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Interaction of kojic acid with gold(III) ions
Analytica Chimica Acta, 106 (1979) 147-150 0 Elsevier Scientific Publishing Company, Amsterdam
- Printed in The Netherlands
Short Communication
INTERACTION
OF KOJIC ACID WITH GOLD(II1) IONS
DA’ITA V. NAIK Department
of Chemistry,
Monmouth
College,
West Long
Branch,
New
Jersey,
07764
(U.S.A.)
(Received 24th August 1978)
Kojic acid (5-hydroxy-(2-hydroxymethyl)-4H-pyran4-one) forms stable complexes with several metal ions, including transition metal [ 11 and lanthanide [2] ions; the chelate ring formed is similar to the chelates formed by related ligands such as tropolone and acetylacetone. Of the metal-kojic acid complexes reported to date, that with iron(II1) is intensely colored and has been used for calorimetric determinations of iron [ 31. Murata and Ujihara [ 41 reported that kojic acid reacts with gold(II1) in the pH range 5.7-6.8 to yield a complex with an intense blue-green fluorescence which they used in a fluorimetric determination of gold. In preliminary spectrofluorimetric analyses, no native fluorescence was detected from the unionized or anionic forms of kojic acid. Heavy metal ions generally quench fluorescence [ 51 and so the reported fluorescence [ 41 of a gold(III)-kojic acid complex was puzzling. Therefore, a study was undertaken to characterize the nature of the interaction of kojic acid with gold(II1). Experimental Kojic acid (Aldrich Chemical Co.) was purified as described by McBryde and Atkinson [3]. The white product melted at 152-153°C (lit. value, 152°C [6] ). Reagent-grade tetrachloroauric acid (J. T. Baker Chemical Co.) and reagent-grade sulfuric acid and sodium hydroxide (Mallinckrodt Chemical Works) were used. Aqueous solutions were prepared with distilled deionized water; other solvents were of spectroscopic quality. The photometric titration procedure for the determination of ionization constants was that described by Capomacchia et al. [ 71. All kojic acid solutions were freshly prepared to avoid chemical decomposition;solutions of different pH and Hammett acidities were prepared; the corrected Hammett acidity scale was used [8] _ To check that the pH of the solutions remained constant, checks were made immediately before and after the spectral measurements. Electronic absorption spectra were obtained with Beckman Model DB and Model DB-GT spectrophotometers, and fluorescence measurements with a Perkin-Elmer Model MPF-2A spectrometer. An Orion Model 701 digital pH meter equipped with a silversilver chloride--glass combination electrode was used.
148
Results and discussion The low-frequency absorption maxima and the ground-state dissociation constants of the prototropic species derived from kojic acid are given in Table 1. The U.V. absorption spectra of kojic acid (N), its monocation (C) and its anion (A) are shown in Fig. 1. Kojic acid has two oxygen atoms capable of participating in two protolytic equilibria, involving the species C, N and A. The acidity constant, pKal, for C * N is -1_5 and the constant pK,,,.for N + A is 7.80_ The value obtained from the photometric titration is in agreement (Table 1) with the literature values determined potentiometrically [2, 3, 9 J . TABLE
1
Ultraviolet absorption maxima, molar absorptivities (E) and ground-state acidity constants for the prototropic species derived from kojic acid Kojic Acid
h,(nm)
log E
PK,
253
3.94
-1.5
283 267
3.84 3.93
Monocation (C) Neutral (N)
I 7.80
313
Anion (A)
OU 220
280
340
3.76
Lit. values
7.75 7.83 7.61
400
X (nm)
Fig. 1. Ultraviolet absorption spectra of prototropic species derived from kojic acid: N = neutral molecule, C = monocation, and A = anion.
[9] [33 [2]
149
(Cl
(N)
(Al
R = -CH20H
The value of -1.5 for pK,, indicates that the carbonyl oxygen of kojic acid is more basic than its counterpart in an aliphatic ketone or aldehyde. The value of p&E, for the 5-hydroxyl proton in kojic acid is comparable to the corresponding values of pK, for tropolone, 6.69, and acetylacetone, 8.86 [2] . The higher acidity of the hydroxyl group in kojic acid and tropolone, compared with that in acetylacetone, may be explained by the fact that the hydroxyl group and the electron-withdrawing carbonyl group are in fl-positions in acetylacetone (enol form); in kojic acid and tropolone the two groups are in e-positions. The higher acidity of tropolone relative to kojic acid is probably due to the higher electron-withdrawing nature of the seven membered unsaturated ring in tropolone compared with the six-membered ring of kojic acid _ Thus, the ability of kojic acid to function as a relatively good anionic bidentate ligand (e.g. Fe(kojate), is a six-coordinate complex with an overall constant log 6 = 25.35 [3] ) towards a variety of metal ions appears to be related to the basicity of its carbonyl oxygen and the acidity of its hydroxyl group. Attempts to detect fluorescence from neutral, anionic or cationic forms of kojic acid in different solvents and in aqueous solution at different pH were unsuccessful. However, as reported by Murata and Ujihara [4], the addition of small amounts of aqueous tetrachloroauric acid to a solution of kojic acid (pH 7) produced an intense green fluorescence (with excitation and emission maxima at 385 and 486 nm, respectively) within a few minutes. When 10.0 ml of 1.0 X 10m4 M kojic acid solution at pH 6.8 was mixed with 10 1_t1 of 2.0 X 10e3 M tetrachloroauric acid solution, maximum fluorescence intensity was obtained in 6 min. Simultaneously, however, the fluorescent solution changed from clear to a colloidal solution with the characteristic purple tint of colloidal gold. Upon centrifugation of the reaction mixture a clear lightyellow solution with intense green fluorescence separated; the absorption spectrum was essentially identical to that of a mixture of kojic acid and its anion with an additional shoulder at 365 nm. Qualitative tests confirmed that the colloidal residue was metallic gold. Since kojic acid reduces ammoniacal silver ions (Tollen’s test) [6] (E” for Ag(NH,); + e * Ag + 2NH3 is 0.‘373 V [lo] ), it is not surprising that it reduces AuCl, ions to gold (II0 for AuCl, + 3e * Au + 4Cl- is 1.00 V [lo] )_ Because the production of fluorescence is accompanied by the reduction of AuCI, ions, the fluorophore may be an oxidation product of kojic acid. This was verified by the fact that when 10.0 ml of 1.0 X 10m4M kojic acid solution was mixed with 10 ~1 of 0.01 M K&O8 (E” for S20s2- + 2e + 2S042- is
150
2.01 V [IO] ) a & een fluorescence developed_ The rate of fluorophore generation was very slow at pH 6.8, but was considerably higher at pH 10. The fluorescence excitation and emission spectra of the fluorophore obtained by the kojic acid-peroxosulfate reaction were identical with the spectra obtained for the fluorophore produced in the kojic acid-gold(III) reaction. The actual identity of the fluorophore is not known. It appears that the fluorophore is not a gold(EII)-kojic acid complex but an oxidation product of kojic acid. Thus the reagents used for the fluorimetric determination of gold by the method of Murata and Ujihara [4] must be free of oxidizing and reducing impurities which may interfere with the formation of the fluorophore. The author is grateful to the College of Pharmacy, University of Florida, and t.o Dr. Stephen G. SchuIman for the use of a fluorescence spectrometer. This work was partly supported by a Grant-in-Aid-for-Creativity from Monmouth College. REFERENCES 1 2 3 4 5 6 7 8
9
10
J_ W. Wiley, G. N. Tyson, Jr. and G. S. Stillner, J_ Am. Chem. Sot., 64 (1942) 963. R. Stampfli and G. R. Choppin, J. Coord. Chem., 1 (1971) 173. W. A. E. McBryde and G. F. Atkinson, Can. J_ Chem., 39 (1961) 510. A. Murata and T. Ujihara, Bunseki Kagaku, 10 (1961) 497; Chem. Abstr., 58 (1964) 6180d_ E. L. Wehry in G. G. Guilbault (Ed.), Fluorescence: Theory, Instrumentation, and Practice, Dekker, New York, 1967, p_ 96_ T. Yabuta. J. Chem. Sot. Tokyo, 37 (1916) 1185. A. C. Capomacchia, J. Casper and S. G. Schulman, J. Pharm. Sci., 63 (1974) 1272. M. J. Jorgensen and D. R_ Hartter. J. Am_ Chem_ Sot., 85 (1963) 878. A. Okac and 2. Kolarik, Collect. Czech. Chem. Commun., 24 (1959) 266. W. M. Latimer, The Oxidation States of the Elements and Their Potentials in Aqueous Solution, 2nd edn., Prentice Hall Inc., Englewood Cliffs, N-J., 1952.
- Printed in The Netherlands
Short Communication
INTERACTION
OF KOJIC ACID WITH GOLD(II1) IONS
DA’ITA V. NAIK Department
of Chemistry,
Monmouth
College,
West Long
Branch,
New
Jersey,
07764
(U.S.A.)
(Received 24th August 1978)
Kojic acid (5-hydroxy-(2-hydroxymethyl)-4H-pyran4-one) forms stable complexes with several metal ions, including transition metal [ 11 and lanthanide [2] ions; the chelate ring formed is similar to the chelates formed by related ligands such as tropolone and acetylacetone. Of the metal-kojic acid complexes reported to date, that with iron(II1) is intensely colored and has been used for calorimetric determinations of iron [ 31. Murata and Ujihara [ 41 reported that kojic acid reacts with gold(II1) in the pH range 5.7-6.8 to yield a complex with an intense blue-green fluorescence which they used in a fluorimetric determination of gold. In preliminary spectrofluorimetric analyses, no native fluorescence was detected from the unionized or anionic forms of kojic acid. Heavy metal ions generally quench fluorescence [ 51 and so the reported fluorescence [ 41 of a gold(III)-kojic acid complex was puzzling. Therefore, a study was undertaken to characterize the nature of the interaction of kojic acid with gold(II1). Experimental Kojic acid (Aldrich Chemical Co.) was purified as described by McBryde and Atkinson [3]. The white product melted at 152-153°C (lit. value, 152°C [6] ). Reagent-grade tetrachloroauric acid (J. T. Baker Chemical Co.) and reagent-grade sulfuric acid and sodium hydroxide (Mallinckrodt Chemical Works) were used. Aqueous solutions were prepared with distilled deionized water; other solvents were of spectroscopic quality. The photometric titration procedure for the determination of ionization constants was that described by Capomacchia et al. [ 71. All kojic acid solutions were freshly prepared to avoid chemical decomposition;solutions of different pH and Hammett acidities were prepared; the corrected Hammett acidity scale was used [8] _ To check that the pH of the solutions remained constant, checks were made immediately before and after the spectral measurements. Electronic absorption spectra were obtained with Beckman Model DB and Model DB-GT spectrophotometers, and fluorescence measurements with a Perkin-Elmer Model MPF-2A spectrometer. An Orion Model 701 digital pH meter equipped with a silversilver chloride--glass combination electrode was used.
148
Results and discussion The low-frequency absorption maxima and the ground-state dissociation constants of the prototropic species derived from kojic acid are given in Table 1. The U.V. absorption spectra of kojic acid (N), its monocation (C) and its anion (A) are shown in Fig. 1. Kojic acid has two oxygen atoms capable of participating in two protolytic equilibria, involving the species C, N and A. The acidity constant, pKal, for C * N is -1_5 and the constant pK,,,.for N + A is 7.80_ The value obtained from the photometric titration is in agreement (Table 1) with the literature values determined potentiometrically [2, 3, 9 J . TABLE
1
Ultraviolet absorption maxima, molar absorptivities (E) and ground-state acidity constants for the prototropic species derived from kojic acid Kojic Acid
h,(nm)
log E
PK,
253
3.94
-1.5
283 267
3.84 3.93
Monocation (C) Neutral (N)
I 7.80
313
Anion (A)
OU 220
280
340
3.76
Lit. values
7.75 7.83 7.61
400
X (nm)
Fig. 1. Ultraviolet absorption spectra of prototropic species derived from kojic acid: N = neutral molecule, C = monocation, and A = anion.
[9] [33 [2]
149
(Cl
(N)
(Al
R = -CH20H
The value of -1.5 for pK,, indicates that the carbonyl oxygen of kojic acid is more basic than its counterpart in an aliphatic ketone or aldehyde. The value of p&E, for the 5-hydroxyl proton in kojic acid is comparable to the corresponding values of pK, for tropolone, 6.69, and acetylacetone, 8.86 [2] . The higher acidity of the hydroxyl group in kojic acid and tropolone, compared with that in acetylacetone, may be explained by the fact that the hydroxyl group and the electron-withdrawing carbonyl group are in fl-positions in acetylacetone (enol form); in kojic acid and tropolone the two groups are in e-positions. The higher acidity of tropolone relative to kojic acid is probably due to the higher electron-withdrawing nature of the seven membered unsaturated ring in tropolone compared with the six-membered ring of kojic acid _ Thus, the ability of kojic acid to function as a relatively good anionic bidentate ligand (e.g. Fe(kojate), is a six-coordinate complex with an overall constant log 6 = 25.35 [3] ) towards a variety of metal ions appears to be related to the basicity of its carbonyl oxygen and the acidity of its hydroxyl group. Attempts to detect fluorescence from neutral, anionic or cationic forms of kojic acid in different solvents and in aqueous solution at different pH were unsuccessful. However, as reported by Murata and Ujihara [4], the addition of small amounts of aqueous tetrachloroauric acid to a solution of kojic acid (pH 7) produced an intense green fluorescence (with excitation and emission maxima at 385 and 486 nm, respectively) within a few minutes. When 10.0 ml of 1.0 X 10m4 M kojic acid solution at pH 6.8 was mixed with 10 1_t1 of 2.0 X 10e3 M tetrachloroauric acid solution, maximum fluorescence intensity was obtained in 6 min. Simultaneously, however, the fluorescent solution changed from clear to a colloidal solution with the characteristic purple tint of colloidal gold. Upon centrifugation of the reaction mixture a clear lightyellow solution with intense green fluorescence separated; the absorption spectrum was essentially identical to that of a mixture of kojic acid and its anion with an additional shoulder at 365 nm. Qualitative tests confirmed that the colloidal residue was metallic gold. Since kojic acid reduces ammoniacal silver ions (Tollen’s test) [6] (E” for Ag(NH,); + e * Ag + 2NH3 is 0.‘373 V [lo] ), it is not surprising that it reduces AuCl, ions to gold (II0 for AuCl, + 3e * Au + 4Cl- is 1.00 V [lo] )_ Because the production of fluorescence is accompanied by the reduction of AuCI, ions, the fluorophore may be an oxidation product of kojic acid. This was verified by the fact that when 10.0 ml of 1.0 X 10m4M kojic acid solution was mixed with 10 ~1 of 0.01 M K&O8 (E” for S20s2- + 2e + 2S042- is
150
2.01 V [IO] ) a & een fluorescence developed_ The rate of fluorophore generation was very slow at pH 6.8, but was considerably higher at pH 10. The fluorescence excitation and emission spectra of the fluorophore obtained by the kojic acid-peroxosulfate reaction were identical with the spectra obtained for the fluorophore produced in the kojic acid-gold(III) reaction. The actual identity of the fluorophore is not known. It appears that the fluorophore is not a gold(EII)-kojic acid complex but an oxidation product of kojic acid. Thus the reagents used for the fluorimetric determination of gold by the method of Murata and Ujihara [4] must be free of oxidizing and reducing impurities which may interfere with the formation of the fluorophore. The author is grateful to the College of Pharmacy, University of Florida, and t.o Dr. Stephen G. SchuIman for the use of a fluorescence spectrometer. This work was partly supported by a Grant-in-Aid-for-Creativity from Monmouth College. REFERENCES 1 2 3 4 5 6 7 8
9
10
J_ W. Wiley, G. N. Tyson, Jr. and G. S. Stillner, J_ Am. Chem. Sot., 64 (1942) 963. R. Stampfli and G. R. Choppin, J. Coord. Chem., 1 (1971) 173. W. A. E. McBryde and G. F. Atkinson, Can. J_ Chem., 39 (1961) 510. A. Murata and T. Ujihara, Bunseki Kagaku, 10 (1961) 497; Chem. Abstr., 58 (1964) 6180d_ E. L. Wehry in G. G. Guilbault (Ed.), Fluorescence: Theory, Instrumentation, and Practice, Dekker, New York, 1967, p_ 96_ T. Yabuta. J. Chem. Sot. Tokyo, 37 (1916) 1185. A. C. Capomacchia, J. Casper and S. G. Schulman, J. Pharm. Sci., 63 (1974) 1272. M. J. Jorgensen and D. R_ Hartter. J. Am_ Chem_ Sot., 85 (1963) 878. A. Okac and 2. Kolarik, Collect. Czech. Chem. Commun., 24 (1959) 266. W. M. Latimer, The Oxidation States of the Elements and Their Potentials in Aqueous Solution, 2nd edn., Prentice Hall Inc., Englewood Cliffs, N-J., 1952.