Rifampicin, 3-[[4-methyl-1-piperazinyl)-imino]-methyl]-rifamycin SV, (RIF) (Fig. 1a) is a semi-synthetic derivative of Rifamycin SV.1,2) It can be used alone or in combination with other drugs, such as isoniazid (INH) and pyrazinamide, in treatment of tuberculosis, leprosy and other infectious diseases specially those resulting from AIDS. The high occurrence of tuberculosis in HIV infected subjects makes the management of HIV treatment complex. RIF is a very active antituberculosis drug that accelerates the metabolism of protease inhibitors. 3,4) Due to the increasing necessity to monitor plasma concentrations in HIV patients with tuberculosis, different methods such as spectroscopy, 5-9) fluorometry, [10][11][12] gas chromatography, 13) polarography, 14) amperometry 15) and high performance liquid chromatography [16][17][18][19][20][21] have been developed to measure RIF alone or in the presence of INH. These methods are expensive and need more expertise in experimentation in comparison with spectrophotometry.Spectrohotometric techniques continue to be the most preferred method for routine analytical work due to their simplicity and reasonable sensitivity with significant economical advantages. On the other hand, it is well known that p-benzoquinones such as 7,7,7,8-tetracyanoquinodimethane (TCNQ), 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and 2,3,5,6-Tetrachloro-1,4-benzoquinone (p-chloranil) as pelectron acceptors (Fig. 1b) often form highly colored electron-donor-acceptor (EDA) or charge transfer (CT) complexes with various donors which provides the possibility of determination of drugs by spectrophotometric methods.In order to develop a new method for determination of RIF in pure and pharmaceutical forms as well as characterize the behavior of RIF in EDA chemistry, we investigated spectrophotometrically the reactions of RIF with organic p-acceptors DDQ, TCNQ and p-chloranil. The absence of an extraction step in these assays and the efficiency of the method offer the ease and rapidity desired for measurement of RIF. Also, an extractive-spectrophotometric method based on the reaction of RIF with iron(III) was developed. The results were compared with other similar methods. The reaction pathways were investigated by IR and 1 H-NMR spectra. Results and DiscussionRIF was found to yield intense colors with p-acceptors in acetonitrile solution, most probably due to the CT complexation of RIF as n-electron donor with DDQ, TCNQ and pchloranil as p-electron acceptors. Figure 2 shows the absorption spectra of RIF, DDQ, TCNQ and p-chloranil and their complexes in acetonitrile. Acetonitrile was found to be an ideal solvent for the formation of charge transfer (CT) complex. Methanol, dichloromethane, chloroform and mixtures of methanol-acetonitrile were examined and produced lower absorbance reading. Acetonitrile was chosen as the best solvent due to intensely colored CT complex with high molar absorptivity values and more reproducibility of the measure- Two simple and accurate spectrophotometric methods for deter...

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