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Acid dissociation of 2-mercaptohistamine and its related compounds
Talanto, Vol. 24, pp 531-532.
Pergamon
Press., 1977 Prtnted in Great Bomb.
ANALYTICAL
DATA
ACID DISSOCIATION OF Z-MERCAPTOHISTAMINE AND ITS RELATED COMPOUNDS H. SAKURAI Faculty of Pharmaceutical
Sciences, University of Tokushima, Tokushima 770, Japan and
S. TAKESHIMA Kyoto College of Pharmacy. Kyoto 607, Japan (Received 26 October 1976. Accepted 2 February 1977)
acid dissociation constants of 2-mercaptohistamine hydrochloride (2-MH) and its reiated compounds ergothioneine (Erg), 2-mercaptoimid~ole (MI), ~-methyl-Z-m~captoim~d~ole (MMI) and histamine dihydrochloride were determined by potentiometry at 15”, 25” and 35” and F = 0.1 (NaClO,). The values of AH and AS for the ionization equilibria were calculated. The magnitude of AH (9.5 kcal/mole) for pk,, of 2-MH is similar to the value AH = 9-10 kcal/mole found for the ionization of a number of amino groups. The value AH = 7.4 k&/mole for pk,, of 2-MH is similar to the values for the ionization of a variety of thiols (AH = _%7kcal/mole). Such values were not obtained for Erg, MI and MMI, which suggests that 2-MH exists mainly in the thiol form, whereas Erg, MI and MM1 are in the thione form. Summary-The
2-Mercaptohistamine (2-MH), which contains both mercaptoirnidazole and amino groups, is an analogue of ergothioneine (2-mercaptohistidine trimethyl betaine, Erg), and is estimated as a biologically active substance.’ The thiol (C-SH) or thione (CLS) forms of Erg are presumed to be related to its biological activity.* The physicochemical properties of Erg36 and such related compounds as ~-methyl-Z-mercaptoimid~ole’ 2-mercaptoimidazole.3-~ and 2-t~olhistidine’ in aqueous solution have been investigated by potentrometry and spectrometry. 2-MH, which is the most important analogue of Erg, was synthesized in 1932,’ but its physicochemical properties have not previously been determined. This paper reports the acid dissociation constants and thermodynamic parameters for 2-MH, determined by potentiometry. The involvement of the thiol rather than the thione form in the dissociation of 2-MH is strongly Indicated. EXPERIMENTAL
with a Denkikagaku-keiki MC-30 combination electrode adjusted with standard solutions. A solution containing 2 mmole of the compound in 10 ml was titrated with carbonate-free O.lM potassium hydroxide, under nitrogen and with magnetic stirring. The ionic strength was adjusted to 0.1 with sodium perchlorate. The potentiometric measurements were made at 15 + 0.t”. 25 i 0.1” and 35 i 0.1”. Ionization constants were calculated according to the method of Schwarzenbach.” When AH was constant over the temperature range used, AH was calculated from l”+_~
The potentiometric titrations were done with a Hiranuma Titrator TB-101 and Titrigraph Rat 101 equipped
11
and AS from AG = AH - TAS where K, and Kr are the equdibrium constants at absolute temperatures Ti and Tz respectively. Otherwise AH and AS were calculated by the method of Fujishiro et aLi3 RESULTS
Materials
2-Mercaptohist~ine hydrochloride was prepared according to the method of Fraser and Raphael,” and recrystallized from 80y0 acetic acid, mp. 241-2” (uncorrected; Fraser and Raphael give 244-S”). This compound gave only one spot on thin-layer chromatography (silica ammonia solution 3: 1; gel ; solvent, ethanol-28% 4 E 0.64), and the structure was identified by both infrared and nuclear magnetic resonance spectroscopy. 2-Mercaptoimidazole (MI), recrystallized from water, m.p. 221~2”, N-methyl-2-mercaptoimidazole (MMI), recrystallized from alcohol, m.p. 2434”, ergothioneine and histamine dihydrochloride were obtained commercially. Carbonate-free potassium hydroxide solution was prepared by the method of Armstrong. I1 Other reagents used were guaranteed grade.
f-f
( 2
L
AND
DISCUSSION
The ionization constants and thermodynamic parameters are summarized in Table 1. The ionization constants were not intluenced by the speed of titration within the range used. The ionization constants of Erg, MI and MM1 agreed well with the values previously reported.%’ The dissociation constants of thiolhistidine were determined as p&,, = 1.84, pk,, = 8.47 and pk,, = 11.4,’ pk,, and pk,, being assigned to the deprotonation of the amino and thiol groups, respectively, by analogy with the constants for cysteine and histidine. As shown in Table 1, the pk., values of Erg, MI and MM1 are similar to those for the thiol groups of various types of thiol compound, but the thermodynamic parameters, especially the entropy change, are different from those for thiol groups. Therefore, a simple comparison of the dissociation constants alone may lead to an erroneous assignment of the pk, values. A plot of TAS against AH is linear for Erg, MI and MMI, but the point for 2-MH is well away from the line. This result shows that for 2-MH there is an additional factor besides 531
532
ANALYTICAL
Chart 1. Probable ionization equilibrium of 2-mercaptohistamine. the simple solute-solvent interaction” which is presumed to exist in the other three compounds. The magnitude of AH1 = 9.5 kcal/mole for pk,, of 2-MH compares favourably with values of AH = 9-12 kcal/mole for the ionization of a number of a-amino-acid esters,” ethylamine,16 glycine” and histamine.*18 Also, the values of AHz = 7.4 kcal/mole and AS2 = -28 Cal. mole- I. deg-’ for pk,, are similar to the values AH = >7 kcal/mole and AS = 20-30 cal.mole- ‘. deg-’ for the ionization of a variety of thio1s.t ‘9-*’ On the basis of the thermodynamic data, the predominant ionization processes appear to be those given in Chart 1. Therefore, it would be concluded that the first and second deprotonation steps of 2-MH are those of the amino and thiol groups, respectively. On the other hand, with such compounds as Erg, MI and MMI, the thermodynamic data imply that the thiol forms are not involved in the deprotonation process, but that the thione forms are, as described previously.3-7 REFERENCES 1.
D. B. Melville, Vitam.Harm., 1959, 17, 155.
*For histamine, pk., was related to the deprotonation of imidazole from the results in Table 1 and reference 18. t On the basis of thermodyn~i~ data, pk,, for cysteine esters and ~ysteamine was related to the deprotonation of the thiol group.20,z’
DATA
2. A. Hama, K. Okumura, N. Tamaki and K. Konishi, f. Pkurm. sot. Japan, 1973,93, 369. 3. 3. Stanovik and M. Tisler, Anal. Biockem., 1964, 9, 68. 4. D. P. Hanlon, J. Med. Ckem., 1971, 14, 1084. 5. J. Carlsson, M. P. J. Kierstan and K. Brochlehurst, Biockem. J., 1974, 139, 221. 6. N. Motohashi, I. Mori, Y. Sugiura and H. Tanaka, Ckem. Pkarm. Bull., 1974, 22, 654. 7. A. Lawson and H. V. Morley, J. Ckem. Sot., 1956, 1103. 8. C. M. Richardson, Biockem. J., 1933, 27, 1036. 9. S. Akabori and S. Numano, Nippon Kagaku-kaishi, 1932, 53, 13. 10. M. M. Fraser and R. A. Raphael, J. Chem. Sot., 1952, 226. 11. D. M. G. Armstrong Ckem. Ind. (~~a~), 1955, 1405. Hefu. Ckim. Acta, 1950, 33, 947. 12. G. Schw~enbach, 13. R. Fujishiro, S. Yada and R. Tamamushi, The Properties ofSofutions,p. 121. Kagaku Dozine, Tokyo, 1968. 14. K. J. Laider, Trans. Faraday Sot., 1959, 55, 172.5. 15. R. W. Hay and L. J. Porter, J. Ckem. Sot. (B), 1967, 1261. 16. A. C. Andrews and B. L. Mickel, J. Am. Ckem. Sot., 1955, 77, 5291. 17. C. B. Murphy and A. E. Martell, J. Biol. Ckem., 1957, 226, 37. 18. C. Tanford and L. Wagner, J. Am. Ckem. Sot., 1953, 75, 435. 19. R. J. Irving, L. Nelander and J. Wadso, Acta Ckem. Scand., 1964, 18, 769. 20. L. J. Porter, I). D. Perrin and R. W. Hay, J. Ckem. Sot. (A), 1969, 118. 21. H. Sakurai, unpublished data.
Table 1. Acid dissociation constants and thermodynamic parameters of 2-mercaptohistamine (p = 0.1 NaClO,) Temp. “C
Compound 2-Mercaptohistamine.
HCl
Ergothioneine
2-Mercaptoimidazole
N-Methyl-2-mercaptoimidazole Histamine’2HCl
Imidazole’s Ethylamine16 Glycinate” Cysteine methyl esterzO Cysteine ethyl este? Ethylmercaptan” Mercaptoethanol’4 fl-Mercaptopropionate” Mercaptoacetic acid”
-
35 15 25 35 15 25 35 :i: 35 15 25 35 15 25 35 0 25 0.4 30 48.8 25 25 :: ::
AG1 pk,, 9.36 f 0.01 9.12 f 0.02 8.86 &-0.04 10.72 i 0.04 10.44 * 0.04 10.11 f 0.02 11.47 + 0.03 11.21 f 0.03 10.74 + 0.03 12.15 + 0.06 11.64 If: 0.04 11.31 + 0.03 6.40 f 0.01 6.13 + 0.01 6.05 + 0.01 7.31 7.12 6.93 11.4 10.8 10.25 9.44 9.00 6.59 6.56 10.61 9.72 10.84 10.68
pk,, 11.74 $ 0.05 11.62 + 0.03 11.37 f 0.06
10.10 4 0.02 9.82 + 0.01 9.62 + 0.01
9.10
9.10 8.98
AGz
kcallmole
12.3 12.4 12.5 14.1 14.2 14.3 15.1 15.3 15.1 16.0 15.9 15.9 8.3 8.4 8.5
15.5 15.8 16.0
AH,
and related compounds AIf*
kcal/mole
9.5
AS,
7.4
11.6 14.9 16.8 13.3 13.4 13.6
7.0
9.5
-9.7 -9.7 -9.6 -9.0 -8.7 -8.5 -0.7 - 1.0 -1.5 + 2.8 + 2.8 + 2.8 -4.8 -4.9 -4.8
7.5
-6.7
9
-19
10.2
-9.5
9.15 8.0 6.42 6.21 6.10 6.28
ASI
cal.mole-‘.deg-’
5.28 4.9
-28.6 -28.2 -27.9
-11.0 -13.0 -15.0
0.8 -24 -2.4 -24.7 -27 - 23.1 -29.2 - 27.8
Pergamon
Press., 1977 Prtnted in Great Bomb.
ANALYTICAL
DATA
ACID DISSOCIATION OF Z-MERCAPTOHISTAMINE AND ITS RELATED COMPOUNDS H. SAKURAI Faculty of Pharmaceutical
Sciences, University of Tokushima, Tokushima 770, Japan and
S. TAKESHIMA Kyoto College of Pharmacy. Kyoto 607, Japan (Received 26 October 1976. Accepted 2 February 1977)
acid dissociation constants of 2-mercaptohistamine hydrochloride (2-MH) and its reiated compounds ergothioneine (Erg), 2-mercaptoimid~ole (MI), ~-methyl-Z-m~captoim~d~ole (MMI) and histamine dihydrochloride were determined by potentiometry at 15”, 25” and 35” and F = 0.1 (NaClO,). The values of AH and AS for the ionization equilibria were calculated. The magnitude of AH (9.5 kcal/mole) for pk,, of 2-MH is similar to the value AH = 9-10 kcal/mole found for the ionization of a number of amino groups. The value AH = 7.4 k&/mole for pk,, of 2-MH is similar to the values for the ionization of a variety of thiols (AH = _%7kcal/mole). Such values were not obtained for Erg, MI and MMI, which suggests that 2-MH exists mainly in the thiol form, whereas Erg, MI and MM1 are in the thione form. Summary-The
2-Mercaptohistamine (2-MH), which contains both mercaptoirnidazole and amino groups, is an analogue of ergothioneine (2-mercaptohistidine trimethyl betaine, Erg), and is estimated as a biologically active substance.’ The thiol (C-SH) or thione (CLS) forms of Erg are presumed to be related to its biological activity.* The physicochemical properties of Erg36 and such related compounds as ~-methyl-Z-mercaptoimid~ole’ 2-mercaptoimidazole.3-~ and 2-t~olhistidine’ in aqueous solution have been investigated by potentrometry and spectrometry. 2-MH, which is the most important analogue of Erg, was synthesized in 1932,’ but its physicochemical properties have not previously been determined. This paper reports the acid dissociation constants and thermodynamic parameters for 2-MH, determined by potentiometry. The involvement of the thiol rather than the thione form in the dissociation of 2-MH is strongly Indicated. EXPERIMENTAL
with a Denkikagaku-keiki MC-30 combination electrode adjusted with standard solutions. A solution containing 2 mmole of the compound in 10 ml was titrated with carbonate-free O.lM potassium hydroxide, under nitrogen and with magnetic stirring. The ionic strength was adjusted to 0.1 with sodium perchlorate. The potentiometric measurements were made at 15 + 0.t”. 25 i 0.1” and 35 i 0.1”. Ionization constants were calculated according to the method of Schwarzenbach.” When AH was constant over the temperature range used, AH was calculated from l”+_~
The potentiometric titrations were done with a Hiranuma Titrator TB-101 and Titrigraph Rat 101 equipped
11
and AS from AG = AH - TAS where K, and Kr are the equdibrium constants at absolute temperatures Ti and Tz respectively. Otherwise AH and AS were calculated by the method of Fujishiro et aLi3 RESULTS
Materials
2-Mercaptohist~ine hydrochloride was prepared according to the method of Fraser and Raphael,” and recrystallized from 80y0 acetic acid, mp. 241-2” (uncorrected; Fraser and Raphael give 244-S”). This compound gave only one spot on thin-layer chromatography (silica ammonia solution 3: 1; gel ; solvent, ethanol-28% 4 E 0.64), and the structure was identified by both infrared and nuclear magnetic resonance spectroscopy. 2-Mercaptoimidazole (MI), recrystallized from water, m.p. 221~2”, N-methyl-2-mercaptoimidazole (MMI), recrystallized from alcohol, m.p. 2434”, ergothioneine and histamine dihydrochloride were obtained commercially. Carbonate-free potassium hydroxide solution was prepared by the method of Armstrong. I1 Other reagents used were guaranteed grade.
f-f
( 2
L
AND
DISCUSSION
The ionization constants and thermodynamic parameters are summarized in Table 1. The ionization constants were not intluenced by the speed of titration within the range used. The ionization constants of Erg, MI and MM1 agreed well with the values previously reported.%’ The dissociation constants of thiolhistidine were determined as p&,, = 1.84, pk,, = 8.47 and pk,, = 11.4,’ pk,, and pk,, being assigned to the deprotonation of the amino and thiol groups, respectively, by analogy with the constants for cysteine and histidine. As shown in Table 1, the pk., values of Erg, MI and MM1 are similar to those for the thiol groups of various types of thiol compound, but the thermodynamic parameters, especially the entropy change, are different from those for thiol groups. Therefore, a simple comparison of the dissociation constants alone may lead to an erroneous assignment of the pk, values. A plot of TAS against AH is linear for Erg, MI and MMI, but the point for 2-MH is well away from the line. This result shows that for 2-MH there is an additional factor besides 531
532
ANALYTICAL
Chart 1. Probable ionization equilibrium of 2-mercaptohistamine. the simple solute-solvent interaction” which is presumed to exist in the other three compounds. The magnitude of AH1 = 9.5 kcal/mole for pk,, of 2-MH compares favourably with values of AH = 9-12 kcal/mole for the ionization of a number of a-amino-acid esters,” ethylamine,16 glycine” and histamine.*18 Also, the values of AHz = 7.4 kcal/mole and AS2 = -28 Cal. mole- I. deg-’ for pk,, are similar to the values AH = >7 kcal/mole and AS = 20-30 cal.mole- ‘. deg-’ for the ionization of a variety of thio1s.t ‘9-*’ On the basis of the thermodynamic data, the predominant ionization processes appear to be those given in Chart 1. Therefore, it would be concluded that the first and second deprotonation steps of 2-MH are those of the amino and thiol groups, respectively. On the other hand, with such compounds as Erg, MI and MMI, the thermodynamic data imply that the thiol forms are not involved in the deprotonation process, but that the thione forms are, as described previously.3-7 REFERENCES 1.
D. B. Melville, Vitam.Harm., 1959, 17, 155.
*For histamine, pk., was related to the deprotonation of imidazole from the results in Table 1 and reference 18. t On the basis of thermodyn~i~ data, pk,, for cysteine esters and ~ysteamine was related to the deprotonation of the thiol group.20,z’
DATA
2. A. Hama, K. Okumura, N. Tamaki and K. Konishi, f. Pkurm. sot. Japan, 1973,93, 369. 3. 3. Stanovik and M. Tisler, Anal. Biockem., 1964, 9, 68. 4. D. P. Hanlon, J. Med. Ckem., 1971, 14, 1084. 5. J. Carlsson, M. P. J. Kierstan and K. Brochlehurst, Biockem. J., 1974, 139, 221. 6. N. Motohashi, I. Mori, Y. Sugiura and H. Tanaka, Ckem. Pkarm. Bull., 1974, 22, 654. 7. A. Lawson and H. V. Morley, J. Ckem. Sot., 1956, 1103. 8. C. M. Richardson, Biockem. J., 1933, 27, 1036. 9. S. Akabori and S. Numano, Nippon Kagaku-kaishi, 1932, 53, 13. 10. M. M. Fraser and R. A. Raphael, J. Chem. Sot., 1952, 226. 11. D. M. G. Armstrong Ckem. Ind. (~~a~), 1955, 1405. Hefu. Ckim. Acta, 1950, 33, 947. 12. G. Schw~enbach, 13. R. Fujishiro, S. Yada and R. Tamamushi, The Properties ofSofutions,p. 121. Kagaku Dozine, Tokyo, 1968. 14. K. J. Laider, Trans. Faraday Sot., 1959, 55, 172.5. 15. R. W. Hay and L. J. Porter, J. Ckem. Sot. (B), 1967, 1261. 16. A. C. Andrews and B. L. Mickel, J. Am. Ckem. Sot., 1955, 77, 5291. 17. C. B. Murphy and A. E. Martell, J. Biol. Ckem., 1957, 226, 37. 18. C. Tanford and L. Wagner, J. Am. Ckem. Sot., 1953, 75, 435. 19. R. J. Irving, L. Nelander and J. Wadso, Acta Ckem. Scand., 1964, 18, 769. 20. L. J. Porter, I). D. Perrin and R. W. Hay, J. Ckem. Sot. (A), 1969, 118. 21. H. Sakurai, unpublished data.
Table 1. Acid dissociation constants and thermodynamic parameters of 2-mercaptohistamine (p = 0.1 NaClO,) Temp. “C
Compound 2-Mercaptohistamine.
HCl
Ergothioneine
2-Mercaptoimidazole
N-Methyl-2-mercaptoimidazole Histamine’2HCl
Imidazole’s Ethylamine16 Glycinate” Cysteine methyl esterzO Cysteine ethyl este? Ethylmercaptan” Mercaptoethanol’4 fl-Mercaptopropionate” Mercaptoacetic acid”
-
35 15 25 35 15 25 35 :i: 35 15 25 35 15 25 35 0 25 0.4 30 48.8 25 25 :: ::
AG1 pk,, 9.36 f 0.01 9.12 f 0.02 8.86 &-0.04 10.72 i 0.04 10.44 * 0.04 10.11 f 0.02 11.47 + 0.03 11.21 f 0.03 10.74 + 0.03 12.15 + 0.06 11.64 If: 0.04 11.31 + 0.03 6.40 f 0.01 6.13 + 0.01 6.05 + 0.01 7.31 7.12 6.93 11.4 10.8 10.25 9.44 9.00 6.59 6.56 10.61 9.72 10.84 10.68
pk,, 11.74 $ 0.05 11.62 + 0.03 11.37 f 0.06
10.10 4 0.02 9.82 + 0.01 9.62 + 0.01
9.10
9.10 8.98
AGz
kcallmole
12.3 12.4 12.5 14.1 14.2 14.3 15.1 15.3 15.1 16.0 15.9 15.9 8.3 8.4 8.5
15.5 15.8 16.0
AH,
and related compounds AIf*
kcal/mole
9.5
AS,
7.4
11.6 14.9 16.8 13.3 13.4 13.6
7.0
9.5
-9.7 -9.7 -9.6 -9.0 -8.7 -8.5 -0.7 - 1.0 -1.5 + 2.8 + 2.8 + 2.8 -4.8 -4.9 -4.8
7.5
-6.7
9
-19
10.2
-9.5
9.15 8.0 6.42 6.21 6.10 6.28
ASI
cal.mole-‘.deg-’
5.28 4.9
-28.6 -28.2 -27.9
-11.0 -13.0 -15.0
0.8 -24 -2.4 -24.7 -27 - 23.1 -29.2 - 27.8