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Electrochemical Behavior and Polarographic Determination of Auronofin
Electrochemical 13ehavior and Polarographic Determination of Auronofin J. HERNANDEZM ~ N D E ZA. , SANCHEZ PEREZ', AND M. DELGADOZAMARREmO Received May 23, 1988, from the Department of Analytical Chemistry, Nutrition and Bromatology, University of Salamanca, Salamanca 37008, Accepted for publication December 12, 1988. Spain. ~
_ _ _ _ ~ Abstract 0 Auronofin in a 50%, ethanokwater solution yields, on the ~~
dropping mercury electrode, a reduction wave whose is a function of pH in the 2 < pH < 9 range (& = -0.036-0.054 pH), and acquires a constant value above pH !3 (€,,,? = -0.50 V, versus saturated calomel electrode).The influence of the variables which affect the reduction process of auronofin in tripolassium phosphate medium was studied.The electrochemical process is reversible and diffusion controlled. Coulometry showed that one electron is involved in the reduction of the drug. Analytical methods are proposed for the determination of auronofin at micromolar levels by DC and DP polarography, with relative standard deviations of 3.2 and 2.4%., respectively. .
-____.-
Auronofin (see structure) is a compound of Au(1) with tetraacetylglucothiopyranoseand triethylphosphine that has recently been introduced as a crisotherapeutic agent for oral treatment of rheumatoid arthritis. Analysis of this drug in biological fluids is normally performed by the determination of gold using AAS and a graphite oven.13 No electrochemical method for its determination has yet been reported. In the present work we describe the electrochemical behavior of auronofin in a 50% ethano1:water solution (viv). The different variables affecting the reduction process of auronofin on the dropping mercury electrode were studied to explain its mechanism and to optimize the conditions for its determination by conventional and dilfferential pulse polarography.
to -2.2-V potential range (versus SCE). When the differential pulse technique was used a AE of -50 mV was applied. The drop time used in both techniques was 2 stdrop.
Results and Discussion Polarographic Behavior of Auronofin-In ethano1:water medium, auronofin gave a reduction wave (DC) or peak (DP) in the -0.2 to -0.5-V (versus SCE) region in accord with the pH conditions used. Using alkaline pH, with K,PO, as the preferred supporting electrolyte, a well-defined wave or peak appears a t -0.5 V (Figure 1). It is well known that protons are involved in the polarographic reduction of most organic molecules in protogenic media; consequently, the pH of the medium must be carefully controlled. The influence of pH on the shape of the polarograms was studied using solutions containing auronofin at a concentration of 1.45 x lo-, M, and varying the pH value in the 13 > pH > 2 range by addition of HC1. The pH of the ethano1:water solution was measured with a combined glass and SCE, adjusting pH with a n aqueous buffer pH solution; accordingly, the value measured was a pH* value. In many organic electrode processes when protons are involved in the electrochemical reaction (0 + mH+ + ne- + R), the El,,-pH dependence (E,,, = E:,, - 0.059 m/n pH) is used as a means for characterization of the electrode process because the above equation can be used for the determination of the number of protons (m) and number of electrons (n) involved in the electrodic reaction. Figure 2 shows the variation in El,, of the reduction wave of auronofin versus the pH* of the ethano1:water solution. The El,, decreases linearly with pH* in the 2 < pH* < 9 range, and acquires a constant
Ac-0"
Experimental Section Apparatus-A Metrohrn 506 polarograph and an E-505 polarographic stand with three electrodes [Metrohm E-A 102911 dropping mercury electrode, Metrohm E.A-285 platinum counterelectrode, and a saturated calomel electirode (SCE)] were constructed in house. A Radiometer potentiometer (pH. meter 51) and a Metrohm AG-9100 combined glass and SCE were also used. Reagents-Solutions of auronofin were prepared in a 50% ethanolic solution using the pure solid product supplied by Smith Kline and French Laboratories S.A.IE. (Spain). The rest of the chemicals used were of R.A. grade. P r o c e d u r d o l u t i o n s of auronofin a t the desired concentrations were prepared using 0.06 M K,PO, as the supporting electrolyte (higher concentrations of this salt produce a precipitate in the ethano1:water medium used). The solutions were placed in the polarographic cell and, after removing oxygen by bubbling nitrogen through it, the corresponding polarograms were recorded in the -0.20
0022-3549/89/0700-0589$tll. OIL0 0 1989, American Pharmaceutical Association
0.2
0.4
0.6
V, SCE
Figure 1-Electrochemical behavior of auronofin. Journal of Pharmaceutical Sciences t 589 Vol. 78, No. 7, July 1989
value for pH* values >9, according to the following equations:
El,, (V) = -0.0316 - 0.543 pH* (2 < pH* < 9) r = 0.999
(1)
Evz (V) = -0.500 (pH* > 9) r
=
(2)
0.950
Taking into account that the number of electrons involved in the electrochemical reduction of auronofin is one, as shown by the coulometric study (see below), and considering the slope of the straight line El,, = -0.0316 - 0.0543pH*, it may be inferred that the number of protons participating in the electrodic reduction process of auronofin at values of pH* that are <9 is one. At higher pH values, the electrodic process does not include protons. It is very likely that the proton exchanged in the electrochemical process corresponds to the protonation of the triethylphosphine ligand of the Au(1) complex, whose p K a 4 is very close to the pH* value (8.63) of the intersection of the two straight lines of Figure 2:
Reversibility-By logarithmic analysis it was ascertained that the process under study is reversible only i n alkaline medium (pH* > 9) when the electrochemical reaction proceeds directly, interchanging one electron without involving protons. Otherwise, the polarographic reduction of auronofin, as with many metal complexes, becomes more irreversible for lower pH* values, probably because protons are also involved in the electrochemical-controlling electrodic reaction. As mentioned above, hydrogen ions participate in the electrode process, the oxidized form of the auronofin molecule has a basic function [we assumed that it is the triethylphosphine ligand of the Au(1) complex], and their protonation appears to implicate a slow establishment of the equilibrium between the oxidized and reduced forms at the electrode surface, causing irreversibility in the overall electrode process. Coulometric Determination of t h e Number of Electrons Involved in the Electrodic Reaction-In order to determine the number of electrons involved in the electrodic process, coulometry (at a constant potential -0.80 V) of a n 1.5 x M auronofin solution was performed. The number of electrons calculated for this process proved to be 0.9, which suggests that the electrodic process corresponds to the reduction of monovalent gold to elemental gold. Influence of T e m p e r a t u r e T h e influence of temperature on the limiting current was studied in the 18 to 49 "C range.
0. 2
0.3 0.4
0.5
'. I
4
6
8 PH
Figure 2-Effect
of pH on €,,*.
590 I Journal of Pharmaceutical Sciences Vol. 78, No. 7, July 1989
10
12
Polarograms of a 1.5 x M auronofin solution were recorded. A temperature coefficient of 2.5%was obtained from which it is inferred that the process is diffusion controlled. Influence of Auronofin Concentration-The influence of auronofin concentration was studied in the 3.63 x to 9.87 x M range. A linear relationship was observed between the limiting current and the auronofin concentration in the 3.63 x lop5 to 5.10 x M range, in accordance with the following expression:
id (PA)
= 0.020
r
=
+ 5.47
x 10% (M)
0.996
(4)
For auronofin concentrations >5 x M, the proportionality between the limiting diffusion current and auronofin concentration is not linear. For auronofin concentrations >3 x lop4M, a postwave appears which is probably generated by an adsorption process. (It is well known that polarographic prewaves and postwaves are caused by the adsorption of the many organic molecules on the electrode surface5.) Differential Pulse Polarography-In order to corroborate the conclusions obtained with conventional polarography, a study was made on the effect of different parameters on the polarographic peak obtained using differential pulse polarography. It should be noted that from the study of the influence of pulse magnitude on the shape of the polarographic peak, the optimum hE is -50 mV. A study was also carried out on the dependence of peak height on the drop time (t).Polarographic recordings corresponding to a 1.7 x low4M auronofin solution were made. A linear relationship was observed between the peak intensity and t2'3 lip (PA) = -0.04 0.257i?'3, r = 0.999); this confirms that the polarographic process is diffusion controlled. The reversibility of the polarographic process in alkaline medium was also studied by the dpp technique, applying the following criteria: AE = E; - E; and iYi; = 1. The results obtained point out the reversible nature of the process. From the study of the influence of the auronofin concentration on peak intensity, a linear dependence was observed between the peak intensity and the concentration of the AufI)complex in the range between 3.62 x lop5 and 9.89 x lop4M:
+
i, (PA)
=
0.023 + 8.64 x r = 0.997
lo2 C (M) (5)
Also of interest was the appearance of a postpeak for auronofin concentrations >3 x lop4M. This postpeak did not prevent measurement of the peak originated by the diffusion of auronofin, although it does point to the existence of an adsorption process. Analytical Characteristics of t h e Method-The relationships between the limiting current, or the peak intensity, and the auronofin concentration permits the polarographic determination of auronofin by the direct current and the differential pulse techniques using the wave, or peak, that appears a t -0.500 V (versus SCE) in alkaline conditions. The detection limits observed upon applying the Kaiser criterion are 2.15 X lop5 M (direct current polarography) and 3.42 x M (differential pulse polarography). The polarographic methods proposed have acceptable precision. The relative standard deviations are 3.2 and 2.4%using conventional and differential pulse polarography, respectively. Auronofin is marketed in tablet form under the name Ridaura. Successful determination of the auronofin content in Ridaura tablets was carried out by the standard addition method without excipient materials separation. The mean content obtained was 2.9 mg of auronofin per tablet (content declared was 3 mgitablet).
References and Notes
4. Henderson, W. A,; Streuli, C. A. J . Am. Chem. Soc. 1960,82,5791-
1. Johnsen A. c.; Wibetoe, G.; Langmyhr, F. J.; Aaseth, J. Anal. Chim. Acta 1982,135, :243-248. 2. Rodgers, A. I. A.; Brown, D. H.; Smith, W. E.; Lewis, D.; Capell, H. A. Anal. I’roc. 1982,19, 87-88. 3. Dahl, S. L.; C!oleman,M. L.; Williams, H. L.; Altz-Smith, M.; Kay, D. R.; Paulus, H. E.; Weinstein, A,; Kaplan, S. Arthritis and Rheumatism 1985,28, 1211-1218.
Acknowledgments
5794. 5. Mairanovski, S. G. Catalytic and Kinetics Waves i n Polarography; Plenum: New York, 1968; Chapter 111.
The authors express their appreciation to S.K.F. (S.A.E.) for donation of pure samples of auronofin.
Journal of Pharmaceutical Sciences I 591 Vol. 78, No. 7, July 1989
_ _ _ _ ~ Abstract 0 Auronofin in a 50%, ethanokwater solution yields, on the ~~
dropping mercury electrode, a reduction wave whose is a function of pH in the 2 < pH < 9 range (& = -0.036-0.054 pH), and acquires a constant value above pH !3 (€,,,? = -0.50 V, versus saturated calomel electrode).The influence of the variables which affect the reduction process of auronofin in tripolassium phosphate medium was studied.The electrochemical process is reversible and diffusion controlled. Coulometry showed that one electron is involved in the reduction of the drug. Analytical methods are proposed for the determination of auronofin at micromolar levels by DC and DP polarography, with relative standard deviations of 3.2 and 2.4%., respectively. .
-____.-
Auronofin (see structure) is a compound of Au(1) with tetraacetylglucothiopyranoseand triethylphosphine that has recently been introduced as a crisotherapeutic agent for oral treatment of rheumatoid arthritis. Analysis of this drug in biological fluids is normally performed by the determination of gold using AAS and a graphite oven.13 No electrochemical method for its determination has yet been reported. In the present work we describe the electrochemical behavior of auronofin in a 50% ethano1:water solution (viv). The different variables affecting the reduction process of auronofin on the dropping mercury electrode were studied to explain its mechanism and to optimize the conditions for its determination by conventional and dilfferential pulse polarography.
to -2.2-V potential range (versus SCE). When the differential pulse technique was used a AE of -50 mV was applied. The drop time used in both techniques was 2 stdrop.
Results and Discussion Polarographic Behavior of Auronofin-In ethano1:water medium, auronofin gave a reduction wave (DC) or peak (DP) in the -0.2 to -0.5-V (versus SCE) region in accord with the pH conditions used. Using alkaline pH, with K,PO, as the preferred supporting electrolyte, a well-defined wave or peak appears a t -0.5 V (Figure 1). It is well known that protons are involved in the polarographic reduction of most organic molecules in protogenic media; consequently, the pH of the medium must be carefully controlled. The influence of pH on the shape of the polarograms was studied using solutions containing auronofin at a concentration of 1.45 x lo-, M, and varying the pH value in the 13 > pH > 2 range by addition of HC1. The pH of the ethano1:water solution was measured with a combined glass and SCE, adjusting pH with a n aqueous buffer pH solution; accordingly, the value measured was a pH* value. In many organic electrode processes when protons are involved in the electrochemical reaction (0 + mH+ + ne- + R), the El,,-pH dependence (E,,, = E:,, - 0.059 m/n pH) is used as a means for characterization of the electrode process because the above equation can be used for the determination of the number of protons (m) and number of electrons (n) involved in the electrodic reaction. Figure 2 shows the variation in El,, of the reduction wave of auronofin versus the pH* of the ethano1:water solution. The El,, decreases linearly with pH* in the 2 < pH* < 9 range, and acquires a constant
Ac-0"
Experimental Section Apparatus-A Metrohrn 506 polarograph and an E-505 polarographic stand with three electrodes [Metrohm E-A 102911 dropping mercury electrode, Metrohm E.A-285 platinum counterelectrode, and a saturated calomel electirode (SCE)] were constructed in house. A Radiometer potentiometer (pH. meter 51) and a Metrohm AG-9100 combined glass and SCE were also used. Reagents-Solutions of auronofin were prepared in a 50% ethanolic solution using the pure solid product supplied by Smith Kline and French Laboratories S.A.IE. (Spain). The rest of the chemicals used were of R.A. grade. P r o c e d u r d o l u t i o n s of auronofin a t the desired concentrations were prepared using 0.06 M K,PO, as the supporting electrolyte (higher concentrations of this salt produce a precipitate in the ethano1:water medium used). The solutions were placed in the polarographic cell and, after removing oxygen by bubbling nitrogen through it, the corresponding polarograms were recorded in the -0.20
0022-3549/89/0700-0589$tll. OIL0 0 1989, American Pharmaceutical Association
0.2
0.4
0.6
V, SCE
Figure 1-Electrochemical behavior of auronofin. Journal of Pharmaceutical Sciences t 589 Vol. 78, No. 7, July 1989
value for pH* values >9, according to the following equations:
El,, (V) = -0.0316 - 0.543 pH* (2 < pH* < 9) r = 0.999
(1)
Evz (V) = -0.500 (pH* > 9) r
=
(2)
0.950
Taking into account that the number of electrons involved in the electrochemical reduction of auronofin is one, as shown by the coulometric study (see below), and considering the slope of the straight line El,, = -0.0316 - 0.0543pH*, it may be inferred that the number of protons participating in the electrodic reduction process of auronofin at values of pH* that are <9 is one. At higher pH values, the electrodic process does not include protons. It is very likely that the proton exchanged in the electrochemical process corresponds to the protonation of the triethylphosphine ligand of the Au(1) complex, whose p K a 4 is very close to the pH* value (8.63) of the intersection of the two straight lines of Figure 2:
Reversibility-By logarithmic analysis it was ascertained that the process under study is reversible only i n alkaline medium (pH* > 9) when the electrochemical reaction proceeds directly, interchanging one electron without involving protons. Otherwise, the polarographic reduction of auronofin, as with many metal complexes, becomes more irreversible for lower pH* values, probably because protons are also involved in the electrochemical-controlling electrodic reaction. As mentioned above, hydrogen ions participate in the electrode process, the oxidized form of the auronofin molecule has a basic function [we assumed that it is the triethylphosphine ligand of the Au(1) complex], and their protonation appears to implicate a slow establishment of the equilibrium between the oxidized and reduced forms at the electrode surface, causing irreversibility in the overall electrode process. Coulometric Determination of t h e Number of Electrons Involved in the Electrodic Reaction-In order to determine the number of electrons involved in the electrodic process, coulometry (at a constant potential -0.80 V) of a n 1.5 x M auronofin solution was performed. The number of electrons calculated for this process proved to be 0.9, which suggests that the electrodic process corresponds to the reduction of monovalent gold to elemental gold. Influence of T e m p e r a t u r e T h e influence of temperature on the limiting current was studied in the 18 to 49 "C range.
0. 2
0.3 0.4
0.5
'. I
4
6
8 PH
Figure 2-Effect
of pH on €,,*.
590 I Journal of Pharmaceutical Sciences Vol. 78, No. 7, July 1989
10
12
Polarograms of a 1.5 x M auronofin solution were recorded. A temperature coefficient of 2.5%was obtained from which it is inferred that the process is diffusion controlled. Influence of Auronofin Concentration-The influence of auronofin concentration was studied in the 3.63 x to 9.87 x M range. A linear relationship was observed between the limiting current and the auronofin concentration in the 3.63 x lop5 to 5.10 x M range, in accordance with the following expression:
id (PA)
= 0.020
r
=
+ 5.47
x 10% (M)
0.996
(4)
For auronofin concentrations >5 x M, the proportionality between the limiting diffusion current and auronofin concentration is not linear. For auronofin concentrations >3 x lop4M, a postwave appears which is probably generated by an adsorption process. (It is well known that polarographic prewaves and postwaves are caused by the adsorption of the many organic molecules on the electrode surface5.) Differential Pulse Polarography-In order to corroborate the conclusions obtained with conventional polarography, a study was made on the effect of different parameters on the polarographic peak obtained using differential pulse polarography. It should be noted that from the study of the influence of pulse magnitude on the shape of the polarographic peak, the optimum hE is -50 mV. A study was also carried out on the dependence of peak height on the drop time (t).Polarographic recordings corresponding to a 1.7 x low4M auronofin solution were made. A linear relationship was observed between the peak intensity and t2'3 lip (PA) = -0.04 0.257i?'3, r = 0.999); this confirms that the polarographic process is diffusion controlled. The reversibility of the polarographic process in alkaline medium was also studied by the dpp technique, applying the following criteria: AE = E; - E; and iYi; = 1. The results obtained point out the reversible nature of the process. From the study of the influence of the auronofin concentration on peak intensity, a linear dependence was observed between the peak intensity and the concentration of the AufI)complex in the range between 3.62 x lop5 and 9.89 x lop4M:
+
i, (PA)
=
0.023 + 8.64 x r = 0.997
lo2 C (M) (5)
Also of interest was the appearance of a postpeak for auronofin concentrations >3 x lop4M. This postpeak did not prevent measurement of the peak originated by the diffusion of auronofin, although it does point to the existence of an adsorption process. Analytical Characteristics of t h e Method-The relationships between the limiting current, or the peak intensity, and the auronofin concentration permits the polarographic determination of auronofin by the direct current and the differential pulse techniques using the wave, or peak, that appears a t -0.500 V (versus SCE) in alkaline conditions. The detection limits observed upon applying the Kaiser criterion are 2.15 X lop5 M (direct current polarography) and 3.42 x M (differential pulse polarography). The polarographic methods proposed have acceptable precision. The relative standard deviations are 3.2 and 2.4%using conventional and differential pulse polarography, respectively. Auronofin is marketed in tablet form under the name Ridaura. Successful determination of the auronofin content in Ridaura tablets was carried out by the standard addition method without excipient materials separation. The mean content obtained was 2.9 mg of auronofin per tablet (content declared was 3 mgitablet).
References and Notes
4. Henderson, W. A,; Streuli, C. A. J . Am. Chem. Soc. 1960,82,5791-
1. Johnsen A. c.; Wibetoe, G.; Langmyhr, F. J.; Aaseth, J. Anal. Chim. Acta 1982,135, :243-248. 2. Rodgers, A. I. A.; Brown, D. H.; Smith, W. E.; Lewis, D.; Capell, H. A. Anal. I’roc. 1982,19, 87-88. 3. Dahl, S. L.; C!oleman,M. L.; Williams, H. L.; Altz-Smith, M.; Kay, D. R.; Paulus, H. E.; Weinstein, A,; Kaplan, S. Arthritis and Rheumatism 1985,28, 1211-1218.
Acknowledgments
5794. 5. Mairanovski, S. G. Catalytic and Kinetics Waves i n Polarography; Plenum: New York, 1968; Chapter 111.
The authors express their appreciation to S.K.F. (S.A.E.) for donation of pure samples of auronofin.
Journal of Pharmaceutical Sciences I 591 Vol. 78, No. 7, July 1989