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

The suitability of using fluorescence spectroscopy to assess tryptophan (TRYP) depletion by quantifying the loss of natural TRYP fluorescence during metal-catalyzed oxidation (MCO) was assessed. To analyze the effect of the oxidation conditions, N-acetyl-l-tryptophanamide (NATA) solutions were oxidized in the presence of Cu2+ and Fe2+ (40 and 80 μM), with and without 0.8 mM H2O2. Also, the effect of selected phenolic compounds (PhC), namely, catechin, genistein, quercetin, rutin, gallic acid and chlorogenic acid (0.08, 0.8 and 8.0 μM), on TRYP MCO was studied. Oxidation systems catalyzed by Fe2+ had a major oxidative potential as a result of a higher amount of free radicals formed through the Fenton reaction. PhC could exert both antioxidant and pro-oxidant effects depending on their concentration, chemical structure of the PhC and the oxidation system. In general, all PhC acted as pro-oxidant agents at the highest dose. The major pro-oxidant effect was displayed by gallic and chlorogenic acid in the presence of Fe2+, Cu2+ or Cu2+/H2O2. Quercetin, rutin and gallic acid showed antioxidant effect against TRYP oxidation at low concentrations in Fe2+/H2O2 systems. Plausible mechanisms for the antioxidant and pro-oxidant effects of PhC on TRYP oxidation are discussed in the present article. The antioxidant efficiency of PhC or PhC-rich extracts must be studied before being used as functional food ingredients.