Molecular conformation of ammonium 8-anilino-1-naphthalenesulfonate hemihydrate. A fluorescent probe for thyroxine binding to thyroxine binding globulin.

The crystal and molecular structure of the ammonium hemihydrate salt of the fluorescent dye, 8-anilino-1-naphthalenesulfonic acid (ANS), has been determined. There are two conformationally distinct molecules in the triclinic P1 lattice. The anilino nitrogen of one molecule has slightly distorted planar geometry, and the overall conformation of the molecule is similar to that observed for the potassium salt of the fluorescent dye 2-p-toluidinyl-6-naphthalenesulfonic acid (TNS). The anilino nitrogen of the other molecule has slightly distorted tetrahedral geometry and the overall conformation of the molecule is similar to that observed for the thyroid hormones T3 and T4. The observation of two distinct conformational modifications of ANS in this crystal structure determination has shed light on the conformational flexibility of the ANS molecule itself and on the mode by which its acts as a competitive inhibitor in thyroid hormone transport proteins and as a signal for hydrophobic areas in macromolecular systems.