Distal conformation of the thyroid hormone 3,5,3'-triiodo-L-thyronine

In the crystal structure of the thyroid hormone, 3,5,3'-triiodo-L-thyronine, the 3' iodine is observed in the distal position, away from the alaninebearing ring of the thyroid hormone. This result had been anticipated from stereochemical and biological activity studies. However, previous observations of structures in which the 3' iodine was proximal had cast some doubt on the stability of the 3' distal conformation. This observation suggests that the relative energies of the two conformers is similar and that perhaps the barrier to rotation is not as great as previously supposed since both the distal and proximal conformers have now been observed in the solid state.