Conformational changes of 3,5,3'-triiodo L-thyronine induced by interactions with phospholipid: physiological speculations.

The conformational changes of 3,5,3'-triiodo L-thyronine induced by interaction with phospholipids were analyzed by Raman spectroscopy. The spectra were interpreted in terms of two conformers of this hormone in equilibrium in the lipid medium, depending on the orientation of the 3'-iodine with respect to the ring alpha. Theoretical geometry optimizations on both conformers in vacuo and in different solvents, together with the respective calculated energies support the experimental results. The presence of only one iodine atom in the phenolic ring allows assumption of a higher flexibility of 3,5,3'-triiodo L-thyronine and a better accommodation into the lipid medium compared to 3,5,3',5'-tetraiodo L-thyronine. The possible physiological implications of structural differences that appear in membrane models between 3,5,3'-triiodo L-thyronine and 3,5,3',5'-tetraiodo L-thyronine are discussed.