A simple and sensitive spectrophotometric method is described for the assay of the drugs; niacin, glibenclamide, erythromycin, thiamine and 4-aminobenzoic acid. The method is based on charge transfer complexation (CT) reaction of niacin, glibenclamide, erythromycin, thiamine and 4-aminobenzoic acid as n-electron donors with 2,3-dichloro-5,6-dicyno-1,4-benzoquinone(DDQ) as л-electron acceptor in methanol. Intensely coloured charge transfer complexes with niacin (reddish brown, lmax ;464 nm; εmax, 1.02×103 dm3mol-1cm-1) thiamine (reddish brown ,lmax ;474 nm; εmax, 1.08×103 dm3mol-1cm-1), glibenclamide (reddish brown , lmax ;474 nm; εmax,0.99×103 dm3mol-1cm-1) erythromycin(reddish brown , lmax ;464 nm; εmax, 1.27×103 dm3mol-1cm-1) 4-aminobenzoic acid(reddish brown, lmax ;474nm; εmax, 1.06×103 dm3mol-1cm-1) all in a 1:1 stoichiometric ratio. Condition for complete reactions and optimum stability of complexes were niacin (70 min, 60 OC) thiamine (25 min, 40 OC), glibenclamide (35 min, 40 OC), erythromycin (15 min, 60 OC) and 4-aminobenzoic acid (15 min, 60 OC) as absorbances of the complexes remained invariant within these conditions. Formation and stability of the complexes of niacin, thiamine, 4-aminobenzoic acid and erythromycin were optimum at pH 8. For glibenclamide pH 2.0 favoured optimum stability and formation. The bands distinguished for the donors to donor-acceptor CT complexes displayed small changes in band intensities and frequency values in the IR spectra ,The –NH2 group vibration occurring at 3609 cm-1 shifted to 3610 cm-1 in thiamine, PABA (3222 cm-1 to 3183 cm-1), ѵ (N-H) occurring at 3331cm-1 shifted to 3371 cm-1 in glibenclamide, ѵ(C N) occurring at 2936 cm-1 shifted to 2944 cm-1 in niacin, ѵ (CH3-N) occurring at 2948 cm-1 shifted to 2939 cm-1 in erythromycin. The vibration ѵ (C O) of DDQ observed at 1665 cm-1 shifted to 1669 cm-1 in the CT complex for thiamine, PABA(1665 cm-1 to 1670 cm-1), glibenclamide(1675 cm-1 to 1676 cm-1), erythromycin(1665 cm-1 to 1674 cm-1), niacin(1665 cm-1 to 1655 cm-1) respectively. Adherence to Beer’s Law was within the concentration range for niacin (5-130 μg/cm3), thiamine (5-80 μg/cm3), glibenclamide (9-100 µg/cm3), erythromycin
(5-150 µg/cm3), 4-aminobenzoic acid(5-90 µg/cm3). Limit of detection and quantification of the drugs based on this method is niacin (1.78 and 5.4), thiamine (1.23 and 3.37), glibenclamide (3.47 and 10.5), erythromycin (2.11 and 6.40), 4-aminobenzoic acid (0.55 and 1.67) respectively. Evaluation of the degree of interference by excipients used in the drugs manufactured indicates tolerance to certain concentrations. A detailed study on the interference of different excipients was made. No significant interference was observed in magnesium stearate (30 µg/cm3), Talc (15-25µg/cm3, 35-40 µg/cm3) with thiamine-DDQ complex. There were no significant interference in stearic acid (35 µg/cm3) but tolerable interference was seen in magnesium stearate (20 µg/cm3) and calcium phosphate (15 µg/cm3) with niacin-DDQ complex. For glibenclamide – DDQ complex, no significant interference was seen with calcium phosphate (30 µg/cm3) but there were tolerable interference present in stearic acid (40 µg/cm3). In 4-aminobenzoic acid, no significant interference was observed with magnesium stearate (30 µg/cm3) and talc (35 -40µg/cm3) but tolerable interference was observed in corn starch (15 µg/cm3). Also no significant interference was seen in corn starch (35 µg/cm3) with erythromycin-DDQ complex but there was tolerable interference in talc (10 µg/cm3). The Pearson correlation coefficient for the compliance of the method as regards the pure and commercial forms of niacin, thiamine, glibenclamide, erythromycin and 4-aminobenzoic acids are 0.993, 0.977, 0.987, 0.998 and 0.993 respectively which shows significance with p < 0.01. The analysis of variance test revealed the non-significance of niacin, thiamine, glibenclamide, erythromycin and 4-aminobenzoic acid with p > 0.01. The mean percentage recoveries were 98.94 0.016, 96.2 0.016, 98.24 0.011, 107.4 0.023 and 102.35 0.014 for niacin, thiamine, glibenclamide, erythromycin and 4-aminobenzoic acid respectively. Kinetics of the reactions infer that the rate of formation of the CT complexes did not vary significantly with increase in concentration of glibenclamide, erythromycin, thiamine, niacin and 4-aminobenzoic acid indicating likely zeroth order dependence of the rate with respect to concentration of the drugs. However, the linearity of the pseudo-first order plot points to first order dependence of rate on [DDQ].The overall rate equation for the reactions can be given as
Based on the limit of detection and quantification, adherence to Beer-Lambert’s law and low degree of interference, the method is recommended for the analysis of these drugs.
1.1 Charge Transfer Complexation
Acceptors are aromatic systems containing electron withdrawing substituents such as nitro, cyano and halogen groups (Foster, 1967). Electron donors are systems that are electron rich (Ajali and Chukwurah, 2001). The interaction between electron donor and electron acceptor results in formation of charge transfer complex (Ajali et al, 2008). The term charge transfer denotes a certain type of complex which results from interaction of an electron acceptor and an electron donor with the formation of weak bonds (Hassib and Issa, 1996). However the nature of the interaction in a charge transfer complex is not a stable chemical bond and is much weaker than covalent forces. It is better characterized as a weak electron resonance. As a result, the excitation energy of this resonance occurs very frequently in the visible region of the electromagnetic spectrum. This produces the usually intense colour characteristic for these complexes. These optical absorption bands are often referred to as charge transfer bands. Molecular interactions between electron donors and acceptors are generally associated with the formation of intensely coloured charge transfer complexes which absorb radiation in the visible region.Charge transfer (CT) complexes have been widely studied (Ezeanokete et al, 2013; Hala et al, 2013; Frag et al, 2011; Ramzin et al, 2012; Farha, 2013). Charge transfer complexes are known to take part in many chemical reactions like addition, substitution and condensation reactions (Van et al, 2006).
Donor acceptor properties are prerequisites for the formation of charge transfer complexes. Most drugs have –NH or –NH2 groups which behave as bases (electron donors) and could form complexes with acids (electron acceptor).Various cases have been reported. The charge-transfer complexes formed between the ephedrine (Eph) drug as a donor with picric acid (Pi) and quinol (QL) as –acceptors have been synthesized in methanol as a solvent at room temperature and spectroscopically studied as shown in scheme 1:
Scheme 1: Interaction of Ephedrine with Quinol to form the charge transfer complex
Spectrophotometry is widely used to monitor the progress of reactions and the position of equilibrium. Its measurement is often straight forward to make and the technique is sensitive and precise provided that relevant limitations (such as the regions over which Beer’s law is valid) are recognized. Spectrophotometric technique continues to be the most preferred methods for routine analytical work due to their simplicity and reasonable sensitivity with significant economical advantages (Raza, 2006).
1.1.2: Analysis of Drugs
A spectrophotometric method has been employed for the determination of allopuriol using DDQ through charge transfer formation. The absorption spectra of allopuriol-DDQ complex in acetonitrile solvent showed three maxima at (ʎmax = 450 nm; ε1 = 1.95 x103 Lmol-1cm-1), 540 nm (ε2 = 0.80 x 103 Lmol-1cm-1) and 580 nm (ε3 = 0.69 x 103 Lmol-1cm-1) with a 1:1 stoichiometric ratio between allopuriol and DDQ. The charge transfer complex formation is shown in scheme 2: