Enzymatic oxidation of dipyridamole in homogeneous and micellar solutions in the horseradish peroxidase–hydrogen peroxide system

Abstract

Enzymatic oxidation of dipyridamole (DIP) by horseradish peroxidase–hydrogen peroxide system (HRP–H 2O 2) in aqueous and micellar solutions was carried out. The reaction was monitored by optical absorption and fluorescence techniques. In aqueous solution at pH 7.0 and pH 9.0, the disappearance of the characteristic bands of DIP centered at 400 nm and 280 nm was observed. A new strong band at 260 nm is observed for the oxidation product (s) with shoulders at 322 nm and 390 nm. A non-fluorescent product is formed upon oxidation. In cationic cethyl trimethyl-1-ammonium chloride (CTAC) and zwitterionic 3-( N-hexadecyl- N, N-dimethylammonium) propane sulfonate (HPS) micellar solutions the same results are observed: three, well-defined, isosbestic points in the optical spectra suggest the transformation between two species. In anionic micellar sodium dodecylsulfate solution (SDS), the appearance of a new band centered around 506 nm was observed, associated to a solution color change from the usual yellow to deep blue/violet, characteristic of a radical species associated to the one-electron oxidation of DIP to its cation radical ( DIP· +), observed previously in electrochemical oxidation. Experiments of radical decay kinetics monitoring the absorbance change at 506 nm were performed and analyzed in the frame of a kinetic model taking into account the species both in homogeneous and micellar media. The reaction medium is composed of bulk solution, SDS micelle/solution interface and enzyme catalytic site (s). The variation of DIP· + concentration was analyzed assuming: (1) synthesis of DIP· + by HRP through one-electron oxidation; (2) decomposition of DIP· + by further one-electron oxidation; (3) direct two-electron oxidation of DIP by HRP; (4) bimolecular DIP· + disproportionation. The main results of the analysis are as follows: (1) kinetic data can be divided in two phases, an HRP active phase and another phase which proceeds in the absence of enzyme activity due to consumption of all H 2O 2; (2) the reactions of DIP· + formation, DIP· + decomposition and DIP two-electron oxidation are HRP concentration dependent; (3) since DIP· + formation constant seems to be overestimated, it is proposed that two-electron oxidation is another source of DIP· +, through the comproportionation reaction. Evidences for this reaction were also observed previously in electrochemical experiments; and (4) the kinetic analysis provides evidences that the bimolecular reaction of DIP· + takes place mainly in the absence of active HRP and in this phase the combination of, at least, two second-order kinetic processes is needed to model the experimental data. Our data suggest that HRP oxidizes DIP in general by a two-electron process or that the cation radical is very unstable so that the one-electron process is only detected in the presence of anionic surfactant, which stabilizes significantly the DIP· + intermediate.