Analysis of tryptophan oxidation by fluorescence spectroscopy: Effect of metal-catalyzed oxidation and selected phenolic compounds

► Copper and iron promote tryptophan oxidation by the formation of free radicals. ► Oxidation systems catalyzed by Fe2+ had major tryptophan degradation potential. ► Structure and environment modify the effect of phenolics on tryptophan oxidation. ► Quercetin, rutin and gallic acid protected tryptophan in Fe2+/H2O2 systems.