Triiodothyronine: the 3'iodine is proximal to the -ring in crystal structure conformation

The crystal and molecular structure of the thyroid hormone L-triiodothyronine has been determined by x-ray diffraction. The two phenyl rings are almost perpendicular to each other, the acute angle between their normals being 82 degrees. The 3'-iodine is situated proximal to the alpha-ring, rather than distal as inferred from chemical studies. Theoretical calculations indicate this proximal conformation to be energetically favored over the distal one.