H-1 and C-13 NMR relaxation studies of molecular dynamics of the thyroid hormones thyroxine, 3,5,3'-triiodothyronine, and 3,5-diiodothyronine

H-1 and C-13 NMR spin-lattice relaxation times and C-13{H-1} nuclear Overhauser enhancement factors have been measured for the thyroid hormones thyroxine, 3,5,3'-triiodothyronine and 3,5-diiodothyronine, with the aim of determining the internal molecular dynamics in these molecules. Spin-lattice relaxation times of protons on the two aromatic rings of these hormones show remarkable differences, with values for the hydroxyl-bearing ring being a factor of 4-12 times larger than those for the alanyl-bearing ring. This difference is not mirrored in the C-13 relaxation times, which are identical within experimental error for the two rings. The C-13 data show that the mobility of the two rings is similar, and therefore the difference in proton spin-lattice relaxation times arises because the protons of the alanyl-bearing ring are efficiently relaxed by interactions with neighboring protons on the side chain. Quantitative analysis of the C-13 relaxation data shows that there must be a significant degree of internal flexibility in the thyroid hormone molecules. The NMR data suggest that in methanol the molecules tumble with an overall correlation time of approximately 0.35 ns, but that rapid internal motion (in the form of jumps between two stable conformations) occurs on a 30-fold faster time scale. When combined with previous variable temperature NMR studies that show interconversion between proximal and distal forms of the outer ring on the microsecond time scale the results provide a complete description of the conformations and both fast and slow internal motions in the thyroid hormones. The findings suggest that modeling studies of thyroid hormone interactions with receptor proteins should take into account the possibility that these internal motions are present. In effect, the thyroid hormones may likely populate a larger range of conformations in the bound state than might be inferred from just the lowest energy forms seen in the crystal and solution states.