The classic analysis of metal transfer between ligands suggests that the metal-binding ligand (M- Ll) and the uncomplexed ligand (L2) form a mixed complex (L2-M-L1), and that transfer is effected with the dissociation of this complex to L2-M and LI. Spectroscopic data suggested that such mixed complexes formed when pyrophosphate-Fe and acetohydroxamate-Fe were added to apotransferrin: the initial species had a different absorbance maximum than the final transferrin-iron complex. We now show that similar spectroscopic changes are seen when free ferrous iron or iron liganded to ATP, citrate or nitrilotriacetate are added to apotransferrin. The evolving spectrum on the addition of iron to apotransferrin may thus reflect iron binding per se rather than the formation of a mixed ligand complex.
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