Redox reactions of ortho-tyrosine and 3-nitro-tyrosine: A comparative study with para-tyrosine

The electrochemical behaviour of ortho-tyrosine (o-Tyr) and 3-nitro-tyrosine (3-NO2-Tyr) in aqueous media, over a wide pH interval, at glassy carbon electrode (GCE), was investigated and compared withpara-tyrosine (p-Tyr), using cyclic, differential pulse and square wave voltammetry, as well as electrochemical impedance spectroscopy. In general, the p- and o-Tyr undergo oxidation in a single irreversible step pH-dependent, with the transfer of one electron and one proton, from the phenolic group to formation of Tyr phenoxy radical (Tyr). However, while the p-Tyr radical preferably polymerizes, forming a resistive film on the GCE surface, the o-Tyr center dot reacts preferentially with water with formation of o- and p-quinone derivatives, that are adsorbed and reversibly reduced on the GCE surface. In relation the 3-NO2-Tyr its oxidation occurs, in general, in two irreversible steps. The first step is pH-dependent, while the second is pH-independent, indicating the absence of protons in the process. The first process correspond to the oxidation of the phenolic group to form 3-NO2-Tyr center dot, which reacts in different ways, polymerizing, forming a resistive film on the GCE surface and/or being directly electro-oxidized to a cationic product (second step). The voltammetric data also showed that the 3-NO2-Tyr phenol group is more difficult to oxidize when compared to p- and o-Tyr molecules. Moreover, unlike p-Tyr and o-Tyr that present no cathodic peak, 3-NO2-Tyr suffers in acid medium electro-reduction in a single irreversible step with formation of two electroactive products. Such processes were assigned to the reduction of the nitro group to form hydroxylamine and amine. Thus, is clearly demonstrated that the nitro group attached, as well as the phenolic group position at the Tyr molecule, strongly influence its redox properties. The redox mechanism of o-Tyr and 3-NO2-Tyr are presented and discussed.