Rationalization of the relative hydrophobicity of some common bile acids by infrared and Raman spectroscopy

The analysis of some bile acids [lithocholic acid (LC), cholic acid (C), chenodeoxycholic acid (CDC), hyodeoxycholic acid (HDC), ursodeoxycholic acid (UDC), β-muricholic acid (β-MC)] by Raman and infrared spectroscopy reveals that hydrophobic bile acids (LC, CDC, C) have their 3α OH bonded by strong intermolecular interactions. Furthermore, the most hydrophobic bile acid (LC), which is practically insoluble in water at room temperature, may be directly related to a polymeric association of its molecules. The hydrophilic bile acids (HDC, UDC, β-MC) possess some free OH bonds. Generally, however, the carboxylic group is implied in a dimeric association. Infrared spectra of diluted bile acids in chloroform give further confirmation because intermolecular bonded line vanishes for the hydrophilic bile acids and remains for hydrophobic ones. Thus, Raman and infrared spectroscopy provide new tools for establishing a rational hydrophobicity/hydrophilicity scale of bile acids. © 1997 John Wiley & Sons, Inc. Biospectroscopy 3: 393–401, 1997