Study on Micelle Formation of Bile Salt Using Nuclear Magnetic Resonance Spectroscopy

¹H-nuclear magnetic resonance (¹H-NMR) measurements can identify the specific protons that contribute to interactions between molecules. Using this technique, micelles formed by four bile salts: sodium taurocholate (NaTC), taurodeoxycholate (NaTDC), taurochenodeoxycholate (NaTCDC), and tauroursodeoxycholate (NaTUDC) were measured and compared in viewpoint of molecular interactions. Rotating-frame nuclear Overhauser effect and exchange spectroscopy (ROESY) analysis of the four bile salts showed differences with respect to the type of micelle formation. For all four bile salts, the key protons contributing to hydrophobic interactions were found to be the methyl protons at positions 18 and 19. The cross-peak patterns for the four bile salt species indicated pairs of characteristic proton depending on a bile salt species. The spin-lattice relaxation time (T₁) of the alkyl side-chain in NaTC was relatively long compared to that of the three other bile salts, even when the concentration was higher than the critical micelle concentration (cmc). This unique behavior indicates that the hydrophilic region of NaTC molecules is flexible within their micelles. Moreover, T₁ values for the typically hydrophobic methyl protons at sites C18 and C19 of NaTC were almost constant above the cmc. These results may suggest that NaTC micelles remain as small primary structures in solution unlike the three other bile salt molecules investigated in the study.