Structure of conjugated bile salt-fatty acid-monoglyceride mixed colloids:Studies by small-angle neutron scattering

The structures of particles found in isotropic phases of mixed surfactant s ystems consisting of conjugated bile salts and fatty lipids were assessed u sing small-angle neutron scattering. The conjugated bile salts were either cholylglycine or chenodeoxycholylglycine. The fatty lipids were mixtures of oleate and oleic acid either alone or with monoolein. The scattering data suggested that both particle interactions and polydispersity must be modele d in these systems. Particle interactions were modeled using the reduced me an spherical approximation and the decoupling approximation. Maximum entrop y was used to characterize the polydispersity. A self-consistent analysis o f the scattering was arrived at by making an initial estimate of particle s ize and shape using derivative-log and Guinier analysis and refining the es timates by analyzing the particle interactions and polydispersity and itera ting. The scattering at high total lipid concentrations was consistent with globular mixed micelles with repulsive electrostatic interactions. The glo bular mixed micelles in these systems were similar in size and shape to tho se observed previously in conjugated bile salt mixtures with either egg yol k phosphatidylcholine or monoolein. Solutions of cholylglycine with monoole in and oleate/oleic acid underwent a transition to vesicles at lower concen trations. This behavior was similar to those observed in conjugated bile sa lts with either egg yolk phosphatidylcholine or monoolein. Cholylglycine mi xtures with oleate/oleic showed somewhat different behavior at lower concen trations, since there was also evidence for coexistence of elongated and ta bletlike micelles. Despite these differences, there were sufficient similar ities in the particle morphologies of these and other conjugated bile salt- fatty lipid systems to suggest a common mode of self-assembly. These soluti ons are models for bile in the bilary system and intestine content during t riglyceride digestion; the common themes of self-assembly have implications for the physiology of lipid solubilization in bile as well as intestinal a bsorption of dietary lipids.