Two-dimensional NMR study on the structures of micelles of sodium taurocholate

The aggregation behavior of sodium taurocholate (TC) in deuterium oxide without salt was investigated by one- and two-dimensional NMR spectroscopy. Analysis of the concentration dependence of the chemical shift suggests that TC forms a dimer and a pentamer. The equilibrium constants of dimerization and pentamerization are close to those already determined by chromatography in the presence of 154 mM sodium chloride. The structure of the dimer is estimated from the NOESY and ROESY spectra of a 8 mM TC solution and molecular mechanics calculations. The inter-proton distances calculated from the molecular mechanics structure are consistent with the NOE and ROE intensities, whereas those calculated from the X-ray crystal structure (hydrogenbonded structure) are inconsistent. The molecular mechanics structure is stabilized by hydrophobic interactions between the steroid nuclei and by reduced electrostatic repulsion between the sulfonate ions. The local structures of the pentamer are estimated on the basis of the ROESY spectrum of a 30 mM TC solution. The pentamer of TC is formed mainly by hydrophobic interactions. Thus, a novel NMR method in surfactant chemistry has provided the first step to resolve the 20-year debate about the structures of dimers and micelles of TC. This novel approach in surfactant chemistry will serve to estimate the structures of micelles of other natural and synthetic surfactants.