Determination of Atomic Local Order in Thyroid Hormones by Extended X-Ray Absorption Fine Structure {Exafs} For Radiation Dose Estimates

Abstract

Dosimetry from ¹²⁵I emitting Auger electrons in the follicles of the thyroid will improve when their complete interaction with the molecular structure about the Auger emitting atoms is known to Ångstrom distances. Extended x-ray absorption fine structure (EXAFS) can provide this information. EXAFS experiments gave intermolecular and intramolecular distance for pure solid L-thyroxine (T₄) (C₁₅H₁₁I₄NO₄) using a model based on crystalline T₄ hydrochloride monohydrate (C₁₅H₁₁I₄NO₄HCl.H₂O). For a solution of L-thyroxine the structure consists of the intramolecular distances found for T₄ and an additional shell of four carbon atoms distances of 2.01 Å. Atomic density functions from Fourier transformed EXAFS measurements are suitable for estimating very short range interactions of Auger electrons in thyroid thyroglobulin over a radial distance of 7.71 Å. Examination of density functions provided a plausible argument for the difference in biological effect between ¹³¹I and ¹²⁵I in the thyroid.