Characterization of Lecithin-Taurodeoxycholate Mixed Micelles Using Small-Angle Neutron Scattering and Static and Dynamic Light Scattering

Abstract

We have used small-angle neutron scattering (SANS) to probe the structure and interparticle interactions of lecithin-taurodeoxycholate mixed micelles. The data are fit to a core-shell model that provides the micelle composition and dimensions. The effects on the scattering spectra of electrostatic and excluded-volume interactions are explored in terms of the decoupling approximation (Kotlarchyk, M.; Chen, S.-H. J. Chem. Phys. 1983Phys. ,79, 2461 and the random phase approximation (Shimada, T.; Doi, M.; Okano, K. J. Chem. Phys. 1988,88,28 15).We found the TDC-lecithin micelles are cylindrical particles with an average cross-sectional radius of 26.7 f 0.4 A. The coreshell structure is found to be an appropriate model for the highly hydrated micelles. The micelle length increases dramatically with an increase in added electrolyte, but not with decreasing concentration as previously reported. The SANS data analysis shows that particles in 0.05 M NaCl grow by less than 15% with a 3-fold decrease in the total surfactant concentration. This is in contrast to the simple interpretation of dynamic light scattering of the same samples that shows an apparent doubling of the micelle length with the same decrease in surfactant concentration. This discrepancy is attributed to neglect of the thermodynamic and hydrodynamic interactions in the analysis of the dynamic light scattering data. Corrections for the thermodynamic interactions are determined from the static data and applied to the interpretation of dynamic light scattering measurements. The strength of hydrodynamic and entanglement interactions is also discussed in relation to existing models for both semidilute polymer solutions and spherical particles.Lecithin and bile salt are the two major cholesterol-solubilizing agents found in gallbladder bile. Lecithin is a double-tailed zwitterionic surfactant well-known for its tendency to form bilayers in solution. Bile salts are derived from cholesterol, and the four fused rings of the steroid backbone impose a "planar" structure on the surfactant. One face of the bile salt molecule is hydrophobic, while the other face is hydrophilic because of the presence of hydroxyl groups. Bile salt species differ in the number and position of hydroxyl groups and conjugation of the short hydrophilic tail with taurine or glycine. Like other surfactants, most bile salts self-associate to form either globular or cylindrical micelles.14 In gallbladder and hepatic bile, lecithin and bile salt together will form mixed aggregates (micelles or vesicles) that solubilize cholesterol.The liver continuously produces hepatic bile, which is a dilute (3 g/dL) solution of lecithin, bile salt, and cholesterol. In hepatic bile the insoluble cholesterol is carried by unilamellar, lecithinrich vesicles.s When bile is not needed for digestion, it is stored in the gallbladder for future use. Water is extracted during storage, and the resulting high concentrations of bile salt drive a microstructural transition from vesicular aggregates to mixed micelles of le...

Characterization of colloidal structures during intestinal lipolysis using small-angle neutron scattering

“…This suggests that the molecules are likely not tightly packed, and that there are empty spaces within the micelle. In addition, the presence of high electrolyte concentrations in micellar solutions has been shown to generate highly hydrated micelles [48, 49], further contributing to loose packing. Thus, the presence of NaTDC and high electrolyte concentration in our experiments likely contribute to spacing between NaTDC and PC molecules within the micelles in this study, introducing complications in assessing molecular packing within the micelles.…”

Self-assembly in aqueous bile salt solutions

“…For example, the reversible formation of fibrils and, through the aggregation of these fibrils, tubules can be observed and result in a viscous or gel-like solution [73]. Furthermore, bile salts show a very rich and interesting phase behaviour in mixtures with other amphiphiles, a classical example are lipid-bile salt mixtures [74][75][76][77][78][79][80][81][82] which have important implications for the solubilization of proteins and other materials [82][83][84].…”

Small-angle scattering of interacting particles. II. Generalized indirect Fourier transformation under consideration of the effective structure factor for polydisperse systems

“…A numerical procedure to calculate the structure factor has been described by Maeda (1989). This method has been used to interpret neutron scattering data of micellar systems (Long et al, 1994). Recently, van der Schoot & Odijk (1990) derived an analytical expression, which is almost identical to the numerical results of Maeda (1991).…”

“…5, we present the results for monodisperse particles at three different volume fractions. The values for length and radius were chosen to be identical to the parameters reported in the publication by Long et al (1994) for suspensions of mixed micelles. In this work the authors used a numerical evaluation of the mean-®eld theory for the interpretation of smallangle light and neutron scattering data.…”