This study investigates redox properties of fluorescein (FLSC), a fluorescent tracer with many applications in several areas, markedly in biochemical research and health care diagnosis, on glassy carbon electrode (GCE) at a wide interval of pH by using voltammetric techniques. Three peaks were observed at different potentials. The investigation revealed that FLSC is irreversibly electroxidized under a diffusion‐controled and pH–dependent process. The oxidation process in acid and physiological media occurs in two consecutive steps with formation of a main electroactive oxidation product in acid medium. Both oxidation steps involve the transfer of one electron and one proton, corresponding to the oxidation of phenolic groups with formation of ortho‐quinone derivatives, which are reversibly reduced to form catechol derivatives, and/or polymeric products. One electron and one proton are removed from the phenolic group at the position C6’ at the first step and at position C3’ at the second step. The diffusion coefficient of FLSC was assessed in pH=7.0 phosphate buffer (9.77×10−5 cm2 s−1). A differential pulse voltammetric method for determination of FLSC in physiological medium was also proposed.

Raltitrexede (RTX) is a folate analogue that belongs to the antimetabolites family and has antineoplastic activity correlated to inhibition of the thymidylate synthase (TS) enzyme. This study addresses the electrochemical characterization of RTX at a glassy carbon electrode, on a wide interval of pH, using voltammetric techniques. The interaction between RTX and double helix DNA (dsDNA) in physiological medium is also addressed, using a DNA‐electrochemical biosensor. A mechanism for the electrochemical oxidation of RTX is proposed. RTX is irreversibly electroxidized under a predominantly diffusion controlled and pH‐dependent process. The oxidation process in acid and physiological media occurs in two consecutive steps. The first oxidation step is pH‐independent and associated with the transfer of one electron of the nitro group at N10 position by releasing a methyl cation. The second oxidation step is pH‐dependent and occurs with the transfer of one electron and one proton from the carbon at C9 position, followed by a direct attack by a water molecule leading to an irreversible dissociation of an oxidation product of RTX. A strong interaction between RTX and dsDNA was revealed by using a multilayer dsDNA electrochemical biosensor. The experiments demonstrated that RTX interacts and binds to the dsDNA helix by different interaction modes, suggesting an intercalation in between the DNA base pairs leading to defects in its secondary structure.

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The electrochemical oxidation of 3‐nitro‐tyrosine (3‐NO2‐Tyr) was studied in aqueous media at metallic electrodes (platinum and gold), using voltammetric techniques. The interaction between 3‐NO2‐Tyr and double helix DNA (dsDNA) in a physiological medium was also investigated. Electro‐oxidation of 3‐NO2‐Tyr occurs in one single irreversible pH‐dependent step with the transfer of one electron and one proton from the phenolic group to the formation of radicals, which preferably dimerize, fouling the electrode surfaces. The differential pulse voltammetry and gel electrophoresis results clearly demonstrated a strong interaction of 3‐NO2‐Tyr with the dsDNA for the formation of a stable 3‐NO2‐Tyr‐dsDNA complex.

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