H and ¹³C NMR Relaxation Studies of Molecular Dynamics of the Thyroid Hormones Thyroxine, 3,5,3‘-Triiodothyronine, and 3,5-Diiodothyronine

Abstract

1H and 13C NMR spin-lattice relaxation times and 13C{1H} 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 13C relaxation times, which are identical within experimental error for the two rings. The 13C 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 13C 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.

Structural changes induced by interactions between thyroid hormones and phospholipid membranes: a Raman Spectroscopy study

“…In addition, many of these affected bands show irregular contours that appear as better defined shoulders upon spectral subtraction of the DLPC spectrum. NMR experiments of T3 in methanol solution have shown the presence of two conformers of the hormone, coexisting in equilibrium . They are called T3p and T3d when the iodine atom in the ring‐β is in proximal or distal orientation to the ring‐α, respectively (see Fig.…”

“…The band corresponding to the antisymmetric ν(C α –I) (187 cm −1 in T3) is not shifted but shows a shoulder at ~175 cm −1 in T3/DLPC. The shoulders of the main symmetric and antisymmetric ν(C α –I) bands are interpreted as evidence of the presence of two conformers of T3, in agreement with published reports . The Raman spectrum of pure T4 shows two very strong bands at 221 and 192 cm −1 which are associated to the symmetric ν(C β –I) and ν(C α –I) modes, respectively.…”

Conformational Flexibility and Halogen Bonding in Thyroid Hormones and Their Metabolites

“…The conformational flexibility and internal dynamics in thyroid hormones in solution have been studied earlier by NMR spectroscopy. − Similar to the three-dimensional structures of thyroid hormones observed in the X-ray structures, these studies also indicated the roughly perpendicular arrangement of the two iodinated rings and the rapid movement of the phenolic ring about the diphenyl ether linkage. As discussed earlier, rotation of the phenolic ring about the O3–C1′ bond in T3 gives rise to the formation of proximal and distal conformers.…”

Conformational Dynamics of Thyroid Hormones by Variable Temperature Nuclear Magnetic Resonance: The Role of Side Chain Rotations and Cisoid/Transoid Interconversions

“…A simple calculation based on ring current effects suggests that the maximum possible chemical shift difference of the H2 and H6 protons in such a twist−skewed conformation is 0.4 ppm, and this is achieved when φ‘ = ±30°. However, this is not likely to be the factor responsible for the coalescence process observed in solution because interchange between symmetry-related skewed forms in solution is extremely rapid (as demonstrated by 13 C T 1 measurements) and will result in an averaged resonance for H2 and H6 at all accessible temperatures.…”