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Experimental and theoretical (INDO CI) electronic absorption spectroscopy in the examination of tautomeric phenomena in nitracrine and its nitre isomers
SpectrochimicaAm, Vol. 48A. No. 6, pp. 77-781. Printed in Great Britain
05~8539/92 s5.00+ 0.00 Pergamon PressLtd
1992
Experimental and theoretical (INDO CI) electronic absorption spectroscopy in the examination of tautomeric phenomena in nitracrine and its nitro isomers JANUSZ RAK AND
JERZY B~A&JOWSKI*
Department of Chemistry, University of Gdadsk, 80-952 Gdaftsk, Poland (Receiued 11 November 1991; accepte$3 December 1991) Abstract-Electronic absorption spectroscopy was employed to examine tautomeric phenomena in N,N-dimethyl-N-(l-nitro-9-acridinyl)-1,3-propanediamine (known as nitracrine-WHO, or Ledakrin in Poland) which exhibits antitumour activity, and also to examine its three nitro isomers. The analysis of the experimental absorption spectra reveals that the compounds exist in the liquid-phasein at least two forms (most probably amino and imino tautomeric forms), remaining in an equilibrium which is strongly affected by the features of a solvent. This was qualitatively confirmed by examining frequencies and oscillator strengths of electronic transitions obtained for the lowest energy structures of a given form by two independent methods based on INDO approximation. The latter methods predict that the strongest transitions occurring below 280 nm have a rr-+n* origin, whereas the longest wavelength transitions may be either X+X* or n-s* typ and cause a shift in electron density within the acridine aromatic system and attached nitro group and exocyclic nitrogen atom.
INTRODUCTION
9-acridinamine derivatives have been synthesized and extensively examined over several decades by LED~CHOWSKI and co-workers from the point of view of their agents [l-3]. Such activity is also exhibited by utility as anticancer N,N-dimethyl-N’-(l-nitro-9-acridinyl)-l,3-propanediamine, known as nitracrine (the WHO term) [l, 41. The compound has been admitted into Poland as an antineoplastic drug (under the name of Ledakrin) and successfully applied in the treatment of certain solid tumours, such as mammary, lung, ovarian and colon tumours [4-71. The X-ray investigations which followed synthetic and biological examinations revealed that the prototropic tautomerism occurs in nitracrine and its 2-nitro isomer [4,8-lo]. This led us to think that the latter phenomenon, being general in 9-acridinamine derivatives [lo-131, plays an important role in biological activity of nitracrine. To examine the problem, in our previous work we employed electronic absorption [14-H] and luminescence [17,18] spectroscopies, as well as, to a limited extent, the MNDO method together with the theoretical analysis of the electronic absorption spectra on the PPP level [16]. These studies revealed that nitracrine and its three nitro isomers presumably occur, in the liquid-phase, in two tautomeric forms, i.e. amino and imino, which can sometimes exist as more than one structural isomer (a stereo-isomer). This was fully confirmed by complex theoretical analysis presented in our most recent paper [19]. The aim of the present work is to find further support for the existence of nitracrine and its isomers in various tautomeric and structural forms. To achieve this we have compared experimental spectra recorded in three solvents of various polarity and electron (proton)-donor-acceptor ability with electronic transitions predicted by the methods based on INDO approximation [20,21], using structures obtained from the complete geometry optimizations with the AM1 method [19,22]. The complementary analysis of the electron density changes accompanying electronic excitation was also carried out to enable an insight into the nature of this process.
NUMEROUS
* Author to whom correspondence should be addressed. 771
772
JANUSZ RAKAND JERZY&A~EJOWSKI MATERIALSAND METHODS
Chemicals
The N,N-dimethyl-N’-( l-nitro-9-acridinyl)-1,3-propanediamine and its isomers were synthesized as dihydrochlorides and purified by the method reported in the literature [4,23]. The compounds were kindly supplied by Prof. J. Konopa and his research group from the Technical University of Gdadsk. The free bases were obtained by the alkalization of the aqueous solution of original compounds with K&O3 and extraction with benzene. The extracts were subsequently dried over anhydrous MgSO,, filtered and the solvent evaporated under reduced pressure. Some of the extracts were additionally subjected to lyophilization. Spectral measurements
The UV and visible absorption spectra were measured on a VSU-2P (Carl Zeiss, Jena) spectrophotometer. Solvents, i.e. n-hexane, from Romil Chemicals Ltd, and acetonitrile, from Fluka AG, both of spectral grade were used as received. 1,CDioxane of Analar grade was purified by the previously described procedure [24]. Calculations
Electronic transitions were calculated employing two semiempirical methods based on INDO approximation. The first set of frequencies and oscillator strengths was determined on the basis of the spectroscopic parameterization (INDO/S) of RIDLEY and ZERNER [20]. In these calculations single excited configurations corresponding to the range from 0 to 10 eV of the excitation energy were used. Usually more than 80 singly excited states were used in the CI procedure. The second set of spectroscopic data was derived using the all-valence INDO/spd method (Ghost and Rydberg INDO) developed by LIPINSKI[21]. In this latter case 80 singly excited configurations were also used in the CI procedure. The value of the adjustable parameter (one) scaling p integrals, was assumed to be 1. All calculations were performed on a Flex Cache 331386 (Advanced Logic Research, Inc) computer.
RESULTSAND
DISCUSSION
Molecular geometries
It was revealed in our latest work, on the basis of the AM1 quantum chemistry calculations, that N,iV-dimethyl-N’-( 1-nitro-9-acridinyl)-1,3-propanediamine (C-283) and nitro isomers of the compound (for symbols see caption to Fig. 1) should actually exist in two tautomeric forms, namely amino and imino, which can further occur as several structural isomers (stereo-isomers) [19]. This would imply that in the liquid-phase we are dealing with complex equilibria in which all the forms of comparable energies participate. The most feasible form in which the molecule occurs is, of course, one of the lowest value of free energy or in some cases simply energy. This latter situation takes place when entropies of various forms do not differ substantially from each other. In theoretical spectroscopic calculations the lowest energy structures of the compounds studied, for a given tautomeric form, were considered. The views of these structures are exhibited in Fig. 1. Experimental
and theoretical absorption spectra
The experimental spectra [Figs 2-5(A)] measured in three markedly different solvents exhibit complex patterns which cannot simply be accounted for by solute-solvent
Tautomeric phenomena in nitracrine and its nitro isomers
173
c-283
C-264
C-257
C-265
amino form
imino form
Fig. 1. Structures of N,N-dimethyl-N’-( 1-nitro-9-acridinyl)-1,3_propanediamine (C-283) and its 2-nitro (C-264), 3-nitro (C-257), and 4-nitro (C-265) isomers in amino and imino tautomeric forms, for which spectroscopic calculations were carried out.
interactions. It is much more probable that this complexity originates from the existence of the compounds in more than one structural form. The effect of the solvent on the electronic absorption is more pronounced in C-283, particularly in the range above 330 nm. This region has also been examined earlier in more detail [14]. The constitution of the molecules of the compounds studied is such that each of them should be polar. The measure of this feature is the dipole moment whose values (in D) for the structures shown in Fig. 1 are (first value corresponds to the amino form and the second value to the imino form): 2.92 and 5.86, 6.36 and 7.60, 8.44 and 6.43, 8.66 and 5.00, for C-283, C-264, C-257 and C-265, respectively [19]. Since the free energy and energy differences between these amino and imino structures do not differ substantially (free energy-first value, and energy-second value), differences between the amino and imino forms shown in Fig. 1, resulting from the AM1 calculations (in kJ mol-‘) [19], are equal to: 14.5 and 18.5, 6.6 and 12.7, 6.9 and 12.9, 27.2 and 32.6, for C-283, C-264,
774
JANUSZ
RAK
AND JERZY
B~A~EJOWSKI
s c
* 0.4
3 fI 0
Fig. 2. The experimental absorption spectra (A) of C-283 in n-hexane (a,-), 1,4-dioxane (b, ---). and acetonitrile (c, ----) together with the calculated transition frequencies for amino &I) and imino (Q) forms; (B) transitions predicted using the INDO/S method; and (C) values from INDOlspd method.
C-257 and C-265, respectively) one can expect that these two forms, at least, exist in solutions in equlibrium. The equilibrium state, and thus relative amounts of both forms will, of course, be affected by solute-solvent interactions. In non-polar solvent (i.e. n-hexane) the equilibrium should, therefore, be moved towards more polar structure (e.g. the imino in the case of C-283). If this indeed took place it would mean that the long wavelength band of C-283 would be ascribed to the amino form, and the band with maximum at ca 385 nm to the imino form (Fig. 2A) [16]. In the remaining isomers of nitracrine the differences between the dipole moment values become smaller, which may
Tautomeric phenomena in nitracrine and its nitro isomers
ns
0.0
1.0
I
P 1.e
1
4 0.0 ”
4 j 0.4
0.0 810
85a
290
930
370
410
460
490
MO
Fig. 3. The experimental absorption spectra (A) of C-264 in n-hexane (a, -), 1,4-dioxane (b,---), and acetonitrile (c, ----) together with the calculated transition frequencies for amino (g) and imino (4) forms; (B) transitions predicted using the INDOE method; and (C) values from INDOlspd method.
in the effect of a solvent on the absorption spectra also being less pronounced. This applies to the 2-nitro and 3-nitro isomers. In the case of the 4-nitro isomer, the amino form is predicted to be much more polar than the imino one; however, as the latter is thermodynamically more favourable it most probably predominates in solution. Both the above factors result in the features of a solvent having a weaker influence on spectra of C-265 than nitracrine. result
Fig. 4. The experimental absorption spectra (A) of C-257 in n-hexane (a, -), 1,Cdioxane (b, ---). and acetonitrile (c, ----) together with the calculated transition frequencies for amino (q) and imino (4) forms; (B) transitions predicted using the INDOE method; and (C) values from INDO/spd method.
Further insight into the nature of UV absorption is revealed by comparing theoretically predicted frequencies and oscillator strengths of electronic transitions with experimental absorption spectra (Figs 2-5). Generally, transitions calculated by the INDO/S method cover the whole range of observed absorption. However, the INDO/spd method predicts transitions usually at somewhat shorter wavelengths than INDO/S does. This is particularly noted in the long wavelengths spectral region. Both spectral characteristics, especially those predicted by the INDO/S method, correlate quite well with maxima in the experimental spectra, in the case of 2-nitro and 3-nitro isomers, whereas for
Tautomeric phenomena in nitracrine and its nitro isomers
777
I\ -
\ \
L,
B
2.0
2 1.5 1 fi 1.0 BO.5 %
0.0
I’“““““‘““1
1.2
% 0.B
h
Io.4 0
0.0 1.0
1.2
$
4 0.11 ”
4 1 0.4
0.0 210
260
220
330
370
410
460
420
520
Fig. 5. The experimental absorption spectra (A) of C-265 in n-hexane (a, -), 1,Cdioxane (b, ---), and acetonitrile (c, ----) together with the calculated transition frequencies for amino (CJ) and imino (4) forms; (B) transitions predicted using the INDOlS method; and (C) values from INDOlspd method.
nitracrine and its 4-nitro isomer the conformity method generally reproduces the experimental
is less satisfactory. The INDO/spd data less well. Unfortunately, both
INDO methods applied predict transitions for amino and imino forms in roughly the same spectral region. Therefore, theoretical results cannot help too much in deconvolution of the experimental spectra. Moreover, taking into account the frequently observed uncertainties in prediction of transition frequencies and especially oscillator strengths [20,25-271 it does not seem possible to reproduce the experimental spectra and particularly the effect of the solvent on, as we believe, complex tautomeric phenomena.
778
JANUSZ RAK AND JERZY B~A~EJOWSKI
AMINO FORM!3
X=255.8 TypeXl-w*
X-482.9 Type: *‘I*
A=246.3 Type: n*
+ n-u*
x-406.2 Type: n-w*
c-264
A=246.6 Ty-pe: z-m*
A-484.9
Type: I--W*
X=254.2 Type: a-**
C-257
x=390.0 Type: n-w*
C-257
&& A-280.5
h=489.1 Type: w+”
Type: PW*
X=255.8 Type: z-r*
C-265
c-265
:$ x=252.1 Type: FW*
A9402.8 Type: n-w*
X=485.3 Type: r--f=
A-249.2 Type: z-w*
& A-406.2 Type: n-w*
Fig. 6. Changes in electron densities accompanying electronic excitation in amino forms, for the longest wavelength transition and for the strongest transition. Empty circles indicate the decrease and filled circles the increase of the electron density at a given atom; the surface of circles is proportional to the absolute value of the electron density change at a given atom.
Electron density changes upon excitation The transition momentum vectors predicted by the INDO/S method for both amino and imino tautomers lie in the plane of the acridine moiety, which implies that these transitions are of the n--*x* type [28] (Figs 6 and 7). The INDO/spd method generally affords similar results for the strongest transitions. The exception is the strongest transition in amino tautomer of C-283 which can be interpreted as a superposition of n-+x* and n+u*. On the other hand, the longest wavelength transitions obtained with
Tautomeric phenomena in nitracrine and its nitro isomers
779
IMINO FORMS INDOIS
INDOlSpd _
c-283
ache
b X1237.0 Type: z-x*
k X=468.7 Type: s-x*
X=222.4 Type: s-w*
X=425.0 Type: n-x*
h=234.9
Type: s-x*
X=488.5 Type: M’
X=228.0 Type: s-x*
X-422.1 Type: n-w*
X=267.9 Type: s-x*
X-488.5 Type: s-x*
A=243.4 Type: x-w*
A-425.1 Type: n-V*
c-265
& X=238.3 Type: s-x*
A-498.3 Type: s-x*
X=225.7 Type: s-w*
b A=421.6 Type: n--x*
Fig. 7. Changes in electron densities accompanying electronic excitation in imino forms (other information is given under Fig. 6).
the latter method should have the IZ+~* origin (such transitions characterize transition momentum vectors perpendicular to the acridine skeleton [28]). Analogous analyses carried out in the case of 9-acridinamine revealed similar regularities [29]. Figures 6 and 7 also visualize changes in the electron density, accompanying electronic excitation. All these changes are predicted to be asymmetrical due to the unsymmetrical constitution of the molecules. According to the INDO/S method the longest wavelength transitions in both amino and imino tautomeric forms cause a shift of the negative charge mainly from oxygen atoms to the nitrogen atom of the -NO2 group. Contrarily, the INDO/spd method predicts that the longest wavelength transitions cause a change in
780
JANUSZRAK AND JERZYB~A~EJOWSKI
electron density from endocyclic nitrogen towards carbon (9), in amino forms, and from exocyclic nitrogen towards carbon (9), in imino forms. However, this latter method predicts that in the overall electron density changes the nitro group is also involved. This is seen in particular in amino forms of C-283 and C-265. Both theoretical methods demonstrate that in the longest wavelengths transitions, some of the atoms of the aromatic system participate more or less. In the case of the strongest transitions electron density changes concern the acridine aromatic system, nitro group and exocyclic nitrogen. Atoms constituting the aliphatic substituent are not involved, as one may expect, in electronic excitation in the spectral region examined. Somewhat unexpected, therefore, is the result of INDO/spd calculations for the amino form of nitracrine corresponding to 246.3 nm (Fig. 6). The latter one appears to be a superposition of n+~c* and IZ+U* transitions realized in two separate sites of the molecule. One thing seems unquestionable, however, namely, that spectral characteristics distinguish nitracrine from its three isomers studied.
CONCLUDING REMARKS
Versatile analysis of the electronic absorption reveals that the behaviour of nitracrine and its isomers can be primarily accounted for by the existence of the molecules in two tautomeric forms. As the results of our most recent work have shown each tautomer can further occur in several structural forms (which can be named stereo-isomers). This means that, in the liquid-phase, one deals with a mixture of various structural forms. For each such form appropriate transition frequencies and oscillator strengths can be expected. Indeed, taking various stereo-isomers of a given tautomer one obtains somewhat different values of both above mentioned spectral characteristics. The combination of these does not, however, account for the complex behaviour of the compounds studied in the liquid-phase. Solvents influence specifically equilibria between various forms, the result of which is that theoretically predicted spectra only roughly reproduce the experimental ones. In one of our previous works we used structures not completely optimized, and a less advanced quantum chemistry method (PPP), for the examination of spectral features of nitracrine and its isomers [16]. Quantum chemistry calculations in the present work were based on the INDO approximation (believed to have one of the most advanced spectroscopic parameterizations) and were performed for structures completely optimized by the AM1 method [19]. The results, although not in all cases correlating immediately with the experimental spectra, extend our knowledge on the bahaviour of this group of compounds and may facilitate an insight into the origin of their biological activity. Acknowledgements-We would like to thank Prof. J. Lipiliski for providing the program for INDOlspd calculations. This work was supported by the Polish State Committee for Scientific Research (KBN) under grant BW15-300-4-068-l.
REFERENCES [l] A. Ledochowski and B. Stefanska, Rocr. Chem. 40, 301 (1966). [2] J. Konopa, A. Ledochowski, A. Matuszkiewicz and E. Jereczek-Morawska, Neoplasma 16, 171 (1%9). [3] M. Hrabowska, Z. Mazerska, J. Paradziej-Lukowicz, K. Onoszko and A. Ledochowski, Drug. Res. 32,lO (1982). [4] “Ledakrin”, Meter. Med. Pal. 8, 237 (1976). [5] M. Warwas, B. Mazerska and W. Dobryszycka, Arch. Immunol. Ther. Exp. 25, 235 (1977). [6] M. Gniazdowski, J. Filipski and M. Chorazy, Antibiotics (N. Y.) 5, 275 (1979). [7] W. A. Denny, B. C. Baguley, B. F. Cain and M. J. Waring, in Molecular Aspects of Anticancer Drug Action (Edited by S. Neidle and M. J. Waring), pp. l-34. MacMillan, London (1983). [8] A. Hempel, S. E. Hull, M. Bogucka-Ledochowska and Z. Dauter, Acta Crystallogr. B 35,474 (1979). [9] A. Hempel, S. E. Hull, Z. Dauter, M. Bogucka-Ledochowska and A. Konitz, Acta Crystallogr. B 35,477 (1979).
Tautomeric phenomena in nitracrine and its nitro isomers
781
[lo] J. J. Stezowski, P. Kollat, M. Bogucka-Ledochowska and J. P. Glusker, J. Am. Chem. Sec. 107, 2067 (1985). [ll] A. C. Capomacchia, J. Casper and S. G. Schulman, J. Pharm. Sci. 63, 1272 (1974). [12] J.-P. Galy, R. Faure, J. Barbe and J. Elgurero, Spectrosc. Lett. 21, 809 (1988). [13] J. Rak, J. Blazejowski and R. J. Zauhar, submitted for publication. [14] J. Biedrzycki, J. Blazejowski, A. Ledochowski and J. Szychlinski, Rocz. Chem. 51, 379 (1977). [15] J. Biedrzycki, J. Blazejowski, J. Szychlinski and A. Ledochowski, Z. Nuturforsch. A 39, 195 (1984). [16] A. Tempczyk, J. Rak and J. Blazejowski, J. Chem. Sot., Perkin Trans. 2, 1501 (1990). [17] J. Rak, K. Nowaczyk, J. Blazejowski and A. Kawski, J. Fluoresc. 1, 57 (1991). [18] J. Rak, J. Blazejowski, K. Nowaczyk, and A. Kawski, 1. Photochem. Photobiol. A 61, 343 (1991). [19] J. Rak, J. Blazejowski and R. J. Zauhar, submitted for publication. [20] J. Ridley and M. C. Zerner, Theor. Chim. Actu 32, 111 (1973); 42,223 (1976). [21] J. Lipinski, ht. J. Quant. Chem. 34, 423 (1988). [22] M. J. S. Dewar, E. G. Zoebisch, E. F. Healy and J. J. P. Stewart, J. Am. Chem. Sot. 107.3902 (1985). [23] A. Ledochowski, Rocz. Chem. 40, 1557 (1966). [24] J. Blazejowski and J. Szychhnski, Anal. Chim. Acta 159, 369 (1984). [25] A.-M. Galy, J.-P. Galy, R. Faure and J. Barbe, Eur. J. Med. Chem.-Chim. Theor. 15, 179 (1980). [26] M. G. Hutchings, B. H. Meyrick and A. J. Nelson, Tetrahedron 40, 5081 (1984). [27] L. Bouscasse, D. Bouin, M. P. Crozet, 0. Jentzer and P. Vanelle, Spectrosc. Len. 22, 693 (1989). [28] J. A. Barltrop and J. P. Coyle, Principles of Photochemistry (Polish edn.). PWN, Warszawa (1987). [29] J. Rak and J. Blazejowski, J. Photochem. Photobiol. A, in press.
05~8539/92 s5.00+ 0.00 Pergamon PressLtd
1992
Experimental and theoretical (INDO CI) electronic absorption spectroscopy in the examination of tautomeric phenomena in nitracrine and its nitro isomers JANUSZ RAK AND
JERZY B~A&JOWSKI*
Department of Chemistry, University of Gdadsk, 80-952 Gdaftsk, Poland (Receiued 11 November 1991; accepte$3 December 1991) Abstract-Electronic absorption spectroscopy was employed to examine tautomeric phenomena in N,N-dimethyl-N-(l-nitro-9-acridinyl)-1,3-propanediamine (known as nitracrine-WHO, or Ledakrin in Poland) which exhibits antitumour activity, and also to examine its three nitro isomers. The analysis of the experimental absorption spectra reveals that the compounds exist in the liquid-phasein at least two forms (most probably amino and imino tautomeric forms), remaining in an equilibrium which is strongly affected by the features of a solvent. This was qualitatively confirmed by examining frequencies and oscillator strengths of electronic transitions obtained for the lowest energy structures of a given form by two independent methods based on INDO approximation. The latter methods predict that the strongest transitions occurring below 280 nm have a rr-+n* origin, whereas the longest wavelength transitions may be either X+X* or n-s* typ and cause a shift in electron density within the acridine aromatic system and attached nitro group and exocyclic nitrogen atom.
INTRODUCTION
9-acridinamine derivatives have been synthesized and extensively examined over several decades by LED~CHOWSKI and co-workers from the point of view of their agents [l-3]. Such activity is also exhibited by utility as anticancer N,N-dimethyl-N’-(l-nitro-9-acridinyl)-l,3-propanediamine, known as nitracrine (the WHO term) [l, 41. The compound has been admitted into Poland as an antineoplastic drug (under the name of Ledakrin) and successfully applied in the treatment of certain solid tumours, such as mammary, lung, ovarian and colon tumours [4-71. The X-ray investigations which followed synthetic and biological examinations revealed that the prototropic tautomerism occurs in nitracrine and its 2-nitro isomer [4,8-lo]. This led us to think that the latter phenomenon, being general in 9-acridinamine derivatives [lo-131, plays an important role in biological activity of nitracrine. To examine the problem, in our previous work we employed electronic absorption [14-H] and luminescence [17,18] spectroscopies, as well as, to a limited extent, the MNDO method together with the theoretical analysis of the electronic absorption spectra on the PPP level [16]. These studies revealed that nitracrine and its three nitro isomers presumably occur, in the liquid-phase, in two tautomeric forms, i.e. amino and imino, which can sometimes exist as more than one structural isomer (a stereo-isomer). This was fully confirmed by complex theoretical analysis presented in our most recent paper [19]. The aim of the present work is to find further support for the existence of nitracrine and its isomers in various tautomeric and structural forms. To achieve this we have compared experimental spectra recorded in three solvents of various polarity and electron (proton)-donor-acceptor ability with electronic transitions predicted by the methods based on INDO approximation [20,21], using structures obtained from the complete geometry optimizations with the AM1 method [19,22]. The complementary analysis of the electron density changes accompanying electronic excitation was also carried out to enable an insight into the nature of this process.
NUMEROUS
* Author to whom correspondence should be addressed. 771
772
JANUSZ RAKAND JERZY&A~EJOWSKI MATERIALSAND METHODS
Chemicals
The N,N-dimethyl-N’-( l-nitro-9-acridinyl)-1,3-propanediamine and its isomers were synthesized as dihydrochlorides and purified by the method reported in the literature [4,23]. The compounds were kindly supplied by Prof. J. Konopa and his research group from the Technical University of Gdadsk. The free bases were obtained by the alkalization of the aqueous solution of original compounds with K&O3 and extraction with benzene. The extracts were subsequently dried over anhydrous MgSO,, filtered and the solvent evaporated under reduced pressure. Some of the extracts were additionally subjected to lyophilization. Spectral measurements
The UV and visible absorption spectra were measured on a VSU-2P (Carl Zeiss, Jena) spectrophotometer. Solvents, i.e. n-hexane, from Romil Chemicals Ltd, and acetonitrile, from Fluka AG, both of spectral grade were used as received. 1,CDioxane of Analar grade was purified by the previously described procedure [24]. Calculations
Electronic transitions were calculated employing two semiempirical methods based on INDO approximation. The first set of frequencies and oscillator strengths was determined on the basis of the spectroscopic parameterization (INDO/S) of RIDLEY and ZERNER [20]. In these calculations single excited configurations corresponding to the range from 0 to 10 eV of the excitation energy were used. Usually more than 80 singly excited states were used in the CI procedure. The second set of spectroscopic data was derived using the all-valence INDO/spd method (Ghost and Rydberg INDO) developed by LIPINSKI[21]. In this latter case 80 singly excited configurations were also used in the CI procedure. The value of the adjustable parameter (one) scaling p integrals, was assumed to be 1. All calculations were performed on a Flex Cache 331386 (Advanced Logic Research, Inc) computer.
RESULTSAND
DISCUSSION
Molecular geometries
It was revealed in our latest work, on the basis of the AM1 quantum chemistry calculations, that N,iV-dimethyl-N’-( 1-nitro-9-acridinyl)-1,3-propanediamine (C-283) and nitro isomers of the compound (for symbols see caption to Fig. 1) should actually exist in two tautomeric forms, namely amino and imino, which can further occur as several structural isomers (stereo-isomers) [19]. This would imply that in the liquid-phase we are dealing with complex equilibria in which all the forms of comparable energies participate. The most feasible form in which the molecule occurs is, of course, one of the lowest value of free energy or in some cases simply energy. This latter situation takes place when entropies of various forms do not differ substantially from each other. In theoretical spectroscopic calculations the lowest energy structures of the compounds studied, for a given tautomeric form, were considered. The views of these structures are exhibited in Fig. 1. Experimental
and theoretical absorption spectra
The experimental spectra [Figs 2-5(A)] measured in three markedly different solvents exhibit complex patterns which cannot simply be accounted for by solute-solvent
Tautomeric phenomena in nitracrine and its nitro isomers
173
c-283
C-264
C-257
C-265
amino form
imino form
Fig. 1. Structures of N,N-dimethyl-N’-( 1-nitro-9-acridinyl)-1,3_propanediamine (C-283) and its 2-nitro (C-264), 3-nitro (C-257), and 4-nitro (C-265) isomers in amino and imino tautomeric forms, for which spectroscopic calculations were carried out.
interactions. It is much more probable that this complexity originates from the existence of the compounds in more than one structural form. The effect of the solvent on the electronic absorption is more pronounced in C-283, particularly in the range above 330 nm. This region has also been examined earlier in more detail [14]. The constitution of the molecules of the compounds studied is such that each of them should be polar. The measure of this feature is the dipole moment whose values (in D) for the structures shown in Fig. 1 are (first value corresponds to the amino form and the second value to the imino form): 2.92 and 5.86, 6.36 and 7.60, 8.44 and 6.43, 8.66 and 5.00, for C-283, C-264, C-257 and C-265, respectively [19]. Since the free energy and energy differences between these amino and imino structures do not differ substantially (free energy-first value, and energy-second value), differences between the amino and imino forms shown in Fig. 1, resulting from the AM1 calculations (in kJ mol-‘) [19], are equal to: 14.5 and 18.5, 6.6 and 12.7, 6.9 and 12.9, 27.2 and 32.6, for C-283, C-264,
774
JANUSZ
RAK
AND JERZY
B~A~EJOWSKI
s c
* 0.4
3 fI 0
Fig. 2. The experimental absorption spectra (A) of C-283 in n-hexane (a,-), 1,4-dioxane (b, ---). and acetonitrile (c, ----) together with the calculated transition frequencies for amino &I) and imino (Q) forms; (B) transitions predicted using the INDO/S method; and (C) values from INDOlspd method.
C-257 and C-265, respectively) one can expect that these two forms, at least, exist in solutions in equlibrium. The equilibrium state, and thus relative amounts of both forms will, of course, be affected by solute-solvent interactions. In non-polar solvent (i.e. n-hexane) the equilibrium should, therefore, be moved towards more polar structure (e.g. the imino in the case of C-283). If this indeed took place it would mean that the long wavelength band of C-283 would be ascribed to the amino form, and the band with maximum at ca 385 nm to the imino form (Fig. 2A) [16]. In the remaining isomers of nitracrine the differences between the dipole moment values become smaller, which may
Tautomeric phenomena in nitracrine and its nitro isomers
ns
0.0
1.0
I
P 1.e
1
4 0.0 ”
4 j 0.4
0.0 810
85a
290
930
370
410
460
490
MO
Fig. 3. The experimental absorption spectra (A) of C-264 in n-hexane (a, -), 1,4-dioxane (b,---), and acetonitrile (c, ----) together with the calculated transition frequencies for amino (g) and imino (4) forms; (B) transitions predicted using the INDOE method; and (C) values from INDOlspd method.
in the effect of a solvent on the absorption spectra also being less pronounced. This applies to the 2-nitro and 3-nitro isomers. In the case of the 4-nitro isomer, the amino form is predicted to be much more polar than the imino one; however, as the latter is thermodynamically more favourable it most probably predominates in solution. Both the above factors result in the features of a solvent having a weaker influence on spectra of C-265 than nitracrine. result
Fig. 4. The experimental absorption spectra (A) of C-257 in n-hexane (a, -), 1,Cdioxane (b, ---). and acetonitrile (c, ----) together with the calculated transition frequencies for amino (q) and imino (4) forms; (B) transitions predicted using the INDOE method; and (C) values from INDO/spd method.
Further insight into the nature of UV absorption is revealed by comparing theoretically predicted frequencies and oscillator strengths of electronic transitions with experimental absorption spectra (Figs 2-5). Generally, transitions calculated by the INDO/S method cover the whole range of observed absorption. However, the INDO/spd method predicts transitions usually at somewhat shorter wavelengths than INDO/S does. This is particularly noted in the long wavelengths spectral region. Both spectral characteristics, especially those predicted by the INDO/S method, correlate quite well with maxima in the experimental spectra, in the case of 2-nitro and 3-nitro isomers, whereas for
Tautomeric phenomena in nitracrine and its nitro isomers
777
I\ -
\ \
L,
B
2.0
2 1.5 1 fi 1.0 BO.5 %
0.0
I’“““““‘““1
1.2
% 0.B
h
Io.4 0
0.0 1.0
1.2
$
4 0.11 ”
4 1 0.4
0.0 210
260
220
330
370
410
460
420
520
Fig. 5. The experimental absorption spectra (A) of C-265 in n-hexane (a, -), 1,Cdioxane (b, ---), and acetonitrile (c, ----) together with the calculated transition frequencies for amino (CJ) and imino (4) forms; (B) transitions predicted using the INDOlS method; and (C) values from INDOlspd method.
nitracrine and its 4-nitro isomer the conformity method generally reproduces the experimental
is less satisfactory. The INDO/spd data less well. Unfortunately, both
INDO methods applied predict transitions for amino and imino forms in roughly the same spectral region. Therefore, theoretical results cannot help too much in deconvolution of the experimental spectra. Moreover, taking into account the frequently observed uncertainties in prediction of transition frequencies and especially oscillator strengths [20,25-271 it does not seem possible to reproduce the experimental spectra and particularly the effect of the solvent on, as we believe, complex tautomeric phenomena.
778
JANUSZ RAK AND JERZY B~A~EJOWSKI
AMINO FORM!3
X=255.8 TypeXl-w*
X-482.9 Type: *‘I*
A=246.3 Type: n*
+ n-u*
x-406.2 Type: n-w*
c-264
A=246.6 Ty-pe: z-m*
A-484.9
Type: I--W*
X=254.2 Type: a-**
C-257
x=390.0 Type: n-w*
C-257
&& A-280.5
h=489.1 Type: w+”
Type: PW*
X=255.8 Type: z-r*
C-265
c-265
:$ x=252.1 Type: FW*
A9402.8 Type: n-w*
X=485.3 Type: r--f=
A-249.2 Type: z-w*
& A-406.2 Type: n-w*
Fig. 6. Changes in electron densities accompanying electronic excitation in amino forms, for the longest wavelength transition and for the strongest transition. Empty circles indicate the decrease and filled circles the increase of the electron density at a given atom; the surface of circles is proportional to the absolute value of the electron density change at a given atom.
Electron density changes upon excitation The transition momentum vectors predicted by the INDO/S method for both amino and imino tautomers lie in the plane of the acridine moiety, which implies that these transitions are of the n--*x* type [28] (Figs 6 and 7). The INDO/spd method generally affords similar results for the strongest transitions. The exception is the strongest transition in amino tautomer of C-283 which can be interpreted as a superposition of n-+x* and n+u*. On the other hand, the longest wavelength transitions obtained with
Tautomeric phenomena in nitracrine and its nitro isomers
779
IMINO FORMS INDOIS
INDOlSpd _
c-283
ache
b X1237.0 Type: z-x*
k X=468.7 Type: s-x*
X=222.4 Type: s-w*
X=425.0 Type: n-x*
h=234.9
Type: s-x*
X=488.5 Type: M’
X=228.0 Type: s-x*
X-422.1 Type: n-w*
X=267.9 Type: s-x*
X-488.5 Type: s-x*
A=243.4 Type: x-w*
A-425.1 Type: n-V*
c-265
& X=238.3 Type: s-x*
A-498.3 Type: s-x*
X=225.7 Type: s-w*
b A=421.6 Type: n--x*
Fig. 7. Changes in electron densities accompanying electronic excitation in imino forms (other information is given under Fig. 6).
the latter method should have the IZ+~* origin (such transitions characterize transition momentum vectors perpendicular to the acridine skeleton [28]). Analogous analyses carried out in the case of 9-acridinamine revealed similar regularities [29]. Figures 6 and 7 also visualize changes in the electron density, accompanying electronic excitation. All these changes are predicted to be asymmetrical due to the unsymmetrical constitution of the molecules. According to the INDO/S method the longest wavelength transitions in both amino and imino tautomeric forms cause a shift of the negative charge mainly from oxygen atoms to the nitrogen atom of the -NO2 group. Contrarily, the INDO/spd method predicts that the longest wavelength transitions cause a change in
780
JANUSZRAK AND JERZYB~A~EJOWSKI
electron density from endocyclic nitrogen towards carbon (9), in amino forms, and from exocyclic nitrogen towards carbon (9), in imino forms. However, this latter method predicts that in the overall electron density changes the nitro group is also involved. This is seen in particular in amino forms of C-283 and C-265. Both theoretical methods demonstrate that in the longest wavelengths transitions, some of the atoms of the aromatic system participate more or less. In the case of the strongest transitions electron density changes concern the acridine aromatic system, nitro group and exocyclic nitrogen. Atoms constituting the aliphatic substituent are not involved, as one may expect, in electronic excitation in the spectral region examined. Somewhat unexpected, therefore, is the result of INDO/spd calculations for the amino form of nitracrine corresponding to 246.3 nm (Fig. 6). The latter one appears to be a superposition of n+~c* and IZ+U* transitions realized in two separate sites of the molecule. One thing seems unquestionable, however, namely, that spectral characteristics distinguish nitracrine from its three isomers studied.
CONCLUDING REMARKS
Versatile analysis of the electronic absorption reveals that the behaviour of nitracrine and its isomers can be primarily accounted for by the existence of the molecules in two tautomeric forms. As the results of our most recent work have shown each tautomer can further occur in several structural forms (which can be named stereo-isomers). This means that, in the liquid-phase, one deals with a mixture of various structural forms. For each such form appropriate transition frequencies and oscillator strengths can be expected. Indeed, taking various stereo-isomers of a given tautomer one obtains somewhat different values of both above mentioned spectral characteristics. The combination of these does not, however, account for the complex behaviour of the compounds studied in the liquid-phase. Solvents influence specifically equilibria between various forms, the result of which is that theoretically predicted spectra only roughly reproduce the experimental ones. In one of our previous works we used structures not completely optimized, and a less advanced quantum chemistry method (PPP), for the examination of spectral features of nitracrine and its isomers [16]. Quantum chemistry calculations in the present work were based on the INDO approximation (believed to have one of the most advanced spectroscopic parameterizations) and were performed for structures completely optimized by the AM1 method [19]. The results, although not in all cases correlating immediately with the experimental spectra, extend our knowledge on the bahaviour of this group of compounds and may facilitate an insight into the origin of their biological activity. Acknowledgements-We would like to thank Prof. J. Lipiliski for providing the program for INDOlspd calculations. This work was supported by the Polish State Committee for Scientific Research (KBN) under grant BW15-300-4-068-l.
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Tautomeric phenomena in nitracrine and its nitro isomers
781
[lo] J. J. Stezowski, P. Kollat, M. Bogucka-Ledochowska and J. P. Glusker, J. Am. Chem. Sec. 107, 2067 (1985). [ll] A. C. Capomacchia, J. Casper and S. G. Schulman, J. Pharm. Sci. 63, 1272 (1974). [12] J.-P. Galy, R. Faure, J. Barbe and J. Elgurero, Spectrosc. Lett. 21, 809 (1988). [13] J. Rak, J. Blazejowski and R. J. Zauhar, submitted for publication. [14] J. Biedrzycki, J. Blazejowski, A. Ledochowski and J. Szychlinski, Rocz. Chem. 51, 379 (1977). [15] J. Biedrzycki, J. Blazejowski, J. Szychlinski and A. Ledochowski, Z. Nuturforsch. A 39, 195 (1984). [16] A. Tempczyk, J. Rak and J. Blazejowski, J. Chem. Sot., Perkin Trans. 2, 1501 (1990). [17] J. Rak, K. Nowaczyk, J. Blazejowski and A. Kawski, J. Fluoresc. 1, 57 (1991). [18] J. Rak, J. Blazejowski, K. Nowaczyk, and A. Kawski, 1. Photochem. Photobiol. A 61, 343 (1991). [19] J. Rak, J. Blazejowski and R. J. Zauhar, submitted for publication. [20] J. Ridley and M. C. Zerner, Theor. Chim. Actu 32, 111 (1973); 42,223 (1976). [21] J. Lipinski, ht. J. Quant. Chem. 34, 423 (1988). [22] M. J. S. Dewar, E. G. Zoebisch, E. F. Healy and J. J. P. Stewart, J. Am. Chem. Sot. 107.3902 (1985). [23] A. Ledochowski, Rocz. Chem. 40, 1557 (1966). [24] J. Blazejowski and J. Szychhnski, Anal. Chim. Acta 159, 369 (1984). [25] A.-M. Galy, J.-P. Galy, R. Faure and J. Barbe, Eur. J. Med. Chem.-Chim. Theor. 15, 179 (1980). [26] M. G. Hutchings, B. H. Meyrick and A. J. Nelson, Tetrahedron 40, 5081 (1984). [27] L. Bouscasse, D. Bouin, M. P. Crozet, 0. Jentzer and P. Vanelle, Spectrosc. Len. 22, 693 (1989). [28] J. A. Barltrop and J. P. Coyle, Principles of Photochemistry (Polish edn.). PWN, Warszawa (1987). [29] J. Rak and J. Blazejowski, J. Photochem. Photobiol. A, in press.