Date on Master's Thesis/Doctoral Dissertation

5-2013

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemistry

Committee Chair

Yappert, Marta C.

Author's Keywords

Bile salts; Salt concentration; NMR spectroscopy; pH; Sodium cholate; Micelles

Subject

Bile; Nuclear magnetic resonance; Surface active agents

Abstract

Bile salts are biosurfactants that aid in the digestion and absorption of lipids. Unlike most classical surfactants, they are facial amphiphiles with a rigid steroid backbone whose hydrophobic and hydrophilic faces are on opposite sides. As the concentration of bile salts increases, primary micelles are formed. Despite decades of studies of the molecular organization of these micelles, there is no agreement on their arrangement. To bridge this gap, one- and two-dimensional NMR studies of sodium cholate (NaCho) monomers and primary micelles were carried out. The experimental changes in chemical shift were interpreted with the aid of theoretical predictions. The observed trends and the presence of new through-space interactions observed upon micellization indicate that four (or six) monomers are arranged in an anti-parallel fashion. The top and bottom of the barrel-like micelles are held by ionic interactions and water-mediated hydrogen bonds. A cooperative hydrogen-bond ‘belt’ is formed with the hydroxyl groups in the central region of NaCho and surrounds the micelle. Our results point to the importance of both hydrophobic interactions and hydrogen bonding in the formation of micelles. Larger aggregates (secondary micelles) form at higher concentrations (> 50 mM). Little is known about their molecular arrangement. Our NMR studies enabled the postulation and partial validation of a model for these aggregates in which primary micelles are stacked together via ion-dipole and H-bonding interactions. The stacks interact with each other in a staggered fashion where the top/bottom of a primary micelle is in the vicinity of the central H-bond belt of its neighboring micelle. Both pH and salt concentration affect primary and secondary micelles of bile salts. We investigated the effect of increasing concentrations of NaCl, NH4Cl, CaCl2 and MgCl2 on both primary and secondary NaCho micelles. Due to its smaller charge density, NH4+ had the least impact because of its interactions with hydroxyl and carboxylate groups are weaker relative to those of the other cations. On the other hand, the higher charge density of Ca2+ and Mg2+ caused the greatest tightening (Mg2+) and even aggregation (Ca2+) as these cations interact with the electronegative moieties of NaCho more effectively.

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