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Anal. chem. 7904, 56, 2827-2830
CD determination of secobarbital in seconal sodium suppositories (100 mg) was done by using two sampling techniques. The mass of the contents averaged around 200 mg; the balance of the mass was insoluble in pH 9.8 buffer solution. In one procedure the entire content of a single capsule was placed in a 10-mL aliquot of buffer. After centrifugation 0.1-mL portions of this were diluted to 10 mL with a standard buffer-BCD solution, in which the BCD concentration was M, and used for CD spectral measurements. The assay was within &0.8% of the prescribed amount. In the other procedure replicate aliquots ( 2 mg) by weight were taken and each dissolved in a 10-mL portion of the standard buffer-BCD solution. Variance among the extracts from a given capsule was measured to be within &2% of the average, which in turn was & l % from the prescription amount. In summary, it has been demonstrated that CD can effectively be used to measure the formation constants for complexation reactions and the procedure is simple because the N
spectra are uncomplicated. Formation constants calculated can then, in turn, be used for quantitative assays of achiral analytes without mixture separation. The method has enormous potential in applications to quality control.
LITERATURE CITED (1) Han, S. M.; Purdie, N. Anal. Chem., preceding paper in this Issue. (2) Thakkar, A. L.; Kuehn, P. B.; Perrin, J. H.; Wiiham, W. L. J. Phafm. Scl. 1972, 6 1 , 1841. (3) Kortum, G.; Vogei, W.; Andrussow, K. "Dissociation Constants of Organic Acids in Aqueous Solution"; Butterworths: London, 1961. (4) Saenger, V. W. Angew. Chem. 1080, 92, 343. 15) . . Monk, C. B. "Electrolytic Dissociation"; Academic Press: New York, 1961; p 186. (6) Bender, M. L.; Domiyama, M. "Cyciodextrin Chemistry"; Springer-Verlag: New York, 1978.
RECEIVED for review April 13, 1984. Resubmitted August 9, 1984. Accepted August 27,1984. Support of this work was from the National Science Foundation under Grant NSF CHE-8240564.
Solute- Induced Circular Dichroism: Drug Discrimination by Cyclodextrin Soon M. Han, W. Marc Atkinson, and Neil Purdie*
Chemistry Department, Oklahoma State University, Stillwater, Oklahoma 74078
Seventeen drugs wtth the potential to associate wlth the chiral oligosaccharide 6-cyclodextrin have been investlgated for induced clrcular dlchrolsm spectra. The selection of drugs includes both chlral and achlrai compounds. Racemic mixtures are Included In the latter category. Formation constants have been determlned where possible, and values for enantiomeric pairs are compared. The problems of Interferences in the determination of achlral analytes are dlscussed.
In two previous reports from this laboratory complexations of a series of achiral drugs (1)and a series of barbitals (2)with P-cyclodextrin (BCD) were investigated by using circular dichroism (CD). The purpose was to demonstrate that a technique typically associated with the study of chiral compounds could be applied to the determination of achiral compounds with equal ease. The success of the method relies upon the capability of the achiral host (BCD) to induce CD activity into the passive guest. When the formation constants measured for these 1:l complexation processes were used, commercially available forms of meperidine hydrochloride (Demerol) and seconal sodium were successfully determined without a prior separation step. It is improper to assume from these two examples that the method has the potential for general application. Many interferences are possible. In this work we have begun a systematic study of some of these. The list includes chiral drugs, racemic mixtures, compounds with more highly substituted aromatic rings, and commonly encountered additives to commercial products. The results of the study would allow us to compare the inherent CD vs. induced CD for chiral substances, the relative stabilities of the complexes formed between D and L enantiomers and BCD, and the effect of the structure of the 0003-2700/84/0356-2827$01.50/0
guest on the magnitude of the induced Cotton effect.
EXPERIMENTAL SECTION The experimental procedures employed were exactly as described previously ( 1 , 2 ) . The compounds, with the exception of the two penicillamines, contain an aromatic and/or heterocyclic ring chromophore and an occasional carbonyl so the wavelength range of interest was from 220 to 350 nm. Those compounds which showed no evidence of a change in the CD spectra on the addition of BCD to simple solutions in deionized water were also examined in solutions buffered at pH 9.8. The 17 potential guest molecules were DL-methadone and L-isomethadone (Drug Enforcement Administration), quinine sulfate, quinidine hydrobromide, D-, L-, and DL-a-phenethylamine, L-hyoscyamine, atropine, acetylsalicylic acid, and caffeine hydrobromide (Sigma Chemical Co.), D- and DL-penicillamine (Aldrich Chemical Co.), S-(-)-nicotine (Eastman Kodak), and L-cocaine hydrochloride, mescaline, and psilocin (National Institute for Drug Abuse). RESULTS AND DISCUSSION Of the 17 compounds investigated, 9 are inherently chiral, 4 are racemates, and 4 are achiral. These last four, namely aspirin, caffeine, mescaline, and psilocin, show no evidence of chirality induction. I t should not be concluded from this that complexation does not occur. Aspirin and caffeine therefore can be excluded from the list of possible interferences in the direct determination of drugs using CD. For mescaline and psilocin the extent of aromatic substitution might prohibit a strong interaction with BCD, while the comparatively polar alkylamine substituent would be less favored by the hydrophobic center of the host. Others for which no CD evidence for complexation by BCD is observed are quinine and quinidine, D- and L-a-phenethylamine, and D- and DL-penicillamine. For the first two the aromatic ring is trisubstituted and again interaction with 0 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
2828
I
+
+
OOCCH
C
+
U
F
+ (_>yHCH3 "2
e
+
210
L
AI
c
3
-233-\
J?-E
VP
Figure 1. CD spectra of chiral molecules: solid lines represent the inherent CD;broken lines represent induced CD for (A) L-isomethadone, (B) L-hyoscyamine, (C) S-(-)-nicotine, (D) L-cocaine, (E) D and L-a-phenethylamlne, (F) o-penicillamine, (G) quinine, and (H) quinidine.
the host may be greatly restricted. The penicillamines have no phenyl substituent and are polar in aqueous media. Discussion of the enantiomers of a-phenethylamine is reserved until the results for the DL racemate are considered. For reference the CD spectra of the nine chiral compounds are shown in Figure 1. Spectra either altered or induced by the addition of BCD are given in Figure 2. CD easily distinguishes between quinine and quinidine whose spectra are essentially mirror images of each other. Distinction by their identical absorption spectra is of course not possible (3). Optical rotation at the Na-D line would also distinguish between the analogues (4)but only after their total separation from any mixture. The spectra of nicotine (5)and
cocaine (6, 7)were previously described. Cocaine is specifically included here because the mathematical procedure used to calculate complexation constants is less uncertain than the previous one (7). Correspondence is good. It is also included to compare spectra, without comment, with the structurally related compounds hyoscyamine and atropine. The most significant CD spectral change for a chiral guest on complexation is still that observed for cocaine (7). All others show a gradual increase or decrease in signal intensity with no sign inversion at the same wavelength maxima of the free drug, which is proportional to the concentration of added BCD. The same is presumably true for the racemic mixtures for which neither enantiomer was available for wavelength
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
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Table I. Formation Constants and Molar Ellipticity Data
compound
337 DL-methadonea 66 DL-a-phenethylaminea atropine (DL-hyoscyamine)‘ 308
e
L-isomethadone L-cocaine ,$(-)-nicotine
i
-
K
220
L- hyoscyamine
300
D-hyoscyamine quinine
quinidine D-penicillamine D-a-phenethylamine
467 406 358
OD (A, nm) 0
nm)
9.8 (290) 0.6 (270) 65.6 (222)
0
0
-263 -55 -60 -64 308 -268 169 t268
ODs (A,
(300) (245) (268) (264) (222) (222)
-400 +87 -64 -71 -12 -71
(300) (245) (268) (264) (222) (222)
-52 (336) +21 (259) +47 (336) -25 (259) -19 (228) -2.9 (268) -3.3 (262) -2.3 (257)
L-cu-phenethylamine* “ K values calculated as averages for both enantiomers. ues same as for the D-cy-isomer but with positive signs.
-
300
220
I
IC
V QCHCH3
Figure 2. Induced CD spectra for (A) w-methadone, (6) atropine (ol-hyoscyamine), and (C)Dc-a-phenethylamine.
maxima confirmation. CD induction was observed and is reported for DL-a-phenethylamine but not for either enantiomer. In order to observe the induced signal for the racemate, the analyte concentration had to be almost M which is about equal to the maximum concentration of BCD used. Under equivalent concentration conditions for either isomer, the CD signal inherent to the chiral molecule was at least 10 times greater than the anticipated induced signal, which introduces significant error in the measured ellipticity change with BCD concentration. No satisfactory complexation constant could be calculated. As a general rule the induced ellipticities for the racemates appear to be in the positive direction. The molecular interaction between drug (D)and sugar (S) was treated as a 1:l complexation equilibrium. The mathematical treatment was described previously ( I , @ . Equilibrium constants, K,and induced molar ellipticities are given in Table
I. K for the DL-a-phenethylamine complexation compares favorably with that for the 6-phenethylamine isomer ( I ) , implying little to no effect upon the relative stabilities by minor structural changes in the noninteracting side chain. Although ODs values are of similar magnitude, they are of opposite sign. Other K values are uniformly 300-400. ODs values in contrast show no simple regular correlation with structural type. These should not be taken too literally, however, because the values
*OD Val-
might be associated with quite different electronic transitions. Calculations from the data for the racemic mixtures in actuality produce only average values for K and ODS, which assumes both enantiomers are complexed equally. The number of instances where both enantiomers are available and the induced CD is substantial enough for an accurate distinction to be made between the equilibria is very limited. Two cases in point here are the a-phenethylamines and the penicillamines. This is true too of isomers of dopa- and phenyl-substituted amino acids. Other isomers of methadone and isomethadone were unobtainable. The only qualifiers we had access to at this time were atropine (or DL-hyoscyamine) and L-hyoscyamine. The latter occurs naturally and racemizes to atropine (-97%) upon extraction ( 4 ) . The complexation constant for L-hyoscyamine is calculated in the usual way. When the values for K and the induced molar ellipticity OLs for the L isomer (Table I) are used the observed ellipticity rc/ for the racemic mixture is corrected by subtracting the known contribution from the L isomer, leaving a value rc/’ which is descriptive of only the D isomer. K and ODs are then calculated by using the same iterative procedure as before ( I , 8). The results are shown parenthetically in Table I. K values are reproducible to within f20, and variations in ODs are on the order of f0.3 over a series of experiments. The positive induced CD signal for atropine is therefore a consequence of a competitive enantiomeric discrimination by BCD in which both isomers are complexed. For both complexed forms the molar ellipticity coefficient for the induced CD signal is of the same sign as the inherent CD, but both are numerically smaller. The thrust of this work was to investigate the possible sources of interferences in the direct determination of an achiral analyte by BCD-induced CD spectropolarimetry. The first important conclusion is that BCD is a discriminating host. Only 7 of 17 potential guests produced CD evidence for complexation, and only 3 of these were from a subset of 8 achiral compounds. These odds are very good for the direct determination of particular achiral drugs in mixtures in which the other constituents are also achiral. All CD-active chiral compounds would theoretically interfere with the achiral analyte determinations. However, this is less of a problem where the spectrum of a chiral compound is unchanged by the addition of BCD which is the cme for five of the nine chiral compounds included here. For example, in a binary mixture of a CD-active and a CD-inactive compound, the former is
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Anal. Chem. 1984,56,2830-2834
easily determined with no interference.' The latter might be determined by the addition of BCD, and the ease with which this is accomplished depends upon the effect that complexation has on the spectrum of the former analyte. Where CD induction by BCD occurs, the effect appears to be greatest when a carbonyl chromophore is present. Add to this observation the fact that carbonyl electronic transitions usually occur at wavelengths longer than 290 nm and out of range of the aromatic transitions, then suitable target analytes for future study can be identified. LITERATURE CITED (1) Han, S. M.; Purdie, N. Anal. Chem, first of three papers in this issue. (2) Han, S. M.; Purdie, N. Anal. Chem, second of three papers In this issue.
(3) Siek, T. J.; Osiewicz, R . J.; Bath, R. J. J . Forensic Sci. 1976, 21, 525. (4) "The Merck Index", 9th ed.; Merck and Co.: Rahway, NJ, 1976; p 1047.
(5) Atkinson, W. M.; Han, S. M.; Purdie, N. Anal. Chem, 1984, 56, 1947. (8) Bowen, J. M.; Purdie, N. Anal. Chem. 1981, 53, 2237. (7) Bowen, J. M.; Purdie, N. Anal. Chem. 1981, 53, 2239. (8) Monk, C. B. "Electrolytic Dissociation"; Academic Press: New York, 1961; p 186.
RECEIVED for review May 9, 1984. Resubmitted August 9, 1984. Accepted August 27,1984. Support of this work was from the National Science Foundation under Grant NSF CHE-8240564.
Kinetics of the Reduction of Copper(I1) to Copper(1) in Aqueous Solutions and the Complexation of the Copper(I)with Allyl Alcohol Naohisa Yanaghihara a n d Tetsuya Ogura
Departamento de Qulmica, Universidad Autcinoma de Guadalajara, A.P. 1-440 Guadalajara, Jalisco, Mexico Nelson Scott a n d Q u i n t u s Fernando*
Department of Chemistry, University of Arizona, Tucson, Arizona 85721
Aqueous solutions of copper( I I ) are reduced in the presence of copper metal and the olefinic ligand, allyl alcohol, to copper( I). The rate of reductlon was followed by the tltrimetrlc determination of copper( I ) wlth EDTA and also by the spectrophotometric determlnatlm of copper( I ) wlth 2,9-dlmethyl-1,lO-phenanthroline. The rate of reduction Is dependent on the pH of the solutlbn and on the concentration of sulfate ions in solutlon; the rate of reduction Is independent of the concentration of copper(1) species in solution. The klnetlc data suggest that the rate of the reduction process Is controlled by a reversible electron-transfer reaction between copper( 11) Ions in solutlon and the copper metal surface and an irreversible complexatlon of copper( I ) wlth allyl alcohol. The actlvatlon energy for the reduction process Is 20 kJ.
The stabilization of copper(1) by allyl alcohol was exploited for the determination of mixtures containing copper(1) and copper(I1) in aqueous solution. The allyl alcohol was used as a masking agent for copper(1) in the course of the titrimetric determination of copper(I1) with EDTA in an aqueous solution containing copper(1) and copper(I1). In an alternative method, mixtures of copper(1) and copper(I1) containing low concentrations of copper(1) (M) were stabilizied by the addition of allyl alcohol and the copper(1) was determined spectrophotometrically with the selective chelating agent, 2,9-dimethyl-l,lO-phenanthroline (1). In a subsequent article, the equilibria that are established when copper(I1) is reduced by copper metal in the presence of allyl alcohol were investigated and the equilibrium constants of the copper(1)-allyl alcohol complexes were determined (2). In the course of these equilibrium studies it was observed that the rate of the re-
duction reaction was influenced by hydrogen ions and sulfate ions in solution. We have studied the kinetics of this reduction reaction over a wide range of concentrations of copper(I1) and allyl alcohol, and the influence of hydrogen ions and sulfate ions on the rate data has been evaluated. On the basis of these results we have proposed a mechanism for the heterogeneous reduction of copper(I1) in aqueous solutions in the presence of copper metal and allyl alcohol. This type of reaction mechanism may have important implications in the kinetics of metal corrosion. Olefinic as well as acetylinic compounds such as allyl alcohol, propargyl alcohol, and vinylacetylene have been reported to inhibit the corrosion of metals such as iron, nickel and aluminum in acidic solutions (3-5). Corrosion of these metals is inhibited by the formation of a protective organic compound on the metal surface. Unsaturated organic compounds, however, promote the corrosion of copper metal in aqueous solutions by the formation of complexes with copper(1). The various factors that affect the rate of dissolution of copper metal in the presence of the unsaturated organic ligand, allyl alcohol, have been determined, and the results are reported below. EXPERIMENTAL SECTION Materials. Copper granules (J.T. Baker Chemical Co.), 20-30 mesh, were washed in dilute HC104,rinsed thoroughly in deionized water and acetone, and dried before use. Varying lengths of copper wire, 8.0-250 cm and 0.25 mm diameter, were used in the kinetic experiments in which the surface area of the copper metal was kept constant. Acetone was purified by refluxing for about 2 h with KMnO, and distilling over anhydrous K2C03. Allyl alcohol was purified either by distilling over anhydrous K2C03or by distilling in vacuo after drying with molecular sieves. Trace impurities of aldehydes that were present in ethanol were removed by refluxing with aluminum powder and KOH; the pure ethanol distillate was used to make up solutions of the reagent 2,9-di-
0003-2700/84/0356-2830$01.50/0 @ 1984 American Chemical Society
