Oxidation of Aromatic Amino Acids by Ferrate(VI): Kinetics and Mechanisms

Amino acids and proteins are an essential part of all life and are found in both biological and environmental systems. Oxidation is a major degradation pathway for proteins, and understanding how this oxidation occurs by studying individual amino acids will allow for a better overall understanding of the structure and function of proteins. The aromatic amino acids tryptophan (Trp), histidine (His), phenylalanine (Phe), and tyrosine (Tyr) are highly prone to oxidation and are the major sites of oxidative stress to proteins. Understanding how oxidative stress affects these individual aromatic amino acids can provide insight into the overall chemistry of the oxidation of proteins.;Kinetic studies of the oxidation of the aromatic amino acids by ferrate(VI) (Fe(VI), FeO42-) have been completed across the pH range 2 -- 13. The reactions all are first-order with respect to Fe(VI) and the amino acids, with the overall reaction following second-order kinetics. The second-order rate constants show a pH dependence, with Trp and Tyr rates gradually increasing from basic to acidic media, while His and Phe showed the highest rate constants in the pH range 6 -- 8.;Electron paramagnetic resonance (EPR) experiments reveal that a radical species is produced in the reactions of Fe(VI) with Trp, Phe, and His, indicating an initial 1-electron transfer step in the reaction mechanism. The reaction mechanism for the oxidation of Trp has been proposed. The identification of oxidation products was carried out using liquid chromatography-mass spectrometry (LC-MS). N-formylkynurenine (NFK) was identified as the major product of Trp oxidation at pH 7.00, indicating that an oxygen-transfer mechanism is involved after the initial 1-electron transfer step. The origin of the oxygen that is transferred to form NFK was determined to come from both the solvent of the reaction as well as directly from Fe(VI) through isotopically labeled Fe18O42- studies. The final iron species has been identified as Fe(III) through the use of Mossbauer spectroscopy and oxygen evolution experiments. The formation of Fe(III) confirms the overall 3-electron transfer of the reaction mechanism in the oxidation of Trp by Fe(VI).