Complexes of human lactoferrin with vanadium in oxidation states +3, +4 and +5

Abstract

Complexes of human lactoferrin with vanadium in the +3, +4 and +5 oxidation states have been characterised, as biological models for vanadium–protein interactions, by UV difference spectroscopy. The results indicate that the metal binds to lactoferrin in the specific iron(III) binding sites with a metal: protein stoichiometry of 2 : 1. Electron spin resonance spectroscopy shows oxygen or hydrogen peroxide repidly oxidise vanadium(III)–lactoferrin to the vanadium(IV) complex which is then itself oxidised to vanadium(V)–lactoferrin. The metal-binding sites of the three vanadium–lactoferrin complexes have been modelled assuming a common protein–ligand donor set. The results show that the metal ion can be accommodated by the protein in the oxidation states +3, +4 and +5 as the V³⁺, VO²⁺ and VO₂⁺ ions respectively, the increase in V–O(oxo) co-ordination being compensated for by decreasing the V–O(carbonate) interaction.