Self-association of sodium cholate in isotonic saline solutions

Micelle formation of bile salts and zwitterionic derivative as studied by two-dimensional NMR spectroscopy

The self-association of sodium taurodeoxycholate (NaTDC) and a zwitterionic derivative of cholic acid (CHAPS) in deuterium oxide was investigated by one- and two-dimensional nuclear magnetic resonance spectroscopy (NMR) spectroscopy. Analysis of the concentration dependence of the chemical shifts of several protons suggested that NaTDC and CHAPS form nonamers and heptamers, respectively, as well as dimer. The equilibrium constants of dimerization and the micellar aggregation numbers are close to the literature values. From the intensities of intermolecular cross-peaks in the nuclear Overhauser effect spectroscopy (NOESY) and rotating frame nuclear Overhauser effect spectroscopy (ROESY) spectra of NaTDC and CHAPS micellar solutions, partial structures of their micelles were estimated. The CHAPS micelle consists mainly of the back-to-back association, similarly to taurocholate (NaTC). However, the NaTDC micelle consists of the back-to-face association, because the face of NaTDC is rather hydrophobic. Furthermore, the back of bile molecules forms a convex plane and the face forms a concave plane. The back-to-face structure of NaTDC will be stabilized by a close contact between these planes. The chemical shift changes of several protons of CHAPS and NaTC in the micellar state are close to each other, but are different from those of NaTDC. This finding is consistent with the difference in their micellar structures.

Photoproduct selectivity in reactions involving singlet and triplet excited states within bile salt micelles

Generally, photochemical reactions tend to give more than one product. For such reactions to be useful one should be able to control them to yield a single product. Of the many approaches used in this context, the use of reaction media with features different from those of isotropic solutions has been very effective. We provide results of our studies on four reactions within bile salt micelles (cholic acid and deoxycholic acid). These four reactions involve homolytic cleavage of a C-C or C-O bond to yield either a singlet or triplet radical pair. The bile salt micelles control the rotational and translational mobilities of the radical pair, resulting in photoproduct selectivity. The dynamic nature of the bile salt micelles results in differential effects on the singlet and triplet radical pairs.

Spectrophotometric determination of the critical micellar concentration of bile salts using bilirubin monoglucuronide as a micellar probe. Utility of derivative spectroscopy

We have developed a simple biologically non-invasive method for determining the critical micellar concentration (CMC) of bile salts using pure naturally occurring bilirubin IX alpha monoglucuronide (BMG), an important bile pigment present in virtually all mammalian biles. This methodology employs visible absorbance spectroscopy of BMG in bile salts over a range of bile salt concentrations that include the reported CMC. Using 100 microM-BMG in 0.4 M-imidazole buffer at pH 7.8, we calculated that the CMC for sodium taurochenodeoxycholate is between 2.5 and 3.0 mM based on: (1) an abrupt change in lambda max. in this concentration range, (2) a precipitous decrease in the amplitude of the absorbance shoulder at 450 nm, (3) a sudden decrease in the second derivative absorbance of BMG at 400 nm and an increase in absorbance at 470 nm, (4) a sharp change in the 4th derivative absorbance at 375 and 395 nm. In contrast, sodium taurocholate, a bile salt that reportedly does not have a CMC but continuously self-associates over a wide concentration range, exhibited none of these changes. The use of derivative spectroscopy enhances the ability to detect the CMC changes and also indicates the number of BMG species in solution and their relative energy states.

A 1H-NMR study of bilirubin IX alpha solubilization by cholate micelles: application of nuclear Overhauser effects

The solubilization of bilirubin IX alpha in aqueous solution by sodium cholate micelles has been examined by 270 MHz 1H-NMR spectroscopy. Incorporation of bilirubin into the micelles is accompanied by specific shifts of bilirubin vinyl and bridgehead protons and the C18 and C19 methyl groups of the steroid. The observed chemical shifts show a monotonic concentration dependence suggesting that changes in aggregation size are continuous. Nuclear Overhauser effects (NOE) have been shown to be a useful probe or micellization. A 4:1 cholate/bilirubin mixture has been investigated by difference NOE spectroscopy. The observation of intermolecular nuclear Overhauser effects between peripheral protons of bilirubin and cholate are diagnostic of spatially proximate groups. Inter-cholate nuclear Overhauser effects increase in magnitude upon bilirubin incorporation suggesting closer packing of steroid molecules on solubilization of the pigment. Intramolecular nuclear Overhauser effects observed for solubilized bilirubin are consistent with a compact intramolecularly hydrogen-bonded conformation resembling that determined for bilirubin in the solid state.