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 systems consisting of conjugated bile salts and fatty lipids were assessed using 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 modeled in these systems. Particle interactions were modeled using the reduced mean spherical approximation and the decoupling approximation. Maximum entropy was used to characterize the polydispersity. A self-consistent analysis of the scattering was arrived at by making an initial estimate of particle size and shape using derivative-log and Guinier analysis and refining the estimates by analyzing the particle interactions and polydispersity and iterating. The scattering at high total lipid concentrations was consistent with globular mixed micelles with repulsive electrostatic interactions. The globular mixed micelles in these systems were similar in size and shape to those observed previously in conjugated bile salt mixtures with either egg yolk phosphatidylcholine or monoolein. Solutions of cholylglycine with monoolein and oleate/oleic acid underwent a transition to vesicles at lower concentrations. This behavior was similar to those observed in conjugated bile salts with either egg yolk phosphatidylcholine or monoolein. Cholylglycine mixtures with oleate/oleic showed somewhat different behavior at lower concentrations, since there was also evidence for coexistence of elongated and tabletlike micelles. Despite these differences, there were sufficient similarities in the particle morphologies of these and other conjugated bile salt-fatty lipid systems to suggest a common mode of self-assembly. These solutions are models for bile in the bilary system and intestine content during triglyceride digestion; the common themes of self-assembly have implications for the physiology of lipid solubilization in bile as well as intestinal absorption of dietary lipids.