaCl and CaCl₂), the critical micelle concentration (CMC) of FPEG dropped from 15 to 12 and 8 mg·L^?1, respectively, due to the salting-out effect on the alkyl chain. Thermodynamic parameters based on the CMCs indicated that micelle formation was an entropy-driven process. Dynamic light scattering measurements verified that these inorganic salts can decrease the hydrodynamic diameters (D _h) of the micelles. Solubilization experiments with three typical polycyclic aromatic hydrocarbons (PAHs) demonstrated that the system of FPEG with NaCl shows the highest solubilization ability, and the molar solubilization ratio and micelle–water partition coefficient (K _m ) values follow the order pyrene > phenanthrene > acenaphthene. After oxidation, PAHs can be released from the micelles through breaking up of the micelles, and the cumulative release efficiency of pyrene, phenanthrene and acenaphthene are 31.2, 42.8 and 44.6 %; the order of release efficiency is opposite to that of the reduced form for solubilization abilities. All the results suggest that the ferrocene-containing, redox-active surfactant FPEG has the potential to be recycled in SER technology through electrochemistry approaches.     

Effects of micelle properties on the conformation of oligocholates and importance of rigidity of foldamers

The conformation of a cholate hexamer with a clicked tether in between two tricholate units and pyrene groups at the chain ends was studied by fluorescence spectroscopy. In contrast to the parent cholate hexamer that folded in all micelles investigated, the folding of the clicked hexamer was highly dependent on the type of surfactant used to solubilize the compound. The clicked oligocholate folded in the Brij 35 micelle, possibly due to the latter's small size and strong internal hydrophobicity. The oligocholate formed intermolecular aggregates in SDS solutions below the CMC of the surfactant. The aggregates were dissociated by the SDS micelles but the individual oligocholate stayed unfolded. In Triton X-100 and sodium cholate solutions, the aggregated, unfolded, and folded oligocholates coexisted and gradual unfolding occurred with an increasing concentration of the surfactant. The conformation of the clicked oligocholate was sensitive to the nonideal mixing of ionic/nonionic micelles and to the unconventional aggregation of sodium cholate.     

Toluene solubilization induces different modes of mixed micelle growth

The effect of toluene solubilization on the size and mobility of Triton X100 (TX100) micelles and TX100/sodium dodecyl sulfate (SDS) mixed micelles was studied by turbidimetry, dynamic light scattering, and capillary electrophoresis. Micelle growth due to toluene solubilization was observed for both surfactant systems; however, two different modes of growth were seen. Mixed micelles in 0.1 M NaCl are spherical (apparent diameter d(app) = 8 nm) and remain so while taking up 3 mM toluene, with a volume increase per micelle of deltaV(m) = 50 nm3. In 0.5 M NaCl, the large d(app) of both nonionic and mixed micelles (14 and 24 nm, respectively) indicate ellipsoidal or rodlike shapes, and their large increases in d(app) upon addition of 3 mM toluene thus correspond to elongational growth, with the same deltaV(m) = 50 nm3. Further addition of toluene to TX100/SDS in 0.5 M NaCl results in a dramatic increase in micelle size followed by an unexpected bimodal size distribution. The addition of excess toluene leads to the formation of ca. 140 nm toluene droplets, stabilized mainly by monomers of the high critical micelle concentration surfactant, SDS. These microemulsions coexist with the smaller (20 nm) swollen mixed micelles.     

Measurement of bilirubin partition coefficients in bile salt micelle/aqueous buffer solutions by micellar electrokinetic chromatography

The partition coefficients for the distribution of bilirubin between aqueous phosphateborate buffer and cholic, taurocholic, taurodeoxycholic, and taurochenodeoxycholic micelles have been measured by micellar electrokinetic chromatography at pH 8.5. Determination of the partition coefficients required that the critical micelle concentration and partial specific volumes be determined for each bile salt. Critical micelle concentrations were slightly higher for the trihydroxy bile salts. Partial specific volumes of the bile salt micelles differed very little from each other, and for each bile salt they were constant over the concentration range studied, which was typically from slightly above the critical micelle concentration to 35 mM. Capacity factors were corrected for the effects of applied voltage by extrapolation of the capacity factor to zero applied volts. The free solution mobility of bilirubin, determined in the absence of bile salt, was also corrected for the effects of applied voltage. Plots of extrapolated capacity factor versus phase ratio yield the partition coefficient as the slope of a linear fit to the data. Partition coefficients for bilirubin were significantly higher for dihydroxy bile salts than for trihydroxy bile salts.     

Solubilization properties of aqueous solutions of alkyltrimethylammonium halides toward a water-insoluble dye

The solubility of a water-insoluble dye, Sudan Red B, in aqueous sodium halide solutions of tetradecyl-, cetyl-, and stearyltrimethylammonium halides has been measured at different surfactant and salt concentrations, and the dependence of solubilization properties on alkyl chain length has been discussed with reference to the micelle size and shape. At low ionic strengths where only spherical micelles exist, the solubilization power of micellar surfactant slightly increases with increasing the ionic strength, but it sharply increases at high ionic strengths above the threshold value of sphere-rod transition. However, the solubilization power becomes independent of the ionic strength, if their rodlike micelles are sufficiently long. The solubilization capacity increases linearly with increasing the molecular weight, almost independent of counterion species, but the rod-like micelle has a higher solubilization capacity than the spherical micelle. The solubilization capacity is larger for a surfactant with longer alkyl chain, indicating that the dye is solubilized more readily in a larger hydrophobic core. The solubilized dye is situated in a rodlike micelle of alkyltrimethylammonium halides, on average, 4.5–7.5 nm apart from each other.     

Bile salt-phospholipid aggregation at submicellar concentrations

The aggregation behavior of the bile salts taurodeoxycholate (NaTDC) and sodium cholate (NaC), are followed at concentrations below critical micelle concentrations (CMCs) using the environment sensitive, fluorescent-labeled phospholipid, 2-(6-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)hexanoyl-1-hexadecanoyl-sn-glycero-3-phosphocholine (NBD-C₆-HPC). A buffer solution containing NBD-C₆-HPC is titrated with increasing NaC or NaTDC and the fluorescence changes followed. Both bile salts induced fluorescence changes below their critical micelle concentration indicating the presence of a bile salt–phospholipid aggregate. A critical control experiment using 6-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino) hexanoic acid (NBD-X) shows that the bile salts are interacting with the longer, C16 hydrocarbon tail, not the NBD probe. The fluorescence curves were fitted to the Hill equation as a model for cooperative aggregation. The cooperativity model provides a minimum estimate for the number of bile salts to give maximal fluorescence. This number was calculated for NaC and NaTDC to have a minimum value of 2. A small aggregation number supports the existence of primary micellar aggregates at submicellar concentrations for bile salt–phospholipid aqueous solutions.     

Importance of Conjugation of the Bile Salt on the Mechanism of Lipolysis

We aim to advance the discussion on the significance of the conjugation of bile salts (BS) in our organism. We hypothesize that conjugation influences the rate of lipolysis. Since the rate of lipolysis is a compound parameter, we compare the effect of conjugation on four surface parameters, which contribute to the rate. Since deconjugation is due to gut microbiota, we hypothesize that microbiota may affect the rate of lipolysis. A meta-analysis of literature data of critical micelle concentration, β, aggregation number, and molar solubilization ratio has been performed for the first time. In addition, critical micelle concentration (CMC), interfacial tension, and lipolysis rate measurements were performed. It was found that the unconjugated BS in mixed micelles increases the antagonism between the BS, therefore, increasing the CMC. This correlated with the effect of unconjugated BS on the solubilization capacity of mixed micelles. The collected literature information indicates that the role of the BS and its conjugation in our organism is a key factor influencing the functioning of our organism, where too high levels of unconjugated BS may lead to malabsorption of fat-soluble nutrients. The experimental lipolysis results irrevocably showed that conjugation is a significant factor influencing the rate.     

Temperature effect on solubilization of n-alkylbenzenes into sodium cholate micelles

The solubilization of n-alkylbenzenes (benzene, toluene, ethylbenzene, n-propylbenzene, n-butylbenzene, n-pentylbenzene, n-hexylbenzene) into an aqueous micellar solution of sodium cholate was carried out. Solubilizate concentrations at equilibrium were determined spectrophotometrically at 293.2, 298.2, 303.2, 308.2, and 313.2 K. The first stepwise association constants (K(1)) between solubilizate monomers and vacant micelles were evaluated from the equilibrium concentrations and found to increase with increasing hydrophobicity of the solubilizate molecules. From the Gibbs energy change for solubilization at different micelle aggregation numbers and from the molecular structure of the solubilizates, the function of sodium cholate micelles as solubilizer was discussed. Enthalpy and entropy changes of solubilization were calculated from the temperature dependence of the K(1) values, and the solubilization was found to be enthalpy-driven for the solubilizates with shorter alkyl chains. The results obtained were also compared with those for conventional aliphatic micelles.     

Solubilization and precipitation of cholesterol in aqueous solution of bile salts and their mixtures

Solubilization of cholesterol by mixed micelles of sodium chenodeoxycholate with sodium ursodeoxycholate was investigated in carbonate-tetraborate buffer (Kolthoff) solution at pH 10 and 37°C. It was found that the mixing of the two bile salts gives a negatively synergetic effect on solubilization of cholesterol. The solubilizing power of bile salts for cholesterol was remarkably influenced with the change in mole fraction of sodium ursodeoxycholate (X _UDC).The behavior of bile salt solutions saturated with cholesterol was examined by measuring the surface tension. Two break points were observed in the curves of surface tension vs. concentration. The break points seem to correspond to a CMC in the absence of solubilized cholesterol and another CMC in the presence of solubilized cholesterol inside bile salt micelle.     

Solubilization of negatively charged DPPC/DPPG liposomes by bile salts

The interactions of the bile salts sodium cholate (NaC) and sodium deoxycholate (NaDC) in 0.1 M NaCl (pH 7.4) with membranes composed of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC), 1,2-dipalmitoyl-sn-glycero-3-phosphatidylglycerol (DPPG) and mixtures of DPPC and DPPG at molar ratios of 3:1 and 1:1 were studied by means of high-sensitivity isothermal titration calorimetry (ITC), dynamic light scattering (DLS), and differential scanning calorimetry (DSC). The partition coefficients and the transfer enthalpies for the incorporation of bile salt molecules into the phospholipid membranes were determined by ITC. The vesicle-to-micelle transition was investigated by ITC, DLS, and DSC. The phase boundaries for the saturation of the vesicles and their complete solubilization established by ITC were in general agreement with DLS data, but systematic differences could be seen due to the difference in detected physical quantities. Electrostatic repulsion effects between the negatively charged bile salt molecules and the negatively charged membrane surfaces are not limiting factors for the vesicle-to-micelle transition. The membrane packing constraints of the phospholipid molecules and the associated spontaneous curvature of the vesicles play the dominant role. DPPG vesicles are transformed by the bile salts into mixed micelles more easily or similarly compared to DPPC vesicles. The saturation of mixed DPPC/DPPG vesicles requires less bile salt, but to induce the solubilization of the liposomes, significantly higher amounts of bile salt are needed compared to the concentrations required for the solubilization of the pure phospholipid systems. The different solubilization behavior of DPPC/DPPG liposomes compared to the pure liposomes could be due to a specific "extraction" of DPPG into the mixed micelles in the coexistence region.     

Solubilization of triphenylamine, triphenylphosphine, triphenylphosphineoxide and triphenylmethanol in single and binary surfactant systems

Solubilization and co-solubilization of triphenyls (TPs) viz., triphenylphosphine (TPP), triphenylphosphineoxide (TPPO), triphenylamine (TPA) and triphenylmethanol (TPM) were studied in various single and binary surfactant systems at 25 °C using UV-visible spectroscopy and HPLC. The solubilization capacities of different micelles towards TPs were found to be a function of the nature and structure of solubilizates, locus of solubilization, size of micelles and the nature of interactions between the solubilizate and micelles. The effect of surfactant mixing on the solubilization of TPs was evaluated using the Regular Solution Approach (RSA). The solubility enhancement of TPs within mixed micelles relative to that observed in single surfactant systems was explained in light of the structural micellar changes associated with the mixing of ionic and non-ionic surfactants. Moreover, kinetics of oxidation of TPP by hydrogen peroxide investigated in these surfactant systems was found to be sensitive to the nature of micelle and the locus of solubilization of TPP within the micelles.     

Solubilization of bilirubin by cholate micelles. Spectroscopic and gel permeation studies

The interactions of bilirubin with bile salts have been studied using fluorescence, circular dichroism and¹H NMR methods. Enhancement of bilirubin fluorescence and induction of optical activity in bilirubin in the presence of cholate has been observed. Fluorescence enhancement is pronounced above the critical micelle concentration, while induced CD bands are detectable even in the premicellar region. Dehydrocholate and deoxycholate did not cause a fluorescence increase, but induced CD bands were observed for bilirubin in these cases. Gel permeation chromatography on Sephadex G-50 yielded a single bilirubin-cholate species at alkaline pH, while two species were obtained at neutral pH.¹H NMR and CD spectral characterizations of these complexes are reported. A 4∶1 cholate-bilirubin mixture has been analysed by difference (nuclear Overhauser effect) NOE spectroscopy. Observation of strong, negative NOE, both intermolecular and intramolecular leads to the conclusion that specific methyl groups on bilirubin and cholate are proximal in the mixed micelle.     

在胶束中竹红菌甲素的光物理特性

研究了竹红菌甲素在胶束水溶液中的增溶能力及其光物理特性。实验证明,竹红菌甲素可以增溶于非离子型(TX-100),阳离子型(TDPB)和阴离子型(SDS)胶束水溶液中,增溶能力的顺序为:TX-100>TDPB>SDS。观察到在胶束中竹红菌甲素的两种互变异构体比例变化,在TDPB胶束中在高居占度条件下生成了基态相迂复合物,吸收峰位λ_max630nm,其对应的激基缔合物在荧光谱中出现在λ_max670nm处。     

The solubilization of alcohols in micellar solutions

The specific conductivities of dodecyldimethylbenzylammonium bromide (C12BBr) have been determined in aqueous butanol and aqueous benzyl alcohol solutions in the temperature range of 5-40°C. From these data the temperature dependent critical micelle concentration (cmc) was determined. The molar fraction of alcohol in the micelle was estimated using the theory suggested by Motomura et al. for surfactant binary mixtures. The thermal properties such as standard Gibbs free energy, enthalpy and entropy of solubilization of alcohols in the micelles were estimated for the phase separation model. The change in heat capacity upon solubilization of alcohol in the micelle has been estimated form the above properties. This revised version was published online in July 2006 with corrections to the Cover Date.     

Appearance of pure fluorocarbon micelles surveyed by fluorescence quenching of amphiphilic quinoline derivatives in fluorocarbon and hydrocarbon surfactant mixtures

A halide-sensitive fluorescence probe was utilized to evaluate the miscibility of fluorocarbon and hydrocarbon surfactants in aqueous micellar systems. The fluorescence of 6-methoxy-N-1,1,2,2-tetrahydroheptadecafluorodecylquinolinium chloride, FC10MQ, was quenched by halide ions dissociated from the surfactant. The fluorescence in micellar solutions showed an initially rapid decay. This suggests that halide ions effectively quench FC10MQ fluorescence at the micellar surface. The subsequent slow decay corresponds to the quenching of FC10MQ fluorescence in the aqueous bulk phase by the free counterions. The Stern-Volmer plots for fluorescence quenching gave a distinct break at the critical micelle concentration of the cationic surfactants. The abrupt increase in fluorescence quenching is attributed to the solubilization of the probe in the micelles. The fluorescence quenching behavior provides direct information about the immiscibility of fluorocarbon and hydrocarbon species in micelles, and the results indicate that almost pure fluorocarbon micelles appear in surfactants mixtures.     

Electrophoretic mobility of the polymer-like micelles of tetradecyldimethylamine oxide hemihydrochloride

Effects of the surfactant concentration C_d and the NaCl concentration C_s on the electrophoretic mobilities U of the well-characterized polymer-like micelles have been investigated by the electrophoretic light scattering, using tetradecyldimethylamine oxide hemihydrochloride (C14DMAO·1/2HCl). At the high ionic strength of 0.1 mol kg⁻¹ NaCl, the electrophoretic mobilities were independent of C_d (5 mM < C_d < 100 mM), despite the concentration-dependent micelle growth of the polymer-like micelles. This suggests that the electrophoretic mobility of the polymer-like micelle at high ionic strengths is independent of the contour length (i.e., the molecular weight), as found on linear polyelectrolytes. Somewhat surprisingly, the entanglements of the polymer-like micelles gave small effect on the electrophoretic mobilities in the examined range of the surfactant concentration above an overlap concentration. The mobilities of the polymer-like micelle decreased with √C_s in a single exponential manner in the range of C_s from 0.02 to 0.3 mol kg⁻¹. It is suggested that the cylinder model can be applied to the electrophoretic mobilities of the polymer-like micelles at high ionic strengths (i.e. a free-draining behavior), since the persistence length of the polymer-like micelle (20 nm) is much larger than the Debye length at high ionic strength.     

Mixed Micellization of Dodecyldimethylamine Oxide with Bile Salts in Aqueous Solutions

Micellar behavior of dodecyldimethylamine oxide (DMDAO) with bile salts [sodium deoxycholate (NaDC) and sodium cholate (NaC)] with and without NaCl was studied by surface tension. Interaction parameters of the mixed micelles were estimated using Rubingh's theory. Strong synergism observed for each mixed system, which is a common feature shown by anionic-cationic mixtures. The mixed solutions remained clear even at equimolar ratio. Different behavior of the two bile salts is explained on the basis of their orientation in cationic micelles.