Received Date: March 22, 2013; Accepted Date: May 02, 2013; Published Date: May 04, 2013
Citation: Najma S, Saeed Arayne M, Saeeda Nadir A (2013) Charge Transfer Complexes of Gamma Aminobutyric Acid-Analogue, A Neurotransmitter: Synthesis and Spectrophotometric Determination. J Bioanal Biomed 5: 013-021. doi: 10.4172/1948-593X.1000075
Copyright: © 2013 Najma S, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Visit for more related articles at Journal of Bioanalysis & Biomedicine
Here, we report spectrophotometric method for the determination of gabapentin through charge transfer complexes with σ and π acceptors and also with dye stuffs. The method involves ion-pair complex formation of gabapentin with ninhydrin, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone, tetracyanoethylene, methyl orange and picric acid. The optimum conditions for the reactions and validation of described procedure have been studied. Resulting complexes absorb at 580, 315, 340, 464 and 342 nm in the Beer’s law range of 2-22, 32-80, 40-360, 12-96 and 15-135 μg mL-1 with correlation coefficient greater than 0.998 in each case respectively. The proposed method was applied successfully for the determination of gabapentin in pharmaceutical formulations. Satisfactory recovery values suggest that the method is reliable for the determination of gabapentin in pharmaceutical formulations without interference of excipients. In addition, the spectral characteristics including oscillator’s strength, dipole moment, ionization potential, energy of complexes, resonance energy and also the thermodynamic parameters i.e. association constant and Gibb’s free energy changes have been determined. Benesi-Hildebrand plots for each complex have been constructed. Further, solid charge transfer complexes of gabapentin were synthesized and characterized by IR spectroscopy.
Charge transfer complexes; Gabapentin; σ and π acceptors; Dye stuffs; Benesi–hildebrand plot
Gabapentin (GBP) (Figure 1) is the analog of inhibitory neurotransmitter gamma aminobutyric acid (GABA) having a cyclohexyl ring at β-position. Even though, it structurally resembles with gamma aminobutyric acid but it is not mimetic of GABA. GBP is used to treat seizures caused by epilepsy, nerve pain caused by herpes virus or shingles; it may lower the number and intensity of hot flashes each day [1,2]. Administration of GBP does not affect cognitive functioning of patient . It is a drug for post herpetic neuralgia and direct peripheral nerve injuries . Even though, GBP is used as neuropathic pain relieving drug since last decade, but the mechanism by which it exerts its pharmacologic effects is not reported in literature yet. However, GBP is supposed to slow down the release of other neurotransmitters by participating in voltage-gated N-type calcium ion channels which actually causes less amount of calcium to reach the nerve terminals .
Charge transfer complex is a chemical association in which the attraction between the molecules is produced by an electronic transition to excited electronic state, resulting in transfer of electronic charge from donor having high ionization potential to acceptor having high electron affinity. These complexes are strongly colored and stable as solids as well as in solutions [6,7].
UV-vis spectrophotometry is an easy and non-destructive technique for the detection and determination of a number of analytes in very simple manner. It usually involves economic reagents and not as much of analysis time is required for this method. In the past few years, different spectrophotometric methods have been developed by our research fellows [8-11]. Spectrophotometric methods for the determination of GBP in pharmaceutical formulations have also been reported by our research colleagues . To date, the reported spectrophotometric methods for GBP determination include condensation with acetylacetone and formaldehyde by Hantzsch reaction , with vanillin in the presence of McIlvain buffer pH 7.5, ninhydrin (NIN) reagent in DMF medium and p-benzoquinone in ethanol medium , charge transfer complex formation with σ-acceptors: NIN  and iodine , with π-acceptors: 2,3-Dichloro- 5,6-dicyano-1,4-benzoquinone (DDQ), Tetracyanoethylene (TCNE), Tetracyanoquinodimethane (TCNQ), p-chloranilic acid and chloranil by Salem  and also by our research fellows  and with dyestuff bromocresol green and bromothymol blue  and also with picric acid (PA) and 2,4-dinitrophenol . Other spectrophotometric methods include reaction of GBP with hypochloride in presence of Kolthoff buffer of pH 7.0 , simple UV-method  and automated spectrophotometry using piezoelectric pumping . Besides, GBP is determined by liquid chromatography (HPLC) , gas chromatography-mass spectrometry (GC-MS) , capillary electrophoresis , spectrofluorimetry , potentiometry , voltametry , chemiluminometry  and through electrochemical sensors .
This paper describes sensitive and rapid spectrophotometric method for the determination of GBP through derivatization with already reported reagents, like charge transfer complexes of GBP with NIN, DDQ, TCNE [12,15] and PA . Present study differs in the manner that complexation has been carried out in low cost easily available solvent i.e. methanol. Secondly, none of these methods has reported the spectral parameters; however the association constant and standard free energy changes have been reported in our previously reported method. Though, our earlier method involve the use of acetonitrile medium and the present study reports the formation of GBP charge transfer complexes in methanol or in aqueous medium for methyl orange (MO). The method is simple and reports many of spectral characteristics including oscillator’s strength, dipole moment, ionization potential, energy of complexes, resonance energy and also the thermodynamic parameters i.e. association constant and Gibb’s free energy changes which have never been determined yet. Benesi-Hildebrand plots for each complex have been constructed. Further, solid charge transfer complexes of GBP were synthesized and characterized by IR spectroscopy. Moreover, the developed methods have been successfully applied for the determination of GBP in pharmaceutical formulation without interference of excipients.
Materials and reagents
Pure GBP was obtained from Godecke AG/Germany under license of Park-Davis. Pharmaceutical formulations Gabix® capsules 100 mg (Getz Pharmaceuticals (Pvt.) Ltd.) were purchased from the local pharmacy. NIN, DDQ, TCNE and MO were purchased from Merck Schuchardt OHG, Germany, where as and PA was obtained from Sigma Aldrich Chemie GmbH. All the solvents used were of analytical grade and double distilled de-ionized water was used throughout the analysis.
Electronic spectra of GBP and its complex were recorded in the region 200-800 nm using Shimadzu 1800 double beam UV–visible spectrophotometer version 3.9 software with quartz cells of 1.0 cm path length. The IR spectra were obtained from KBr discs using Shimadzu Prestige-21200VEC version 1.2 software.
Standard stock solutions
Stock solution of 0.1% pure GBP was prepared in analytical grade methanol. 1% solution of NIN, DDQ, TCNE and PA were prepared fresh daily in analytical grade methanol. MO was prepared in double distilled deionized water. Phthalate buffer of pH ranging from 2.2-4.0 was prepared by dissolving 20.4 g potassium hydrogen phthalate in 1.0 L distilled water and pH of solution was maintained by adding 0.1M HCl.
Procedure for pure GBP
Method for NIN: Serial volume of drug solution in the concentration range of 2-22 μgmL-1 was transferred into test tubes. To each test tube 2 mL of NIN reagent was added, and then heated on a water-bath at 50 ± 1°C for 5 min. The contents of the tubes were cooled at room temperature and then transferred into 10 mL volumetric flasks. The volume was brought to mark with methanol and the absorbance was measured against reagent blank treated similarly.
Method for DDQ, TCNE and PA: For DDQ, TCNE and PA, 1 mL of them were placed into two different series of 10 ml volumetric flasks. Then an appropriate volume of GBP solution (32-80, 40-360 and 15-135 μgmL-1 for DDQ, TCNE and PA) was added to each flask. All the solutions were allowed to react at room temperature for 15 min. The volume was made up to the mark with methanol and the absorbance was measured against reagent blank treated similarly.
Method for MO: Into a series of 10 mL volumetric flaks, aliquots of GBP solution were transferred to get final concentration range 12-96 μgmL-1. To each flask, 3.0 mL buffer of pH 3.0 and 3 mL MO solution was added and the volume was completed with distilled water. The contents were transferred to a 50 mL separating funnel and extracted with 20 mL chloroform in two portions. The absorbance of extracted organic layer was measured against reagent blank treated similarly. The standard calibration graphs of all Charge transfer complexes were prepared by plotting absorbance vs. concentration of GBP.
Procedures for pharmaceuticals formulation
Twenty Gabix® capsules (100 mg) were finely powdered. The powder equivalent to 100 mg of GBP was dissolved in methanol and shaken well for proper mixing. This solution was allowed to stand for 30 min and then sonicated for complete solubilization of drugs. Then the contents were filtered to separate the insoluble excipients and volume was completed with same solvent to get 0.1% GABA solution. The procedure was continued as described under the procedure for pure GBP.
Synthesis of solid charge transfer Complexes
Synthesis of GBP charge transfer complexes was carried out by refluxing the GBP with each of the complexing agent separately for 2 hrs in the ratio of 1:1 as found through the stoichiometric study by applying Job’s method. The resulting product in each case was filtered off and washed with minimum amount of methanol. The excess solvent was evaporated to dryness. The solid charge transfer complexes were characterized by UV-visible and IR spectroscopy.
We reported charge transfer complexes of GBP with NIN and π acceptors in acetonitrile medium . In the present study, formation of GBP charge transfer complexes with NIN, DDQ, TCNE and PA in methanol or in aqueous medium for MO has been described. The complexes showed Ruhemann purple, red, bluish green, red and dark yellow coloration with λmax 315, 340, 580, 464 and 342 nm respectively. The absorption spectra of these complexes are shown in figure 2.
Stoichiometry of the reaction
The reaction stoichiometries between GBP and complexing agents were determined by measuring the absorbance of complex solutions containing the drug solution and complexing agents in ratios (0:10, 1:9…..10:0). The results indicated that the charge transfer interaction of GBP occurs on an equimolar basis (1:1) with all the complexing agents (Figure 3).
Optimization of reaction time
The optimum heating time for GBP-NIN complex formation was determined by following the absorbance at different time intervals. The contents were refluxed on water bath at 55 ± 1°C. After every minute, one of the tubes was removed; the procedure was applied to obtain ten readings. It is evident from (Figure 4) that complete complexation occurred at 5 min. and the absorbance remained constant up to 10 min. Therefore, the complete reaction time was considered to be 5 min. The optimum reaction time for GBP with π-acceptors and PA were determined by monitoring the color development at different time interval at room temperature (25 ± 1°C). The complete color development was attained instantaneously and was stable for more than 24 hr (Figure 5). The maximum absorption for GBP-MO complex was attained after proper shaking of mixture up to 2 min and it also found to be stable for 24 hr. The effect of reagent concentration is apparent from figure 6 that increased volume of complexing agent does not persuade the absorbance of complex and 2.0 mL of 1% NIN, 1.0 mL of 1% DDQ, TCNE and PA solutions and 3 mL of 1% MO solution are sufficient for complete complexation. For MO method, buffer of suitable pH was selected by testing them in the range of 2.0-4.0 and the maximum absorbance was obtained at pH 3.0 (Figure 7). For proper extraction of complex, solvents like benzene, carbon tetra chloride, dichloromethane and chloroform were tested. Double extraction with chloroform was supposed to be adequate for maximum recovery.
Under the optimized experimental conditions, the calibration graphs plotted between absorbance verses concentration of GBP were found to be linear. Beer’s law ranges were obtained to be 2-22, 32-80, 40-360, 12-96 and 15-135 μg mL-1 for NIN, DDQ, TCNE, MO and PA with correlation coefficient greater than 0.998 for all the complexes. Regression characteristics including slope, intercept, correlation coefficient and also the molar absorptivity values are given in table 1. Limit of detection and quantitation were determined to establish the sensitivity of method. The LOD values were calculated to be 320, 150, 680, 960 and 410 ng mL-1 respectively (Table 1).
|λmax (nm)||580 nm||315 nm||340 nm||464||342|
|Linearity range µgmL-1||2-22||32-80||40-360||12-96||15-135|
Table 1: Optimum conditions and analytical parameters.
The proposed method was applied on five replicates of GBP at seven concentration level to test the precision of method. The measurements of relative standard deviation obtained for each method are reported in table 2. The low %RSD values confirm the sensitivity of method. For the determination of accuracy, the percent recovery of GBP through proposed method was calculated. Satisfactory values of recovery (Table 3) concluded that the method is free from constant error.
Table 2: Precision of method.
|% Rec||% Err||% Rec||% Err||% Rec||% Err||% Rec||% Err||% Rec||% Err|
Table 3: Accuracy of method.
The high values of molar absorptivity of the resulting colored complexes indicate the high sensitivity of the methods. High value of correlation coefficient confirms the best linearity of the calibration curve. The value of detection limit and variance speak of good sensitivity of the proposed method and negligible disperse of the calibration data around the line of regression. Therefore, it is concluded that the proposed method is free from constant error independent of the GBP concentration.
The developed charge transfer methods were successfully applied for the determination of GBP in its dosage formulation by five replicate determinations. The results obtained are shown in table 3. Satisfactory recovery data was obtained in the range of 99.23-100.80%, 98.46-100.64%, 99.32.46-101.82%, 98.02-100.31% and 99.93-101.15% for NIN, DDQ, TCNE, MO and PA. The low percent error indicated the reliability of method. The assay results were in good agreement with the label claim. Commonly present excipients did not found to interfere during the assay.
Effect of excipients
A systemic study was performed to determine the effect of inactive ingredients commonly used in pharmaceutical formulation by scanning the blank solution containing 10 mg GBP and also the placebo solutions prepared by separately mixing 10 mg of GBP with pyrrolidone (10 mg), lactose (10 mg), talc (20 mg), magnesium stearate (15 mg) and starch (10 mg) in 100 mL volumetric flask. The percent recovery values given in table 4 indicate that there is no effect of common excipients present in pharmaceutical formulation.
Table 4: Recovery of ibuprofen in presence of different excipients.
Spectral characteristics of charge transfer complexes
where εmax is the molar extinction coefficient at maximum absorbance, ν1/2 is the band-width at half absorbance in cm-1 and νmax is wave number in cm-1. Relatively large values of oscillator strength indicate the strength of complexes. The ionization potential (Ip) of free donor was calculated by using the equation (3) ,
where νCT is the wave number in cm-1 corresponding to the charge transfer band of complex formed between donor and acceptor. The resonance energy of charge transfer complex in the ground state was determined by Brieglab and Czekalla equation (4) given bellow :
where h is the plank’s constant and νCT is the wave number in cm-1.
The energy of charge transfer complexes was calculated by applying the relationship given in equation (5) :
where λCT is the wavelength of charge transfer band at maximum absorbance. All the spectral data including oscillator strength, dipole moment, ionization potential, resonance energy and energy of charge transfer complexes are presented in table 5.
|Complex||f||µ||Ip||ECT||RN||Kc×102 (lit/mol)||ΔG° (KCal)|
Table 5: Spectrophotometric results.
where, K is the association constant, A is absorbance, ε is molar extinction coefficient and [Ao] and [Do] are the initial concentrations of the acceptor and donor respectively. The values for Kc are given in table 5. Further investigation was made by plotting the values of 1/D0 versus A0/A, which showed linear curve in all the cases. Table 6 presents the values and the graphs are displayed in figure 8.
|Complex||D (M)× 10-3||A (M)× 10-3||Abs||1/D× 103||A/Abs× 10-2|
Table 6: The values of [A0]/Abs and 1/ [D0] for ibuprofen complexes.
The standard free energy changes (ΔG°) associated with GBP complexation reactions were calculated from the association constants by applying equation (2). The values of ΔG° for each complex are given in table 5.
where, ΔG° is the free energy change of the complex in KJ mol−1, R is the gas constant (1.987 cal mol−1 deg−1), T is temperature in Kelvin and Kc is the association constant of drug-acceptor complexes.
Corresponding to the previously reported IR spectra of GBP , it showed twin peak at 2857 and 2927 cm−1 attributing to –NH2, a C–N stretch at 1165 cm−1 and C=O stretch of carboxylic group at 1615 cm−1. The IR spectra of charge transfer complexes of GBP with NIN, DDQ and TCNE have already been reported by our research colleagues earlier . In the IR spectra of MO and PA complexes of GBP, the doublet peak for –NH2 found absent, confirming the formation of complex. Carbonyl peak shifted to 1710 cm−1 and peaks at 1342 and 1161 cm−1 and also at 650 cm−1 appeared owing to the S=O and S–O group respectively in GBP-MO complex, where as in GBP-PA complex, carbonyl peak appeared at 1653 cm−1 and two peaks of medium intensity appeared at 1560 and 1320 cm−1 attributed to asymmetric and symmetric stretch of NO2. The C=C aromatic stretch appeared at 1510 and 1431 cm−1, while aliphatic –CH2 appeared at 1478 cm−1. Figure 9 represents the IR spectra of GBP complexes. On the basis of spectral characteristics, the structure of complexes is given in figure 10.
The determination of GBP in bulk and its formulation has been investigated by means of spectrophotometric analysis. The method is based on charge transfer complexes with NIN, DDQ, TCNE, MO and PA. Linear calibration curves were obtained with correlation coefficient greater than 0.998. Spectral characteristics including oscillator’s strength, dipole moment, ionization potential, energy of complexes, resonance energy, association constant and Gibb’s free energy changes have been determined. Benesi-Hildebrand plots for each complex have been constructed. Further, solid charge transfer complexes of GBP were synthesized and characterized by IR and 1H-NMR spectroscopy.
Mrs Saeeda Nadir Ali deeply acknowledges Higher Education Commission, Pakistan for providing Indigenous Scholarship for this research work.
Make the best use of Scientific Research and information from our 700 + peer reviewed, Open Access Journals