Voltammetric and spectroscopic studies on the interaction of pentoxifylline with cysteine in the presence and absence of UV irradiation

The interaction of pentoxifylline (PTX), a methylxanthine derivative drug, with cysteine on the hanging mercury drop electrode studied by square-wave voltammetry, cyclic voltammetry and UV-Vis spectroscopy at phosphate buffer (pH 7.4) in the presence and absence of UV irradiation. The addition of PTX to cysteine solution results in the decrease of peak currents of mercurous cysteine thiolate and the shift of its voltammetric reversible peak towards positive potentials. Electrochemical data indicated a 1 : 1 molecular complex formation of PTX with cysteine by means of electrostatic interaction and intermolecular forces. For the long UV irradiation times (25-85 min), voltammogram of the cysteine solution gave the reduction peaks of sulphonyl radical (-0.056V), disulfidic anion (-0.684V) and free cystine (-0.800V) due to probably the formation of thiyl radical. With adding of PTX to the UV irradiated cysteine solution, a new peak at -0.292V was observed. The peak at -0.292V was also obtained for the cysteine and PTX mixture under UV irradiation, but the peaks of sulphonyl radical and disulphidic anion were no seen. As a result, this irreversible peak at -0.292V may be assigned to the reduction of an electroactive species which is formed from PTX-cysteine complex under UV irradiation. PTX prevents the formation of thiyl radical and its further oxidation.