Complementary Mössbauer and EPR Studies of Iron(III) in Diferric Human Serum Transferrin with Oxalate or Bicarbonate as Synergistic Anions

Abstract

The spectral and magnetic properties of iron(III) bound in the metal binding sites of human serum transferrin with oxalate or bicarbonate as synergistic anions has been studied with Mössbauer spectroscopy and electron paramagnetic resonance (EPR). The Mössbauer spectra of the iron(III) in diferric transferrin with oxalate have been described using a spin Hamiltonian with the values of the zero field splitting parameter, D = 0.55 ± 0.05 cm⁻¹, and the rhombicity of the crystal field, E/D = 0.045 ± 0.005. The EPR spectrum can be described with D = 0.58 cm⁻¹ and E/D = 0.057, using a g-strain model for the lineshape that is based on a Gaussian distribution of D and E/D with Gaussian widths σ(D) = 0.35 cm⁻¹ and σ(E/D) = 0.013, respectively. The rhombicity of the iron surroundings for the transferrin-oxalate complex is almost one order of magnitude smaller than for the bicarbonate complex and the zero field splitting parameter is twice as large in the oxalate as in the bicarbonate complex. We conclude that the crystal field symmetry of the iron site is almost tetragonal in the oxalate complex but rhombohedral in the bicarbonate complex, reflecting the different geometries of the oxalate and bicarbonate coordination. The isomer shift δ = 0.56 ± 0.01 mm s⁻¹ and the quadrupole splitting ΔE_Q = 0.2 ± 0.1 mm s⁻¹, on the other hand, are very close to the values found for the bicarbonate complex. No differences between the Mössbauer spectra of the two iron(III) ions in diferric transferrin with oxalate were found. The homogeneity of the diferric transferrin samples was controlled by capillary zone electrophoresis in the presence of urea.