ISSN:
0749-1581
Keywords:
Thyroxine
;
Triiodothyronine
;
Variable-temperature
;
1H NMR
;
Activation barriers
;
Chemistry
;
Analytical Chemistry and Spectroscopy
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
1H NMR spectra at 300 and 400 MHz have been recorded for the thyroid hormones thyroxine (T4) and triiodothyronine (T3) over a range of temperatures from 175 to 298 K in MeOH-d4. Above 200 K the aromatic region of the NMR spectrum of T4 consists of two peaks, one for the H-2′, H-6′ protons and one for H-2,H-6. As the temperature is lowered each of these resonances first broadens and then splits into two peaks of equal intensity at low temperature. This is consistent with slowed exchange between two equally populated conformers. Similar behaviour is noted for T3, although in this case the spectrum is complicated by the presence of spin-spin splitting. Consideration of coalescence temperatures and a line-shape analysis yielded activation barriers in the range 35-38 kJ mol-1. It is concluded that the observed spectral changes are due to dynamic interconversion between proximal and distal forms of the hormones, and that the interconversion takes place by cooperative rotations around the bonds between each phenyl ring and the linking oxygen atom.
Additional Material:
7 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/mrc.1260260402
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