CONFORMATIONAL DYNAMICS OF THYROID-HORMONES BY VARIABLE-TEMPERATURE NUCLEAR-MAGNETIC-RESONANCE - THE ROLE OF SIDE-CHAIN ROTATIONS AND CISOID TRANSOID INTERCONVERSIONS/

H-1 NMR spectra of the thyroid hormone thyroxine recorded at low tempe rature and high field show splitting into two peaks of the resonance d ue to the H2,6 protons of the inner (tyrosyl) ring. A single resonance is observed in 600 MHz spectra at temperatures above 185 K. An analys is of the line shape as a function of temperature shows that the coale scence phenomenon is due to an exchange process with a barrier of 37 k J mol(-1). This is identical to the barrier for coalescence of the H2' ,6' protons of the outer (phenolic) ring reported previously for the t hyroid hormones and their analogues. It is proposed that the separate peaks at low temperature are due to resonances for H2,6 in cisoid and transoid conformers which are populated in approximately equal populat ions. These two peaks are averaged resonances for the individual H2 an d H6 protons. Conversion of cisoid to transoid forms can occur via rot ation of either the alanyl side chain or the outer ring, from one face of the inner ring to the other. It is proposed that the latter proces s is the one responsible for the observed coalescence phenomenon. The barrier to rotation of the alanyl side chain is greater than or equal to 37 kJ mol(-1), which is significantly larger than has previously be en reported for Csp(2)-Csp(3) bonds in other Ph-CH2-X systems. The rec ent crystal structure of a hormone agonist bound to the ligand-binding domain of the rat thyroid hormone receptor (Wagner et al. Nature 1995 , 378, 690-697) shows the transoid form to be the bound conformation. The significant energy barrier to cisoid/transoid interconversion dete rmined in the current study combined with the tight fit of the hormone to its receptor suggests that interconversion between the forms canno t occur at the receptor site but that selection for the preferred boun d form occurs from the 50% population of the transoid form in solution .