Electrochimica Acta 54 (2009) 4353–4358 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta Voltammetric determination of trace quantities of 6-thioguanine based on the interaction with DNA at a mercury electrode Elham Mirmomtaz, Ali Asghar Ensafi ∗ Department of Chemistry, Isfahan University of Technology, Isfahan, 84156-83111, Iran article info Article history: Received 8 January 2009 Received in revised form 23 February 2009 Accepted 3 March 2009 Available online 17 March 2009 Keywords: 6-Thioguanine DNA Intercalation Electroanalytical method Clinical analysis abstract Herein, a sensitive square wave voltammetric (SWV) method is described for the quantitative determi- nation of an anticancer drug, 6-thioguanine (6-TG). The interaction of 6-TG with double stranded DNA (ds-DNA) in the solution phase resulted in a well amplified SWV response at the surface of hanging mer- cury dropping electrode (HMDE). Accumulation and stripping steps were made in the sample medium conditioned with acetate buffer (pH 4.8). Optimized conditions for the accumulation step included the deposition potential at -0.10 V, a deposition time of 30 s, a frequency of 50 Hz, a pulse amplitude of 20 mV, and a step potential of 7 mV. In the solution containing 2.0 mg L -1 ds-DNA, determination was performed within a wide concentration range of 2.4 × 10 -9 to 1.8 × 10 -5 mol L -1 , and a detection limit of 2.1 nmol L -1 6-TG. An overall conclusion was that the intercalation of 6-TG into ds-DNA in a solution medium of the acetate buffer is a possible reason for the observed behavior. The method was applied for the determination of 6-TG in 6-thioguanine tablets and spiked blood serum samples. No statistically sig- nificant differences were observed between the expected and obtained concentrations. The new method is sufficiently sensitive to detect ultra trace amounts of 6-TG content. © 2009 Elsevier Ltd. All rights reserved. 1. Introduction Chemically modified bases are frequently studied for their numerous pharmacological, biochemical, and biological capabil- ities. It is well known that thiopurines inhibit the synthesis of DNA and RNA and have been used successfully in the treatment of acute leukemia. 6-Thioguanine (6-TG), 2-amino-7H-purine-6-thiol (Scheme 1) is one of the most important anticancer therapeutic agents used in the clinical treatment of acute childhood lym- phoblastic leukemia, inflammatory bowel disease, Crohn’s disease, AIDS, and some other pathology [1,2]. Several methods have been reported for the determination of 6-TG using voltammetry [2], fluorimetric [3], luminescence [4], and HPLC [5–9]. However, these methods failed to measure low quantities of 6-TG below 45.0 nmol L -1 . One important limitation with HPLC or spectrometric techniques is that 6-TG lacks suffi- cient UV absorption, thus requiring a pre- or post-column derivative procedure that obviously results in increased costs and analytical complexity. In addition, most of these methods are time-consuming and not sensitive enough to allow determination of low quantities of 6-TG in small aliquots of the sample while they also depend on expensive tools for screening. Electrochemical methods have the ∗ Corresponding author. Fax: +98 311 3912350. E-mail address: Ensafi@cc.iut.ac.ir (A.A. Ensafi). advantage of far shorter analysis time compared to spectrometric and chromatographic methods. The therapeutic effects of anticancer compounds such as 6-TG are believed to be related to the sulfur atom incorporation into the DNA [10–15]. Thio-bases influence the structure of DNA, although no clear understanding of such changes at the molecular level is yet available. Therefore, investigation of the interactions between DNA and these anticancer drugs is of great importance both in bio- chemistry and in medical research [16–19] to develop a powerful tool for the recognition and monitoring of these drugs. Nucleic acids can add new and unique dimensions of specificity to the arsenal of electrochemical biosensors, and are expected to play a major role in future from the point of analysis. It follows then that thanks to its rapidity, high sensitivity, and low cost, electrochemical DNA sensing could be a promising technique for the analysis of anticancer com- pounds. These drugs can be quantified by investigating the changes in the analytical signals after their interactions with DNA. Further- more, the changes in DNA or drug signals provide good evidence for the interaction mechanism to be elucidated. Currently, electro- chemical methods are being used to study the interactions between DNA and anticancer molecules [20–23]. A number of researchers have investigated anticancer drug–DNA interactions on different types of electrodes such as gold, carbon paste, and platinum. A self-assembled monolayer of 6-TG supported on Hg has been extensively studied with various electrochemical techniques [24–30]. Recently, the surface electrochemical method 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.03.004