Feature Article Reduction of Lapachones in Aqueous Media at a Glassy Carbon Electrode F.C.Abreu, a,b,c M.O.F.Goulart, b and A.M.OliveiraBrett* a a Departamento de QuÌmica, Faculdade de Cie √ncias e Tecnologia, Universidade de Coimbra, 3004-535 Coimbra, Portugal e-mail: brett@ci.uc.pt b Departamento de QuÌmica/CCEN, Universidade Federal de Alagoas, Campus A. C. Simo ƒes, Tabuleiro do Martins, 57072-970 ± Maceio ¬ ± Alagoas-Brasil c Departamento de QuÌmica Fundamental, Universidade Federal de Pernambuco, Recife, Pernambuco, Brasil Received: March 9, 2001 Final version: May 8, 2001 Abstract The electrochemical reduction of a-lapachone, b-lapachone, b-lapachone-sulfonic acid and 3-bromo-b-lapachone was studied by cyclic voltammetry, square-wave and differential pulse voltammetry in mixed ethanolic (20%) aqueous buffered media (pH 4.5) using a glassy carbon electrode. The reduction showed, for the ortho-quinones, well-defined pH-dependent reversible peaks corresponding to a mechanism involving the same number of electrons and protons, typical of quinones. For the para-quinone, at pH 4.5, an irreversible pair of peaks is evident, corresponding probably to an EC system. Disproportionation of the radical together with a coupled chemical reaction after the first electron transfer seems to occur. The chemical reaction is probably a dihydropyran ring cleavage, leading to intermediates, that are oxidized in its more stable forms, one of them very similar to the one presented by b-lapachone. The hydrogen- bonding-aided stabilization of the radicals generated during b-lapachone and derivatives reduction, after electron and proton capture confers more stability to the reduced ortho-lapachones in comparison to their reduced para-isomers. At pH 7.0, the reduction of a-lapachone is quasi-reversible. In all cases, the ortho-quinones are reduced easier than the para-isomers. The electrochemical method enables the study of the stability of these quinones after reduction, and allows their detection and quantification which confers to this methodology an extra advantage. For the soluble sulfonic acid the detection limit is 1.7  10 6 M. Keywords: b-Lapachone, a-Lapachone, Cathodic reduction, Interconversion, Ring cleavage, Cyclic voltammetry 1. Introduction b-Lapachoneisanaturallyoccurringquinone[1]thatcanbe alsoeasilysynthesizedbysulfuricacidtreatmentoflapachol or lomatiol [2]. It has been shown to have a wide range of biological activities, including antibacterial, antifungal and trypanocidal activities [3±6]. b-lapachone was considered the most active compound against bloodstream forms of T. cruzi, in vitro, but was completely inactive in vivo, differentlyfrom b-allyl-b-lapachonethatremainedeffective in suppressing trypomastigote infectivity, even in the presence of blood [7]. Its mechanism of action as antibacte- rial, cytotoxic and trypanocidal was related to its redox cycling nature, generating reactive oxygen species, which can damage DNA [5, 8 ± 14]. a-Lapachone does not have any anti-trypanosomal activity [6] and is a poor redox cycler [12]. The normal reduction potential of the two isomeric lapachones was obtained, by reduction of the latter com- pounds to their quinols and electrometrical titration with potassium ferricyanide, using an aqueous ethanolic solution [15]. The reduction potential of the para-quinone was lower than its isomer and the broad aspects into the chemistry of naphthoquinones, including the compounds under analysis, were described [16]. In general, ortho-quinones, like b-lapachone, have a better redox cycling ability than para- quinones [12]. The majority of the above mentioned bio- logical activities of b-lapachone is dependent on bioreduc- tion.Establishedrelationshipbetweentheeaseofreduction and biological activities [17], in anticancer [18] and in trypanocidal bioassays [19], shows the relevance of electro- chemicalstudiesastoolsforthecomprehensionofthedrugs× mechanismofactionagainstvariousdiseasesandprediction of biological activities. Correlation is not absolute and is easier to find in the case of ET-OS (electron transfer- oxidative stress) [17, 18]. The electrochemical approach is expected to produce valuable insight into the biological outcome of these compounds. In the present work, the electrochemical reduction of a-lapachone, b-lapachone, b-lapachone-3-sulfonic acid and 3-bromo-b-lapachone (Scheme 1) was studied, using a glassy carbon electrode. 2. Experimental The lapachones described in Scheme 1: a-lapachone (3,4- dihydro-2,2-dimethyl-2H-naphtho[2,3-b]pyrano-5,10-dione), b-lapachone (3,4-dihydro-2,2-dimethyl-2H-naphtho[1,2- b]pyran-5,6-dione), b-lapachone-3-sulfonic acid (3,4-dihy- dro-2,2-dimethyl-2H-naphtho[1,2-b]pyran-5,6-dione-3-sul- 29 Electroanalysis 2002, 14, No. 1 ¹WILEY-VCH Verlag GmbH, 69469 Weinheim, Germany, 2002 1040-0397/02/0101-0029 $ 17.50+.50/0