A tryptophan-substituted cholic acid : expanding the family of labelled biomolecules

The synthesis of a novel cholic acid derivative bearing in the C-3 position a residue of tryptophan linked through an amide bond is herein described. Acidic or basic conditions are needed for the solubilization of the derivative in water. In alkaline solutions the molecule shows a self-association similar to the one of its natural precursor leading to the formation of ellipsoidal micelles which does not involve significant Trp–Trp interactions. On the contrary, in acidic conditions strong interactions between the tryptophan moieties occur, leading to the formation of a gel at low temperature. These interactions are broken upon heating and small micelles similar to those observed at high pH are formed. In both cases, fluorescence spectra suggest a polar environment for the amino acid fluorophore not remarkably affected by the self-assembly.