The crystal structure of a major metabolite of thyroid hormone: 3,3',5'-triiodo-L-thyronine.

Two independent conformations of the thyroinactive thyroid hormone metabolite, 3,3',5'-triiodo-L-thyronine (rT3) were determined by X-ray diffraction methods. The conformations show significant difference in the lettering geometry when compared with those of the thyroactive thyroxine (T4) and 3,5,3'-triiodo-L-thyronine (T3). The diphenyl ether conformation of the two conformers of rT3 is an anti-skewed one, in which the torsion angles, phi (C5-C4-O4-Cl') are 8 degrees and -6 degrees, and phi' (C4-O4-Cl'-O6') are 86 degrees and 87 degrees. This conformation is in contrast to a twist-skewed one of T4 and T3. The difference in the binding abilities between T4, T3 and rT3 to thyroxine binding carrier proteins in serum or to a nuclear receptor protein may be explained by the characteristic solid-state conformations of these metabolites.