Zero-Field Splittings of Iron Complexes of Transferrins

The zero-field splittings in Fe(III) ions specifically bound to human transferrin, human and bovine lactoferrin, and hen conalbumin (ovotransferrin) have been measured by studying the temperature dependence of the signal amplitudes near g' = 4.3. In each case, the zero-field splitting was in the range 1.5 to 1.8°K and the associated spin Hamiltonian parameter D was in the range of 0.10 cm-1 to 0.11 cm-1. When nitrilotriacetate was substituted for the bicarbonate ordinarily occupying the anion-binding site of transferrin, the zero-field splitting increased to 3.2°K. Whether this is due to a change in geometry or a change in ligands at the metalbinding site is not yet clear.