APPLICATION OF ¹²⁹I MOSSBAUER EFFECT TO BIOLOGICAL SYSTEMS: STUDIES WITH HEME MODELS^*

Abstract

The Mossbauer effect associated with ¹²⁹I, ¹²⁵Te, and ⁵⁷Fe has been applied to investigate structural properties of the axial ligands in ferric-high spin hematoheme-I, hematoheme-Te, and hematoheme-histidine-iodide. The magnitude and sign of the ¹²⁹I quadrupole coupling constant (e ² qQ) and the isomer shift, as deduced from the Mossbauer effect spectra, are consistent with an axial, s-p hybridized bond of overwhelming σ-character. Identical coupling constant (e ² qQ(¹²⁷I) = -1826 ± 3 MHz) were measured for both heme-I and hematoheme-histidine-iodide. Implications of this result to the nature of ferric-high spin heme-histidine complexing are discussed. A stable heme ¹²⁵Te species formed in the electron-capture decay of heme-¹²⁵I is observed, and structural properties of this singly bonded telluride ligand are discussed.