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Fourier transform infrared study on the identification of gossypol tautomers
Journal of Molecular Structure, 239 (1990) 23-31 Elsevier Science Publishers B.V., Amsterdam
2:3
FOURIER TRANSFORM INFRARED STUDY ON THE IDENTIFICATION OF GOSSYPOL TAUTOMERS
BOGUMIL BRZEZINSKI,
JERZY OLEJNIK and STEFAN PASZYC
Department of Chemistry, A. Mickiewicz University, 60-780 Poznari (Poland) (Received 8 November 1989 )
ABSTRACT
Gossypol and its adducts with some organic compounds, as well as its two tetrabutylammonium salts (2 : 1 and 4 : 1) , were studied in various solutions by Fourier transform IR (FTIR) spectroscopy. In CH,Cl, solution gossypol exists as the aldehyde tautomer. In the DMSO-d, solution an equilibrium between the aldehyde and lactol tautomers was observed in which the predominant form is the latter. The IR spectrum of the 2: 1 tetrabutylammonium salt indicates that a very complex equilibrium occurs in CD&N solution. In the case of the 4: 1 salt the ketol tautomeric form is associated via very strong intermolecular (0. *H* -0) - hydrogen bonds with large proton polarizability.
INTRODUCTION
Gossypol, 2,2’ -bis (8-formyl-1,6,7-trihydroxy-5-isopropyl-3-methylnaphthalene) and its derivatives are of great interest because of their biological importance. Most significant are their contraceptive and toxic activities. For these reasons extensive investigations of the chemical properties, reactivity and structure of gossypol have been carried out. Various spectroscopic techniques for the determination of the gossypol structure have been used [l-9]. IR studies of gossypol in the solid state and in several solvents have confirmed the observation of Adams et al. that gossypol exists in three tautomeric forms l-3 [lo]. The aldehyde tautomer ( 1) has been observed for gossypol in the solid state [ 111 and in chloroform solutions [ 1,5]. The lactol (2) and the ketol (3) tautomeric forms have been found by ‘H NMR spectroscopy in DMSO-$ and NaOD-D20 solutions, respectively [ 7,8].
0022-2860/90/$03.50
0 1990 -
Elsevier Science Publishers B.V.
24
2
3
It has recently been shown that the equilibrium between tautomers 1 and 2 depends linearly on the nucleophilic number (B) of the solvent used [ 121. This observation encouraged us to investigate, by IR spectroscopy, the nature of gossypol and the hydrogen-bond formation of its hydroxylic groups in some organic solvents. Although a number of gossypol adducts with organic compounds have been investigated by X-ray crystallography [ 13-171, no systematic IR studies of these complexes have been carried out up to now. In this paper IR spectroscopic results on gossypol adducts with some organic compounds, as well as with tetrabutylammonium salts, are also presented. RESULTS AND DISCUSSION
Gossypol, its adducts with some organic compounds, and its two tetrabutylammonium salts (2 : 1 and 4 : 1) were studied in CH.&, DMSO-d, and CD&N solutions by means of FTIR spectroscopy. Gossypol The IR spectra of gossypol in CH&12 and DMSO-$ solutions and in the solid state are shown in Fig. 1. The spectra of gossypol in CH,Cl, solution and in solid state in the region 1800-400 cm-l are almost identical. However, significant differences are observed in the region 4000-3000 cm-’ with regard to the OH vibrations. In the spectrum of the CH,Cl, solution of gossypol only a sharp OH stretching vibration band at 3502 cm-’ are observed, this had been observed by Wood et al. [ 51 and Danilova et al. [ 61. Furthermore, a very weak continuous absorption is seen in the region 3100-1900 cm-l (Fig. 4, dotted line). The presence of this absorption (in the case of classical IR spectroscopy) could be demonstrated only using very high concentrations or very thick layers. This continuum is caused by the intramolecular C=O* - *H-07 +C=O+H ***-07 hydrogen bond. This IR continuum demonstrates that these hydrogen bonds show large proton polarizability and the proton fluctuates between the donor and acceptor group. It has been demonstrated in earlier work that
Fig. 1. FTIR spectra of gossypol in the region 4000-400 cm-‘. (-_) CH&l, solution (----) DMSO& solution; (-.-.-) in solid state (KBr pellets, 2 mg/200 mg KBr).
the IR continua for intramolecular hydrogen bonds vanish to a greater or lesser extent depending on whether or not the donor and the acceptor groups are conjugated [ 18-201. Under these conditions the proton polarizability vanishes as the charge shift caused by the proton motion is in the same direction within the molecule. It is for this reason that the intensity of the continuum is so weak. In the spectrum of crystalline gossypol the 3502-cm-’ band splits into two components. Thus, the intermolecular hydrogen bonds formed by the 06-H and the 01-H groups differ in strength. Furthermore, instead of the continuous absorption, a very broad band of low intensity with a maximum at about 2700 cm-’ is observed (Fig. 1, dashed-dotted line). This band demonstrates that the proton in the intramolecular C=O - - *HO7 bond is localized at the H07 group. The fact that no differences are observed in the vC=O vibrations region between the spectrum of gossypol in CH,Cl, solution and that in the solid state shows that the resonance effect plays a very important role in both cases. The very low YC=O frequency (1625 cm-’ in CH$lP solution) provides further evidence of this type of bonding. The spectrum of gossypol in DMSO-d, solution (Fig. 1, dashed line) is completely different from those in the solid state and in CH,Cl, solution. The stretching vibrations of the OH groups of gossypol are observed as an intense band with a maximum at about 3100 cm-‘, (vH-06 and vH-01) and a weak band with a maximum at about 2700 cm-l ( vH-07). It is well known from ‘H NMR studies that in DMSO-d, solution no intramolecular hydrogen bond is formed in the gossypol molecule and that the equilibrium between the two tautomers, 1 and 2, is shifted to the lactol form (about 80% ) [ 121. Thus, the observed shift of the vO-H bands toward smaller wavenumbers can be explained by formation of relatively strong intermolecular hydrogen bonds with DMSO-d, molecules in which all O-H protons remain localized at the gossypol molecule.
Figure 2 shows that the vC=O vibration observed is shifted in CH&12 solution (solid line) at 1625 cm-’ in DMSO-d, solution (dashed line) to 1647 cm-’ and the intensity of this band strongly decreases, as only about 20% of the aldehyde form remains in this solution. Furthermore, a band existing at about 1575 cm-l, assigned by Scherer [21] and Soares [22] to the ring vibration of naphthalene (4)) vanishes almost completely in the present case.
Recently,
the
same
observations
were
made
for
1,8-bis-
(dimethylamino ) naphthalene and its mono salts with various acids [ 231. For these salts a very strong intramolecular hydrogen bond between two amino groups in peri positions of the naphthalene ring was formed. Therefore no band for the ring vibration 4 at 1575 cm-’ can be observed in the IR spectrum. All these results taken together show that gossypol in DMSO-$ solution occurs in the form of two hydrogen-bonded tautomers, 1 and 2 (see structures 5 and 6)) with intermolecular hydrogen bonds to the solvent molecules. The tautomer 2 is predominant in this solution.
s
5
'HO,ti F--0
5
6
Thus, the structure 5 has the vC=0 vibration at 1647 cm-’ and the ring vibration (4) at 1575 cm-‘. For structure 6 no band at 1575 cm-’ can be observed.
i.’
D
~1800
17w
1800
le.00
WAVENUMBER [l/CM] Fig. 2. FTIR spectra of gossypol in the region 1800-1500 cm-‘. DMSO-d, solution; (-.--.-) in the solid state (KBr pellets).
(--_)
CHzClz solution; (----)
Gossypol adducts with organic compounds Gossypol easily forms adducts with organic compounds when it is recrystallized from the corresponding organic solvents. The FTIR spectra of CH,Cl, solutions of 1: 1 mixtures of gossypol with formic acid, acetic acid, 1,4dioxane and ethyl acetate were taken. The spectra of 1: 1 gossypol mixtures with formic and acetic acid as well as spectra of the pure acids in CH,Cl, are shown in Fig. 3. As can be seen, two vC=O bands (self associated and unassociated) [24,25] are observed in the 1800-1700-cm-’ region. These bands are identical for both solutions, i.e. the free acids and gossypol-acid mixtures. The complete dissociation of gossypol adducts in CH&!& solution is clearly indicated by these spectra. The results obtained for gossypol mixtures with acids and the other compounds investigated demonstrate that the adducts probably do not play an important role in chemical and biochemical properties of gossypol as no interaction between gossypol and organic compounds could be detected by IR spectroscopy in solution. Tetrabutylammonium salts of gossypol Two tetrabutylammonium salts of gossypol, 2: 1 and 4: 1, were studied in CD&N solutions using FTIR spectroscopy. In Fig. 4, the IR spectra of these
b
Fig. 3. FTIR spectra in the region 1800-1000 cm-‘. (. . . . ) Gossypol; (-) gossypol adducts with acids; (----) corresponding acids in CH,Cl, solutions. (a) Formic acid; (b) acetic acid.
Fig. 4. FTIR spectra in the region 4000-400 cm-‘. (-_) Gossypol in CH,Cl,; (----) 2:1 tetrabutylammonium salt of gossypol in CD&N; (-.-.-) 4:1 tetrabutylammonium salt of gossypol in CD&N solution; (.... ) spectrum of gossypol in CH,Cl,, on tenfold expanded ordinate scale.
1000
1700
1000
iwo
WAVENUMBER [l/CM] Fig. 5.FTIR spectra in the region 1800-1500 cm-‘. (-) Gossypol in CH.J& solution and tetrabutylammonium salts of gossypol in CD&N solutions; (----) 2: 1 salt; (-.-.-) 4: 1 salt.
salts are compared with the spectrum of a solution of gossypol in CH,C&. In Figure 5, the carbonyl regions, with an extended wavenumbers scale, are shown for both cases. A comparison of the IR spectra of the two tetrabutylammonium salts with that of the pure gossypol shows that the intense band at 3502 cm-‘, caused by the vH-06 and vH-01 vibrations in the pure gossypol, completely disappears, indicating that another type of hydrogen bond is formed in the case of these salts. In the IR spectrum of the 2 : 1 salt (Fig. 4, dashed line) in the region 31001700 cm-l, two pronounced maxima at about 3000 and 2500 cm-l are found. These are assumed to be the stretching vibrations of the two hydrogen-bonded OH groups of the tautomeric structures 7-9.
29
--.0+-J
w
O .
..H-0
o
;-*
\
10
The intense ring stretching vibration of naphthalene at 1563 cm-’ and the absence of the vC=O vibration in the spectrum of the 2 : 1 salt (Fig. 5, dashed line) show that the tautomeric structure 8 is the predominant form. This re-, sult is in agreement with the ‘H NMR spectrum, which shows that signals for the hydrogen-bonded protons at 12.70 ppm (H-07) and at 8.52 ppm (H-06) are concentration independent [ 121. The behaviour of the hydrogen-bonded system in the 2 : 1 salt is intermediate between that of the systems in 2,6-pyridine-dicarboxylic acid N-oxide [26] or of the tetrabutylammonium salt of 2,6-di (hydroxymethyl)phenol and 2-hydroxyisophthalic acid [ 271. In the spectrum of the 4: 1 tetrabutylammonium salt of gossypol (Fig. 4, dashed-dotted line), instead of the vO-H vibration at 3502 cm-’ an intense continuous absorption in the region 1500-700 cm-’ occurs, whereas the intensity of this continuum in the region 3000-1900 cm-l is only weak, showing a broad band-like structure with a maximum at about 2100 cm-l. The continuum demonstrates that very strong intermolecular (0. *H- -0) -’ hydrogen bonds with proton polarizability are formed in the CD&N solution of the 4 : 1 salt. An analogous continuum has already been observed for very short intraand inter-molecular hydrogen bonds [ 28-351. In the carbonyl vibration region (Fig. 5, dashed-dotted line) in the spectrum of the 4: 1 salt the intense band at 1563 cm-‘, caused by the ring stretching vibration of naphthalene, is also observed. Furthermore, a very intense band at 1680 cm-l, caused by vC=O vibration, indicates that the ketol tautomer 10 is the main form in CD&N solution. This is in agreement with earlier IR results obtained with aromatic ketones [ 36,371. Furthermore, the ‘H NMR signal of the ketol protons is found at 9.80 ppm, which confirms the above conclusion [ 11. CONCLUSIONS
Three tautomeric forms of gossypol in solution can be identified by means of IR spectroscopy. The bands which characterize the aldehyde tautomer 1 are
30
the weak continuous absorption in the region 3100-1900 cm-’ and the vC=O vibration at 1625 cm-‘. Furthermore, the features of the IR spectra in the region below 1700 cm-’ in solution and in the solid state are almost the same. The bands which characterize spectral features of the lactol tautomer 2 are the result of the stretching naphthalene ring vibration at about 1570 cm-’ and the vC=O vibration at 1625 cm-’ no longer being found. The bands which are characteristic in the ketol tautomer 3 are the YC=O vibration at about 1680 cm-’ and the naphthalene stretching ring vibration at about 1570 cm-l. The investigated gossypol adducts with organic compounds which are stable in the solid state are completely dissociated in solution. Thus, the adducts of gossypol cannot affect the chemical and biochemical properties of gossypol in solutions. EXPERIMENTAL
Yellow microcrystalline pure gossypol (m.p. 178’ C ), obtained from the Institute of Bioorganic Chemistry, Academy of Sciences of UzSSR, Tashkent, U.S.S.R., was recrystallized from CH,Cl, solutions. Organic adducts of gossypol were prepared by recrystallization from corresponding organic solvents. In the case of the formic acid-gossypol mixture, the equimolar amount of formic acid was added to the CDC& solution of gossypol. The tetrabutylammonium salts of gossypol were synthesized by addition of equimolar amounts of 0.06 mol dmp3 carefully dried ethanol solutions of tetrabutylammonium hydroxide to the ethanol solutions of gossypol (hydroxide-gossypol ratios 2: 1 and 4: 1). The solvent was removed under reduced pressure at room temperature and the residues were dissolved in acetonitriled 3. All solvents were of spectral grade (DMSO-$, acetonitrile-d, and methylenechloride) and were stored over 3-A molecular sieves for several days. All manipulations with the substances were performed in a carefully dried and CO,-free glove-box. The IR spectra were taken on 0.05 mol dme3 solutions with a Fourier transform IR (FTIR) spectrometer (IFS 113 V; Bruker, Karlsruhe, FRG) using a cell with Si windows (sample thickness 0.400 mm, detector DTGS, resolution 2 and number of scans 500). The cell is described in ref. 38. ACKNOWLEDGEMENT
This work was supported project RP.II.13.2.
by the Ministry
of National
Education,
as part of
31. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
29 30 31 32 33 34 35 36 37 38
R.T. O’Connor, P. van Haar, F.E. DuPre, L.E. Brown and C.H. Pominski, J. Am. Chem. Sot., 76 (1954) 2368. C.Y. Hopkins, J. Am. Chem. Oil Sot., 55 (1968) 778. A.J. Zaki, N.T. Keen, J.J. Sims and D.C. Erwin, Phytopathology, 62 (1972) 1398. O.G. Correa, H.M. Cappi, M. Salem and C. Staffa, J. Am. Chem. Oil Sot., 53 (1966) 678. A.B. Wood, F.V. Robinson and R.C. Lago, Chem. Ind., (1969) 1739. T.A. Danilova and A.N. Mironova, Izv. Akad. Nauk SSSR, Ser. Fiz., 26 (1962) 1308. F.G. Kamaev, N.I. Baram, A.I. Ismailov, V.B. Leontyev and A.S. Sadykov, Izv. Akad. Nauk SSSR, Ser. Khim., (1979) 1003. R.D. Stipanovic, A.A. Bell and C.R. Howell, J. Am. Chem. Oil Sot., 60 (1973) 462. D.H. O’Brien and R.D. Stipanovic, J. Org. Chem., 43 (1978) 1105. R. Adams, T.A. Geissman and J.D. Edwards, Chem. Rev., 60 (1960) 555. S.A. Talipov, B.T. Ibragimov, C.B. Nazarov, T.F. Aripov and A.S. Sadykov, Khim. Prir. Soedin. (1985) 835. B. Brzezinski, J. Olejnik, S. Paszyc and T.F. Aripov, J. Mol. Struct., 220 (1990) 261. X. Changfu, H. Cunheng, B. Guanghong and M. Shantian, Scientia Sinica, Part B 25 (1982) 1194. B.T. Ibragimov, S.A. Talipov, G.B. Nazarov, T.F. Aripov and A.I. Ismailov, Khim. Prir. Soedin. (1984) 663. B.T. Ibragimov, S.A. Talipov, B.N. Dadabaev, G.B. Nazarov and T.F. Aripov, Khim. Prir. Soedin., 212 (1989) 247. B.T. Ibragimov, S.A. Talipov and B.N. Dadabaev, Khim. Prir. Soedin., 212 (1989) 247. M. Gdaniec, B.T. Ibragimov and S.A. Talipov, J. Inclusion Phenom., submitted. B. Brzezinski and G. Zundel, Chem. Phys. Lett., 75 (1980) 500. B. Brzezinski and G. Zundel, J. Phys. Chem., 86 (1982) 5133. B. Brzezinski and G. Zundel, Chem. Phys. Lett., 115 (1985) 212. J.R. Scherer, J. Phys. Chem., 36 (1962) 3308. M.I.V. Soares, Rev. Port. Quim., 10 (1968) 65. B. Brzezinski, E. Grech, Z. Malarski and L. Sobczyk, J. Chem. Sot., Faraday Trans. 1, 86 (1990) 1777. G.A. Allen and M.A. Caldin, Q. Rev. (London), 7 (1953) 255. T.D. Alger, D.M. Grant and J.R. Lyerla, Jr., J. Phys. Chem., 75 (1971) 2539. B. Brzezinski, G. Zundel and R. Kraemer, Chem. Phys. Lett., 124 (1986) 395. B. Brzezinski, G. Zundel and R. Kraemer, J. Phys. Chem., 91 (1987) 3077. D. Hadzi and S. Bratos, in P. Schuster, G. Zundel and C. Sandorfy (Eds.), The Hydrogen Bond-Recent Developments in Theory and Experiments, North-Holland, Amsterdam, 1976, Vol. II, Chap. 12. B. Brycki and M. Szafran, J. Chem. Sot., Perkin Trans. 2, (1984) 223. U. Boehner and G. Zundel, J. Chem. Sot., Faraday Trans. 1,81 (1985) 1425. U. Boehner and G. Zundel, J. Phys. Chem., 90 (1986) 964. A. Koll, M. Rospenk, L. Sobczyk and T. Giowiak, Can. J. Chem., 64 (1986) 1859. B. Brzezinski, J. Olejnik, G. Zundel and R. Kraemer, J. Mol. Struct., 212 (1989) 247. B. Brzezinski and G. Zundel, Z. Phys. Chem., N.F., 111 (1978) 31. B. Brzezinski and G. Zundel, J. Chem. Sot., Faraday Trans. 2,77 (1981) 1101. J.F. Grove and N.R. Willis, J. Chem. Sot., (1951) 877. M.L. Josien and N. Fuson, J. Am. Chem. Sot., 73 (1951) 478. R. Kraemer and G. Zundel, Z. Phys. Chem. (Frankfurt am Main), 144 (1985) 265.
2:3
FOURIER TRANSFORM INFRARED STUDY ON THE IDENTIFICATION OF GOSSYPOL TAUTOMERS
BOGUMIL BRZEZINSKI,
JERZY OLEJNIK and STEFAN PASZYC
Department of Chemistry, A. Mickiewicz University, 60-780 Poznari (Poland) (Received 8 November 1989 )
ABSTRACT
Gossypol and its adducts with some organic compounds, as well as its two tetrabutylammonium salts (2 : 1 and 4 : 1) , were studied in various solutions by Fourier transform IR (FTIR) spectroscopy. In CH,Cl, solution gossypol exists as the aldehyde tautomer. In the DMSO-d, solution an equilibrium between the aldehyde and lactol tautomers was observed in which the predominant form is the latter. The IR spectrum of the 2: 1 tetrabutylammonium salt indicates that a very complex equilibrium occurs in CD&N solution. In the case of the 4: 1 salt the ketol tautomeric form is associated via very strong intermolecular (0. *H* -0) - hydrogen bonds with large proton polarizability.
INTRODUCTION
Gossypol, 2,2’ -bis (8-formyl-1,6,7-trihydroxy-5-isopropyl-3-methylnaphthalene) and its derivatives are of great interest because of their biological importance. Most significant are their contraceptive and toxic activities. For these reasons extensive investigations of the chemical properties, reactivity and structure of gossypol have been carried out. Various spectroscopic techniques for the determination of the gossypol structure have been used [l-9]. IR studies of gossypol in the solid state and in several solvents have confirmed the observation of Adams et al. that gossypol exists in three tautomeric forms l-3 [lo]. The aldehyde tautomer ( 1) has been observed for gossypol in the solid state [ 111 and in chloroform solutions [ 1,5]. The lactol (2) and the ketol (3) tautomeric forms have been found by ‘H NMR spectroscopy in DMSO-$ and NaOD-D20 solutions, respectively [ 7,8].
0022-2860/90/$03.50
0 1990 -
Elsevier Science Publishers B.V.
24
2
3
It has recently been shown that the equilibrium between tautomers 1 and 2 depends linearly on the nucleophilic number (B) of the solvent used [ 121. This observation encouraged us to investigate, by IR spectroscopy, the nature of gossypol and the hydrogen-bond formation of its hydroxylic groups in some organic solvents. Although a number of gossypol adducts with organic compounds have been investigated by X-ray crystallography [ 13-171, no systematic IR studies of these complexes have been carried out up to now. In this paper IR spectroscopic results on gossypol adducts with some organic compounds, as well as with tetrabutylammonium salts, are also presented. RESULTS AND DISCUSSION
Gossypol, its adducts with some organic compounds, and its two tetrabutylammonium salts (2 : 1 and 4 : 1) were studied in CH.&, DMSO-d, and CD&N solutions by means of FTIR spectroscopy. Gossypol The IR spectra of gossypol in CH&12 and DMSO-$ solutions and in the solid state are shown in Fig. 1. The spectra of gossypol in CH,Cl, solution and in solid state in the region 1800-400 cm-l are almost identical. However, significant differences are observed in the region 4000-3000 cm-’ with regard to the OH vibrations. In the spectrum of the CH,Cl, solution of gossypol only a sharp OH stretching vibration band at 3502 cm-’ are observed, this had been observed by Wood et al. [ 51 and Danilova et al. [ 61. Furthermore, a very weak continuous absorption is seen in the region 3100-1900 cm-l (Fig. 4, dotted line). The presence of this absorption (in the case of classical IR spectroscopy) could be demonstrated only using very high concentrations or very thick layers. This continuum is caused by the intramolecular C=O* - *H-07 +C=O+H ***-07 hydrogen bond. This IR continuum demonstrates that these hydrogen bonds show large proton polarizability and the proton fluctuates between the donor and acceptor group. It has been demonstrated in earlier work that
Fig. 1. FTIR spectra of gossypol in the region 4000-400 cm-‘. (-_) CH&l, solution (----) DMSO& solution; (-.-.-) in solid state (KBr pellets, 2 mg/200 mg KBr).
the IR continua for intramolecular hydrogen bonds vanish to a greater or lesser extent depending on whether or not the donor and the acceptor groups are conjugated [ 18-201. Under these conditions the proton polarizability vanishes as the charge shift caused by the proton motion is in the same direction within the molecule. It is for this reason that the intensity of the continuum is so weak. In the spectrum of crystalline gossypol the 3502-cm-’ band splits into two components. Thus, the intermolecular hydrogen bonds formed by the 06-H and the 01-H groups differ in strength. Furthermore, instead of the continuous absorption, a very broad band of low intensity with a maximum at about 2700 cm-’ is observed (Fig. 1, dashed-dotted line). This band demonstrates that the proton in the intramolecular C=O - - *HO7 bond is localized at the H07 group. The fact that no differences are observed in the vC=O vibrations region between the spectrum of gossypol in CH,Cl, solution and that in the solid state shows that the resonance effect plays a very important role in both cases. The very low YC=O frequency (1625 cm-’ in CH$lP solution) provides further evidence of this type of bonding. The spectrum of gossypol in DMSO-d, solution (Fig. 1, dashed line) is completely different from those in the solid state and in CH,Cl, solution. The stretching vibrations of the OH groups of gossypol are observed as an intense band with a maximum at about 3100 cm-‘, (vH-06 and vH-01) and a weak band with a maximum at about 2700 cm-l ( vH-07). It is well known from ‘H NMR studies that in DMSO-d, solution no intramolecular hydrogen bond is formed in the gossypol molecule and that the equilibrium between the two tautomers, 1 and 2, is shifted to the lactol form (about 80% ) [ 121. Thus, the observed shift of the vO-H bands toward smaller wavenumbers can be explained by formation of relatively strong intermolecular hydrogen bonds with DMSO-d, molecules in which all O-H protons remain localized at the gossypol molecule.
Figure 2 shows that the vC=O vibration observed is shifted in CH&12 solution (solid line) at 1625 cm-’ in DMSO-d, solution (dashed line) to 1647 cm-’ and the intensity of this band strongly decreases, as only about 20% of the aldehyde form remains in this solution. Furthermore, a band existing at about 1575 cm-l, assigned by Scherer [21] and Soares [22] to the ring vibration of naphthalene (4)) vanishes almost completely in the present case.
Recently,
the
same
observations
were
made
for
1,8-bis-
(dimethylamino ) naphthalene and its mono salts with various acids [ 231. For these salts a very strong intramolecular hydrogen bond between two amino groups in peri positions of the naphthalene ring was formed. Therefore no band for the ring vibration 4 at 1575 cm-’ can be observed in the IR spectrum. All these results taken together show that gossypol in DMSO-$ solution occurs in the form of two hydrogen-bonded tautomers, 1 and 2 (see structures 5 and 6)) with intermolecular hydrogen bonds to the solvent molecules. The tautomer 2 is predominant in this solution.
s
5
'HO,ti F--0
5
6
Thus, the structure 5 has the vC=0 vibration at 1647 cm-’ and the ring vibration (4) at 1575 cm-‘. For structure 6 no band at 1575 cm-’ can be observed.
i.’
D
~1800
17w
1800
le.00
WAVENUMBER [l/CM] Fig. 2. FTIR spectra of gossypol in the region 1800-1500 cm-‘. DMSO-d, solution; (-.--.-) in the solid state (KBr pellets).
(--_)
CHzClz solution; (----)
Gossypol adducts with organic compounds Gossypol easily forms adducts with organic compounds when it is recrystallized from the corresponding organic solvents. The FTIR spectra of CH,Cl, solutions of 1: 1 mixtures of gossypol with formic acid, acetic acid, 1,4dioxane and ethyl acetate were taken. The spectra of 1: 1 gossypol mixtures with formic and acetic acid as well as spectra of the pure acids in CH,Cl, are shown in Fig. 3. As can be seen, two vC=O bands (self associated and unassociated) [24,25] are observed in the 1800-1700-cm-’ region. These bands are identical for both solutions, i.e. the free acids and gossypol-acid mixtures. The complete dissociation of gossypol adducts in CH&!& solution is clearly indicated by these spectra. The results obtained for gossypol mixtures with acids and the other compounds investigated demonstrate that the adducts probably do not play an important role in chemical and biochemical properties of gossypol as no interaction between gossypol and organic compounds could be detected by IR spectroscopy in solution. Tetrabutylammonium salts of gossypol Two tetrabutylammonium salts of gossypol, 2: 1 and 4: 1, were studied in CD&N solutions using FTIR spectroscopy. In Fig. 4, the IR spectra of these
b
Fig. 3. FTIR spectra in the region 1800-1000 cm-‘. (. . . . ) Gossypol; (-) gossypol adducts with acids; (----) corresponding acids in CH,Cl, solutions. (a) Formic acid; (b) acetic acid.
Fig. 4. FTIR spectra in the region 4000-400 cm-‘. (-_) Gossypol in CH,Cl,; (----) 2:1 tetrabutylammonium salt of gossypol in CD&N; (-.-.-) 4:1 tetrabutylammonium salt of gossypol in CD&N solution; (.... ) spectrum of gossypol in CH,Cl,, on tenfold expanded ordinate scale.
1000
1700
1000
iwo
WAVENUMBER [l/CM] Fig. 5.FTIR spectra in the region 1800-1500 cm-‘. (-) Gossypol in CH.J& solution and tetrabutylammonium salts of gossypol in CD&N solutions; (----) 2: 1 salt; (-.-.-) 4: 1 salt.
salts are compared with the spectrum of a solution of gossypol in CH,C&. In Figure 5, the carbonyl regions, with an extended wavenumbers scale, are shown for both cases. A comparison of the IR spectra of the two tetrabutylammonium salts with that of the pure gossypol shows that the intense band at 3502 cm-‘, caused by the vH-06 and vH-01 vibrations in the pure gossypol, completely disappears, indicating that another type of hydrogen bond is formed in the case of these salts. In the IR spectrum of the 2 : 1 salt (Fig. 4, dashed line) in the region 31001700 cm-l, two pronounced maxima at about 3000 and 2500 cm-l are found. These are assumed to be the stretching vibrations of the two hydrogen-bonded OH groups of the tautomeric structures 7-9.
29
--.0+-J
w
O .
..H-0
o
;-*
\
10
The intense ring stretching vibration of naphthalene at 1563 cm-’ and the absence of the vC=O vibration in the spectrum of the 2 : 1 salt (Fig. 5, dashed line) show that the tautomeric structure 8 is the predominant form. This re-, sult is in agreement with the ‘H NMR spectrum, which shows that signals for the hydrogen-bonded protons at 12.70 ppm (H-07) and at 8.52 ppm (H-06) are concentration independent [ 121. The behaviour of the hydrogen-bonded system in the 2 : 1 salt is intermediate between that of the systems in 2,6-pyridine-dicarboxylic acid N-oxide [26] or of the tetrabutylammonium salt of 2,6-di (hydroxymethyl)phenol and 2-hydroxyisophthalic acid [ 271. In the spectrum of the 4: 1 tetrabutylammonium salt of gossypol (Fig. 4, dashed-dotted line), instead of the vO-H vibration at 3502 cm-’ an intense continuous absorption in the region 1500-700 cm-’ occurs, whereas the intensity of this continuum in the region 3000-1900 cm-l is only weak, showing a broad band-like structure with a maximum at about 2100 cm-l. The continuum demonstrates that very strong intermolecular (0. *H- -0) -’ hydrogen bonds with proton polarizability are formed in the CD&N solution of the 4 : 1 salt. An analogous continuum has already been observed for very short intraand inter-molecular hydrogen bonds [ 28-351. In the carbonyl vibration region (Fig. 5, dashed-dotted line) in the spectrum of the 4: 1 salt the intense band at 1563 cm-‘, caused by the ring stretching vibration of naphthalene, is also observed. Furthermore, a very intense band at 1680 cm-l, caused by vC=O vibration, indicates that the ketol tautomer 10 is the main form in CD&N solution. This is in agreement with earlier IR results obtained with aromatic ketones [ 36,371. Furthermore, the ‘H NMR signal of the ketol protons is found at 9.80 ppm, which confirms the above conclusion [ 11. CONCLUSIONS
Three tautomeric forms of gossypol in solution can be identified by means of IR spectroscopy. The bands which characterize the aldehyde tautomer 1 are
30
the weak continuous absorption in the region 3100-1900 cm-’ and the vC=O vibration at 1625 cm-‘. Furthermore, the features of the IR spectra in the region below 1700 cm-’ in solution and in the solid state are almost the same. The bands which characterize spectral features of the lactol tautomer 2 are the result of the stretching naphthalene ring vibration at about 1570 cm-’ and the vC=O vibration at 1625 cm-’ no longer being found. The bands which are characteristic in the ketol tautomer 3 are the YC=O vibration at about 1680 cm-’ and the naphthalene stretching ring vibration at about 1570 cm-l. The investigated gossypol adducts with organic compounds which are stable in the solid state are completely dissociated in solution. Thus, the adducts of gossypol cannot affect the chemical and biochemical properties of gossypol in solutions. EXPERIMENTAL
Yellow microcrystalline pure gossypol (m.p. 178’ C ), obtained from the Institute of Bioorganic Chemistry, Academy of Sciences of UzSSR, Tashkent, U.S.S.R., was recrystallized from CH,Cl, solutions. Organic adducts of gossypol were prepared by recrystallization from corresponding organic solvents. In the case of the formic acid-gossypol mixture, the equimolar amount of formic acid was added to the CDC& solution of gossypol. The tetrabutylammonium salts of gossypol were synthesized by addition of equimolar amounts of 0.06 mol dmp3 carefully dried ethanol solutions of tetrabutylammonium hydroxide to the ethanol solutions of gossypol (hydroxide-gossypol ratios 2: 1 and 4: 1). The solvent was removed under reduced pressure at room temperature and the residues were dissolved in acetonitriled 3. All solvents were of spectral grade (DMSO-$, acetonitrile-d, and methylenechloride) and were stored over 3-A molecular sieves for several days. All manipulations with the substances were performed in a carefully dried and CO,-free glove-box. The IR spectra were taken on 0.05 mol dme3 solutions with a Fourier transform IR (FTIR) spectrometer (IFS 113 V; Bruker, Karlsruhe, FRG) using a cell with Si windows (sample thickness 0.400 mm, detector DTGS, resolution 2 and number of scans 500). The cell is described in ref. 38. ACKNOWLEDGEMENT
This work was supported project RP.II.13.2.
by the Ministry
of National
Education,
as part of
31. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
29 30 31 32 33 34 35 36 37 38
R.T. O’Connor, P. van Haar, F.E. DuPre, L.E. Brown and C.H. Pominski, J. Am. Chem. Sot., 76 (1954) 2368. C.Y. Hopkins, J. Am. Chem. Oil Sot., 55 (1968) 778. A.J. Zaki, N.T. Keen, J.J. Sims and D.C. Erwin, Phytopathology, 62 (1972) 1398. O.G. Correa, H.M. Cappi, M. Salem and C. Staffa, J. Am. Chem. Oil Sot., 53 (1966) 678. A.B. Wood, F.V. Robinson and R.C. Lago, Chem. Ind., (1969) 1739. T.A. Danilova and A.N. Mironova, Izv. Akad. Nauk SSSR, Ser. Fiz., 26 (1962) 1308. F.G. Kamaev, N.I. Baram, A.I. Ismailov, V.B. Leontyev and A.S. Sadykov, Izv. Akad. Nauk SSSR, Ser. Khim., (1979) 1003. R.D. Stipanovic, A.A. Bell and C.R. Howell, J. Am. Chem. Oil Sot., 60 (1973) 462. D.H. O’Brien and R.D. Stipanovic, J. Org. Chem., 43 (1978) 1105. R. Adams, T.A. Geissman and J.D. Edwards, Chem. Rev., 60 (1960) 555. S.A. Talipov, B.T. Ibragimov, C.B. Nazarov, T.F. Aripov and A.S. Sadykov, Khim. Prir. Soedin. (1985) 835. B. Brzezinski, J. Olejnik, S. Paszyc and T.F. Aripov, J. Mol. Struct., 220 (1990) 261. X. Changfu, H. Cunheng, B. Guanghong and M. Shantian, Scientia Sinica, Part B 25 (1982) 1194. B.T. Ibragimov, S.A. Talipov, G.B. Nazarov, T.F. Aripov and A.I. Ismailov, Khim. Prir. Soedin. (1984) 663. B.T. Ibragimov, S.A. Talipov, B.N. Dadabaev, G.B. Nazarov and T.F. Aripov, Khim. Prir. Soedin., 212 (1989) 247. B.T. Ibragimov, S.A. Talipov and B.N. Dadabaev, Khim. Prir. Soedin., 212 (1989) 247. M. Gdaniec, B.T. Ibragimov and S.A. Talipov, J. Inclusion Phenom., submitted. B. Brzezinski and G. Zundel, Chem. Phys. Lett., 75 (1980) 500. B. Brzezinski and G. Zundel, J. Phys. Chem., 86 (1982) 5133. B. Brzezinski and G. Zundel, Chem. Phys. Lett., 115 (1985) 212. J.R. Scherer, J. Phys. Chem., 36 (1962) 3308. M.I.V. Soares, Rev. Port. Quim., 10 (1968) 65. B. Brzezinski, E. Grech, Z. Malarski and L. Sobczyk, J. Chem. Sot., Faraday Trans. 1, 86 (1990) 1777. G.A. Allen and M.A. Caldin, Q. Rev. (London), 7 (1953) 255. T.D. Alger, D.M. Grant and J.R. Lyerla, Jr., J. Phys. Chem., 75 (1971) 2539. B. Brzezinski, G. Zundel and R. Kraemer, Chem. Phys. Lett., 124 (1986) 395. B. Brzezinski, G. Zundel and R. Kraemer, J. Phys. Chem., 91 (1987) 3077. D. Hadzi and S. Bratos, in P. Schuster, G. Zundel and C. Sandorfy (Eds.), The Hydrogen Bond-Recent Developments in Theory and Experiments, North-Holland, Amsterdam, 1976, Vol. II, Chap. 12. B. Brycki and M. Szafran, J. Chem. Sot., Perkin Trans. 2, (1984) 223. U. Boehner and G. Zundel, J. Chem. Sot., Faraday Trans. 1,81 (1985) 1425. U. Boehner and G. Zundel, J. Phys. Chem., 90 (1986) 964. A. Koll, M. Rospenk, L. Sobczyk and T. Giowiak, Can. J. Chem., 64 (1986) 1859. B. Brzezinski, J. Olejnik, G. Zundel and R. Kraemer, J. Mol. Struct., 212 (1989) 247. B. Brzezinski and G. Zundel, Z. Phys. Chem., N.F., 111 (1978) 31. B. Brzezinski and G. Zundel, J. Chem. Sot., Faraday Trans. 2,77 (1981) 1101. J.F. Grove and N.R. Willis, J. Chem. Sot., (1951) 877. M.L. Josien and N. Fuson, J. Am. Chem. Sot., 73 (1951) 478. R. Kraemer and G. Zundel, Z. Phys. Chem. (Frankfurt am Main), 144 (1985) 265.