Thermal stability of amino acid-(tyrosine and tryptophan) coated magnetites

The thermal stability of two amino acid-(tyrosine and tryptophan) coated magnetite and their corresponding precursors, [Fe-2(III) Fe-II(Tyr)(8)]center dot 9H(2)O and [(Fe2FeII)-Fe-III(Trp)(2)(OH)(4)](NO3)(2)center dot 8H2O (where tyrosine=Tyr and tryptophan=Trp), was analyzed in comparison with free amino acids. The complexes present a lower thermal stability relative to the free ligand, due to the catalytic effect introduced by the iron cation and the presence of NO3- groups. The presence of NO3- group determines also a different degradation's stoichiometry of the amino acid anion comparative with the one expressed by the free ligand molecule. The amino acid bonded to magnetite decomposes in two steps, its presence inducing an increasing of gamma-Fe2O3-->alpha Fe2O3 conversion temperature.