J . Phys. Chem. 1990, 94, 7021-7032
7021
Theoretlcal ab Initio Study of the Protomeric Tautomerism of 2-Hydroxypyrlmidlne, 4-Hydroxypyrimidlne, and Thelr Derivatives Andrzej L e 8 and Ludwik Adamowicz* Department of Chemistry, The University of Arizona. Tucson, Arizona 85721 (Received: March 15, 1990; In Final Form: May 21, 1990)
We investigated the gas-phase protomeric tautomerism of a series of monosubstituted derivatives of two parent compounds, 2-hydroxypyrimidine and 4-hydroxypyrimidine (substituted at the C(4) and C(2) positions, respectively, by H, NH2, OH, SH, and SCH3). We used the ab initio Hartree-Fock method, the many-body perturbation theory (MBPT), and the coupled cluster (CC) method with the double-{ Gaussian basis augmented with polarization functions to calculate the electronic contribution to the molecular total energy. The energy of the zero-point nuclear vibrations was obtained within the harmonic approximation by means of the SCF/3-2 1G* analytical energy derivatives. For each molecule we considered several tautomeric forms corresponding to different protonation sites: at the ring nitrogen atoms and the exocyclic oxygen atom. We estimated the relative temperature-dependent distribution of various tautomeric forms in the gas phase and predicted the environmental influence on the tautomeric equilibrium. We show that the commonly applied MBPT(2) theory for the evaluation of the electron correlation effects should be supplemented by an estimation of the higher order electron correlation effects. For all investigated systems, they appear to favor the oxo tautomeric forms by 1-4 kJ mol-'. In all cases studied in the present paper we achieved a qualitative agreement with the conclusions based on the recent low-temperature matrix-isolation IR spectra. We lend support to the experimentally derived conclusion that 2-hydroxypyrimidine should exist in the gas phase (or in the weakly polar environment) almost exclusively in the hydroxy tautomeric form. The theoretically derived equilibrium constant is equal to K(oxo/hydroxy) = 0.01 at T = 500 K. A coexistence of the hydroxy and oxo forms, with a clear predominance of the hydroxy form, should characterize the cytosine ( K = 0.37), isocytosine ( K = 0.15), and S-methylated 4-thiouracil (K = 0.17) vapors. A nearly equimolar hydroxy:oxo mixture should correspond to the gas-phase 4-hydroxypyrimidine ( K = 1.04) and S-methylated 2-thiouracil ( K = 1.08). According to our estimation of the environment effects, the hydroxy-oxo tautomeric equilibrium should be strongly shifted toward the oxo form in polar solvents. The tautomeric rearrangement corresponding to proton transfer between the ring nitrogen atoms (N( I)-H to N(3)-H and vice versa) appears to be highly unfavorable in the gas phase. O n the other hand, such a process can easily occur in polar solvents due to an additional strong (mostly electrostatic) solvent stabilization of the energetically unfavored tautomeric form.
Introduction
2- and 4-hydroxypyrimidine are parent compounds of numerous biologically important pyrimidine nucleobases. Depending on the substituent at the carbon ring atoms C(2) or C(4) (see Figures 1 and 2), one may obtain cytosine or uracil derivatives that are known to be the components of genetic material. All these molecules may appear in various tautomeric forms, differing by the positions of the protons.' The most common are the oxo forms, identified in the literature as 2(4)-pyrimidinones. Below we briefly sketch the biochemical significance of oxo and hydroxy tautomeric forms of 2(4)-hydroxypyrimidines.
R
b, N,
-
2-hydroxypyrimidine 6-hydroxypyrinidins R H, NH,, OH, SH. SCH,.
2-Hydroxypyrimidine and 4-Hydroxypyrimidine ( R = H). 2and 4-hydroxypyrimidine can form nucleosides, parent compounds of cytidine and uridine. Some of the 2- and 4-hydroxypyrimidine derivatives affect cell-division processes (arrest mitosis).2 They also exhibit antibacterial activity, e.g., the 5-halo derivatives inhibit the cytidine deaminase or are active antiviral agents (antiherpes HSV-I and HSV-2).3 It has recently been suggested4 that for
' Permanent address: Department of Chemistry, University of Warsaw,
Pasteura I , 02-093 Warsaw, Poland.
inhibitory effect the structural fragment -N( l)C(=O)Nis necessary in the heterocyclic base. This also means that the biological activity should be ascribed to the oxo tautomeric form. The hydroxy tautomer of 2( 1H)-pyrimidone should not appear under normal biological conditions, and the reason for that will be discussed later in this paper. However, there are exceptions, e.g., some substrates in the pyrimidone nucleoside synthesis do in fact involve the derivatives of the hydroxy tautomer as, for example, 2-(trimethyl~iloxy)pyrimidine.~ It has been recognized that the tautomeric form adopted by oxopyrimidines depends on the envir~nment.~-*For example, 2-oxopyrimidine occurs in the oxo form in polar solutions and in the crystal, but in a nonpolar environment, such as in n-heptane or isolated in an inert rare-gas matrix, it was found exclusively in the hydroxy form. Also, 4-oxopyrimidine appears solely in the oxo form in polar solutions and in the crystalline state but exists in an approximate 1:1 equilibrium of the oxo and hydroxy forms in an inert e n ~ i r o n m e n t . ~ - ' ~ (1) Tautomerism of Heterocycles; Katritzky, A. R., Linda, P., Eds.; Adv. Heterocycl. Chem., Supplement No. 1; Academic Press: New York, 1976. (2) Undheim, K. In Trends in Medicinal Chemistry '88'; Van der Goot, H., Domany, G., Pallos, L., Timmerman, H., Eds.; Elsevier Science: Amsterdam, 1989. (3) Efange, S. M. N.; Alessi, E. M.;Shih, H. C.; Cheng, Y. C.; Bardos. T. J. J. Med. Chem. 1985, 28, 904. (4) Holy, A.; Ludzisa, A.; Votruba, 1.; Sediva, K.; Pischel, H. Coll. Czech.
Chem. Commun. 1985, 50, 393. (5) Beak, P.; Fry, F. S. J. Am. Chem. SOC.1973, 95. 1700. (6) Beak, P.; Fry, F. S.;Steele, J. J. Am. Chem. Soc. 1976, 98, 171. (7) Beak, P. Acc. Chem. Res. 1977, IO, 186. (8) Person, W. B.; Szczepaniak, K.; Szczesniak, M.; Kwiatkowski, J. S.; Hernandez, L.; Czerminski, R. J . Mol. Struct. 1989, f 9 4 , 239. (9) Shugar, D.; Szczepaniak, K. Int. J. Quantum Chem. 1981, 20, 573. (IO) Czerminski, R.; Kuczera, K.; Rostkowska, H.;Nowak, M. J.; Szczepaniak, K. J. Mol. Strucr. 1986, 140,235. ( 1 1) Szczesniak, M. Ph.D. Thesis, 1985: Institute of Physics, Polish
Academy of Sciences, Warsaw. (12) Kuczera, K.; Szczesniak, M.; Szczepaniak, K. J . Mol. Strucr. 1988, f72, 89.
0022-3654/90/2094-7021%02.50/0 0 1990 American Chemical Society
7022 The Journal of Physical Chemistry, Vol. 94, No. 18, 1990
oxo
hydroxy
Figure 1. Oxo and hydroxy tautomeric forms of 2-hydroxypyrimidine (R = H) and of its derivatives: cytosine (R = NHJ, uracil (R = OH), 4-thiouracil (R = SH), and S-methylated 4-thiouracil (R = SCH,).
0x0
hydroxy
Figure 2. Oxo and hydroxy tautomeric forms of 4-hydroxypyrimidine(R = H) and of its derivatives: isocytosine (R = NH2), uracil (R = OH), 2-thiouracil (R = SH) and S-methylated 2-thiouracil (R = SCH,).
Cytosine and lsocytosine ( R = NH,). Cytosine occurs naturally in all nucleic acids, and it is also a component of many important drugs. A majority of biological and medicinal studies reveal that the cytosine moiety occurs in the oxo tautomeric form.I4*I5 However, it has recently been found cytosine exists as a mixture of the aminooxo and aminohydroxy tautomers in an inert lowtemperature Ar and N2 matrix.1618 Also, theoretical calculations (corresponding to the gas phase) confirmed a considerable stability of the amino-hydroxy tautomer.Is2l It can then be inferred that cytosine tautomerism is very environment dependent. lsocytosine requires some more detailed description since it has not been studied as intensively as cytosine. Isocytosine is an isomer of cytosine with the amino and carbonyl functional groups being interchanged. Nevertheless, this minor transformation causes substantial changes in the biochemical response. Our interest in isocytosine stems from its biological significance and medical applications, some of which we will discuss below. Isocytosine residues appear in some synthetic acyclic pyrimidine nucleosides, for example, in 1-[ [2-hydroxy-l-(hydroxymethyl)ethoxy]methyl]i~ocytosine,~~ which is related to a potent antiviral drug, ganciclovir, used in AIDS therapy. In this compound, the acyclic chain is bound to the N(1) endocyclic atom. Such a bonding fixes the oxo-amino N(1)H tautomeric form of the isocytosine moiety. It will be shown later that the N( l)H tautomer ( I 3) Nowak, M. J.; Szczepaniak, K.; Barski. A.; Shugar, D. J . Mol. Struct. 1980, 62. 47. ( 14) Saenger, W. Principles of Nuclei Acid Structure; Springer-Verlag:
New York, 1983. (IS) Mansuri, M. M.;Martin, J. C. Ann. Rep. Med. Chem. 1987, 22. 147. (16) Szczesniak, M.; Szczepaniak, K.; Kwiatkowski, J. S.; KuBulat, K.; Person, W. B. J . Am. Chem. SOC.1988, 110, 8319. ( I 7) Nowak, M. J.; Lapinski, L.; Fulara, J. Spectrochim. Acta 1989, 45A, 229. __ (18) Radchenko, E. D.; Sheina, G. G.;Smorygo, N. A,; Blagoi, Y. P. J . Mol. Struct. 1984, 116. 387. (19) Kwiatkowski, J. S.; Bartlett, R. J.; Person, W. 8. J . Am. Chem. SOC. 1988. -. - -, .110. . -, -2 -7 -5 -7 . (20) LeS, A.; Adamowicz, L.; Bartlett, R. J. J . Phys. Chem. 1989.93,4001. (21) Gould, I . R.; Hillier, I . H . Chem. Phys. Lett. 1989, 161, 185. (22) Beauchamp, L. M.; Serling, B. L.; Kelsey, J. E.; Biron, K. K.; Collins, P.: Selway, J.; Lin, J.-C.: Schaeffer, H. J . J . Med. Chem. 1988, 31. 144.
Le5 and Adamowicz is very unstable in comparison to the other tautomeric forms, and this fact may imply enhanced reactivity of N ( l ) H tautomers. Although this particular nucleoside exhibits minor antiviral activity (e&, against the Epstein-Barr virus), the investigation of its molecular structure and the correlation with the biochemical response to it are important factors in understanding the molecular structure of receptors of antiviral drugs. For example, the same acyclic chains bound to the 5-halogenated cytosine or to 5-azacytosine result in very potent drugs inhibiting the replication of the Epstein-Barr v i r ~ s . ~ ~ . ~ ~ Isocytosine residues appear in some antisecretory agents exhibiting the H2-receptor histamine antagonist a ~ t i v i t y . * ~Also, *~~ the isocytosine moiety appears in temelastine, an agent that exhibits high affinity to the histamine H I receptor. It is worth mentioning that the isocytosine moiety appears in temelastine in the N( l)H tautomeric form,26similarly as a large group of drugs related to the oxymetidineanother potent H2-receptor histamine a n t a g ~ n i s t . ~The ~ isocytosine residue was also found in the products of guanine cleavage by alkylating agents. After the cleavage of the imidazole ring, the resulting compound (Fapy) had the isocytosine moiety in another tautomeric form, N(3)H, that seems to be more stable than the N( l ) H tautomer. It was discovered that polynucleotides containning Fapy residues inhibit DNA ~ynthesis.~' The isocytosine moiety was found in some tetrahydrofolate derivatives that are responsible for carrying methyl groups during the biosynthesis of thymine from uracil.28 The isocytosine moiety appears in the oxo-amino tautomeric forms, and no other forms (hydroxy, imino) were detected.29 Another interesting feature of isocytosine is found from studies of its crystal structure. Two forms, oxoamino N(3)H and N(l)H, coexist in a 1:l ratio in the crystalline state.30 In some organometallic (Cu")complexes related to metalloenzymes and containing ligands with the isocytosine moiety, the N( l ) H tautomeric form is inferred.3' Also, in the 6-methylisocytosine crystal, the N ( l ) H tautomer occurs e x c l ~ s i v e l y . ~On ~ the other hand, 6-methylisocytosine in a N2 matrix exists mainly in the aminohydroxy form.j3 In the liquid state (ethanol, diethyl ether), only the N(3)H form exists, or two forms, N(3)H and N(l)H, coexist in ~ a t e r . ' ~ These . ~ ~ observations were also confirmed by the magnetic circular dichroism spectra.36 The proton NMR spectra of an isocytosine residue of an acyclic nucleoside, 1-[[2hydroxy- 1-( hydroxymethy1)-ethoxy] methyl] isocytosine, clearly shows that in the dimethyl sulfoxide solution only the oxo-amino tautomeric form appears. No traces of the hydroxy or imino forms were detected.37 A similar conclusion can be drawn from the (23) Martin, J. C.; Jeffrey, G. A.; Mc Gee, D. P. C.; Tippie, M. A.; Smee, D. F.; Mattews, T. R.; Verheyden, J. P. H. J . Med. Chem. 1985, 28, 358. (24) Brown, T. H.; Blakemore, R. C.; Blurton, P.; Durant, G.J.; Ganellin, C. R.; Parsons, M. E.; Rasmussen, A. C.; Rawlings, D. A,; Walker, T. F. Eur. J . Med. Chem. 1989, 24,65. (25) Spengler, J. P.; Schunack, W. Arch. Pharm. (Weinheim, Ger.) 1984, 317, 425. (26) Ganellin, C. R.; Bouthenet, M. L.; Garbarg, M.; Gros, C.; Ife, R. J.; Korner, M.; Ruat, M.; Schwartz, J. C.; Tertiuk, W.; Theobald, C. J. In Trends in Medicinal Chemistry '88';Van der Gwt, H., Domany, G.,Pallos, L., Timmerman, H., Eds.; Elsevier Science: Amsterdam, 1989. (27) Laval, J.; OConnor, T. R.; Boiteux, S. In International Congress on DNA Damage and Repair, Rome. 1987; Castellani, A,. Ed.; Plenum Press: New York, 1989. (28) Stryer, L. Biochemistry; W. H. Freeman: New York, 1981. (29) Birnbaum, G. I.; Watanabe, K. A,; Fox, J. J. Can. J . Chem. 1980, 41, 2793. (30) Sharma, B. D.; Mc Connell, J. F. Acta Crystallogr. 1965, 19, 797. (31) Sakaguchi, H.; Anzai, H.; Furuhata, K.; Ogura, H.; Iitaka, Y. Chem. Pharm. Bull. 1979, 27, 1871. (32) Lowe, P. R.; Schwalbe, C. H.; Williams, G. J. B. Acta Crystallogr. 1987, C43, 330. (33) Szczesniak, M.; Nowak, M. J.; Szczepaniak, K. J . Mol. Struct. 1984, 115, 221.
(34) Helene, C.; Douzou, P. C. R. Acad. Sci. Paris 1964, 259, 4853. (35) Morita, H.; Nagakura, S . Theor. Chim. Acta 1968, 1 1 , 279. (36) Kaito, A.; Hatano, M.: Ueda, T.; Shibuya, S . Bull. Chem. SOC.Jpn. 1980, 53, 3073. (37) Beauchamp, L. M.; Serling, B. L.; Kelsey, J. E.; Biron, K. K.;Collins, P.; Selway, J.; Lin, J.-C.; Schaeffer, H. J. J . Med. Chem. 1988, 31, 144.
Protomeric Tautomerism of Pyrimidines
The Journal of Physical Chemistry, Vol. 94, No. 18, 1990 7023
recent proton N MR studies of 5-methyl-3’-azido-2’,3’,-dideoxy- TABLE I: Total Energy (and Its Components) of the Oxo (N,H)and i~ocytidine,~~ a structural analogue of AZT, a potent anti-AIDS Hydroxy Tautomeric Forms of 2-Hydroxypyrimidine (hartrees) drug. It may be then inferred that in the polar environment the oxo hydroxy biologically active tautomer should have the oxo and amino total energy components functional groups and also an accessible (not protonated) endoSCF/3-21 G -335.665 I68 -335.663 364 cyclic nitrogen atom. An interesting question related to the SCF/DZP -337.629 256 -331.634123 intermolecular interactions of isocytosine with other nucleic bases MBPT(2)/DZPb -1.005 592 -I .006 260 was recently reviewed,j9 namely, why the is0 bases (isocytosine, 0.91 *ZPE/3-21G 0.08 1 084 0.080 234 isoguanine) are not involved as carriers of the genetic code, even total energyC -338.553 164 -338.560749 though they can potentially form complementary hydrogen bonds ‘DZP basis set ( 1 32 basis functions) of Dunning65composed of 9s5p analogous to the normal nucleobases. On the basis of semiprimitive Gaussians contracted to 4s2p and augmented by one d-funcempirical and ab initio calculations (SCF with a small basis set, tion (6d) on carbon, nitrogen, and oxygen atoms; 4s contracted to 2s a crude estimation of the electron correlation effects) of the and augmented by one p-function on hydrogen atoms. Our optimized interaction energy, it was shown that the pairs of is0 bases are geometry was essentially identical with that of La Manna6’ and Ciepas stable as normal pairs. It was then suggested that the reason lak et a1.62. bValence electron correlation only. CTotal energy = for the absence of isobases from the genetic code is probably SCF/DZP + MBPT(Z)/DZP + 0.91*ZPE/3-21G. related to the lack of a sufficient specificity in the is0 base interactions. A very recent enzymatic experiment has proved, derivatives of 2-thiouracil are of particular interedo due to their however, that the oligonucleotides containing isocytosine and structural similarity with PTU (6-propyl-2-thiouracil), which is isoguanine can be copied by RNA and DNA p o l y m e r a ~ e s . ~ ~ * ~a ~widely used antithyroid drug. Detailed studies of its metabolism Also, some synthetic analogues of pyrimidine and purine bases have shown that PTU is excreted in the form of a PTU-gluc(2,6-diaminopyrimidine and 1-methylpyrazolo[4,3-d] pyrimiuronide adduct, where the pyrimidine ring is bound to the sugar dine-S,7(4H,6H)-dione) were shown to form complementary moiety through the sulfur atom51-52 in a way similar to that of nucleic bases in a way similar to that of natural nucleotide^.^^ It the anhydron~cleosides.~~ A presence of S-methyl-PTU in rat was then suggested that the genetic alphabet may contain more urine has been also reportedes4 The metabolic transformation than four letters (A, T, G, C).42 A theoretical study of various of S-methylated thionucleotides and sulfur-containing peptides tautomeric forms in which unusual nucleic bases appear may help is probably followed by S-oxygenation to water-soluble sulfoxides to clarify important problems of mispairings and mutual recogand sulfones.54 Also, the S-demethylation metabolic routes have nition through the hydrogen bonds. been discovered, as, for example, for S-methylated mercaptoUracil in Tautomeric Forms (R= OH). Biological significance purine.54 was discussed in our previous paper.43 It would be in order to Very recently an antileukemic activity of S-methyl and 0mention that uracil does not exhibit any detectable tautomerism methyl derivatives of 5-halouracils bonded at the amino group under known physiological conditions and appears in the dioxo to the glucopyranosyl ring has been reported.55 This observation form. Also, low-temperature matrix e x p e r i m e n t ~ show ~ ~ . ~no ~ confirms early suggestions” that the methylation products of uracil indication of rare hydroxy forms. The theoretical calculations and related nucleic acid constituents might be related to mutapredicted a very low probability of the formation of rare tautomeric genesis and carcinogenesis. Very recently, some 5-substituted forms.43 There is, however, a possibility that in chemically 2-(thiomethy1)uracils were successfully used in the synthesis of modified uracil a considerable tautomerism may appear. Such isocytosine derivatives related to the H2-receptor histamine ana case will be discussed in the present paper. tagonist~.*~ It is a n t i ~ i p a t e dthat ~ ~ in the pyrimidic ring of all these 2-Thiouracil and 4-Thiouracil in Tautomeric Forms ( R = SH). compounds the proton is located at the ring nitrogen N ( 1) atom. The biological relevance of these compounds was discussed The thiouracilato moieties appear in several organometallic e l ~ e w h e r e . ~ ~Similar . ~ ’ to uracil, 2(4)-thiouracil does not exhibit complexes with transition metals (e.g., Pt, Pd). The resulting any detectable tautomerism under known physiological conditions. mono- and bidentate a d d ~ c t P ~are * ~closely * related to the platinum S-Methylated 2-Thiouracil and 4-Thiouracil (R= SCH,).In pyrimidine blue, which is a newly suggested antitumor agent.59 the context of the previous studies on 2- and 4 - t h i o ~ r a c i l sour ,~~ In the present work we investigated the electronic, structural, interest was directed toward the investigation of the molecular and energetic characteristics of various tautomeric forms of Cand electronic structure of their S-methylated derivatives. For(4)-substituted 2-hydroxypyrimidines and C(2)-substituted 4mally, these compounds can be derived from the thiol tautomeric hydroxypyrimidines by ab initio SCF and post-SCF methods. The forms of thiouracils by replacing the S H group by the SCH3 environmental effect on the position of the tautomeric equilibrium counterpart. The resulting structure can be viewed as a system was estimated by using the Kirkwood-Onsager theory. The study composed of the nucleophilic thiouracilato moiety and the methyl undertaken here potentially can lead to predictions of the biogroup. It is interesting that the S-methylated derivatives of logically active tautomeric forms in nonpolar or weakly polar thionucleotides occur naturally. For example, the S-methylated environments and may be helpful in the formulation of molecular 6-thioadenine was recently detected in tRNALyS4*and in mammechanisms of actions. Also, the theoretically calculated IR malian ~ R N A s .Among ~ ~ the thiopyrimidines, the S-methylated vibrational frequencies and intensities can help to resolve the recently obtained matrix-isolation infrared spectra. (38) Lin, T. S.; Shen, Z. Y.; August, E. M.; Brankovan, V.; Yang, H.; Ghazzouli, I.; Prusoff, W. H. J . Med. Chem. 1989, 32, 1891. (39) Jaworski. A.; Kwiatkowski, J. S.; Lesyng, B. Int. J . Quantum Chem. Quantum Biol. 1986, Symp. 12, 209. (40) Switzer, C.; Moroney, S. E.; Benner, S.A. J . Am. Chem. Soc. 1989, 111. 8322. (41) Bhattacharyya, A.; Lilley, D. M. J. J . Mol. Biol. 1989, 209, 583. (42) Piccirili, J. A,; Krauch, T.; Moroney, S. E.; Benner, S. A. Nature 1990, 343, 33. (43) Leg, A.; Adamowicz, L. J . Phys. Chem. 1989, 93, 7078. (44) Szczesniak, M.; Nowak, M. J.; Rostkowska, H.; Szczepaniak, K.; Person, W. B.; Shugar, D.J. Am. Chem. Soc. 1983, 105, 5969. (45) Nowak, M. J. J . Mol. Struct. 1989, 193. 35. (46) Katritzky, A. R.; Baykut, G.; Rachwal, S.; Szafran, M.; Caster, K. C.; Eyler, J. J . Chem. Soc. 1989, 1499. (47) Leg, A.; Adamowicz, L. J . Am. Chem. SOC.1990, 112, 1504. (48) Yamada, Y.; Ishikura, H. J . Biochem. 1981, 89, 1589. (49) Yamaizumi, Z.; Nishimura, S.;Limburg, K.; Raba, M.; Gross, H. J.; Crain, P. F.; Mc Closkey, J. A. J . Am. Chem. Soc. 1979, 101, 2224.
(50) Sarcone, E. J.; Sokal, J. E. J . Biol. Chem. 1958, 231, 605. (51) Lindsay, R. H.; Vaughn, A.; Kelly, K.; Abou-Enein, H. Y.Biochem. Pharmacol. 1977, 26, 833. (52) Lindsay, R. H.; Hill, J. B.; Kelly, K.;Vaughn, A. Endorrimlogy 1974, 94, 1689. (53) Yamagata, Y.; Fujii, S.; Fujiwara, T.; Tomita, K.-I.; Ueda, T. Acta Crystallogr. 1980, 836, 339, 343. (54) Sulfur-containing drugs and related organic compounds; Damani, L. A,, Ed.; Ellis Horwood: Chichester, 1989. ( 5 5 ) Negrillo, J.; Sanchez, A.; Nogueras, M.; Melgarejo, M. An. Quim. 1988, C84, 165. (56) Wong, J. L.; Fuchs, D. S. J . Org. Chem. 1971, 36, 848. (57) Raper, E. S. Coord. Chem. Rev. 1985, 61, 1 15. ( 5 8 ) Goodgame, D. M. L.; Rollins, R. W.; Slawin, A. M.2.;Williams, D. J.; Zard, P. W. Inorg. Chim. Acta 1986, 120, 91. Cros, S.; Francois, G. Eur. (59) Arrizabala, P.; Castan, P.; Laurent, J.-P.; J . Med. Chem. 1984, 19, 501.
Lei and Adamowicz
7024 The Journal of Physical Chemistry, Vol. 94, No. 18, 1990 TABLE 11: Total Energy (and Its Components)' of 0 x 0 ( N $ U and Hydroxy Tautomers of 4-Hydroxypynmidine (hrrtrees) oxo hydroxy total energy components -335.671 765 -335.665 198 SCF/3-2 1G -337.636 583 -337.635 454 SCF/DZP -1.005 692 -1.007 106 MBPT(Z)/DZP 0.91 *ZPE/3-21G 0.08 I 309 0.080 288 total energy -338.560 966 -338.562 272 aSee footnotes to Table I
TABLE 111: Energy Contributions to the Total Energy' of 2-Amino-4-hydroxypyrimidine (Isocytosine) Tautomers (hartrees) hydroxyoxo-aminob oxo-imino amino total energy components -390.41 7 907 -390.408 195 -390.414 748 SCF/3-21G SCF/DZP -392.700079 -392.689653 -392.702 615 -1.168976 -1.169386 -1.168 IO3 M BPT( 2) / DZPbsC 0.91 *ZPE/3-21Gb 0.097 603 0.098 204 0.096 989 total energy -393.770579 -393.760425 -393.775 012 "See footnotes to Table 1. *Another oxo-amine form with hydrogen atom at N , lies 48 kJ mol-' (SCF/3-21G = -390.399673 hartrees) above the normal form ( H at N,) and has dipole moment of 9.0 D (SCF/3-21G). With the DZP basis set the N , H tautomer is predicted to lie at 45.8 kJ mol-' (SCF/DZP//3-21G = -392.682616 hartrees, MBPT(2)/DZP/3-2IG = -1 .I68 005 hartrees) and having the dipole moment of 9.3 D. 'Valence electron correlation only.
Methodology The present work involved several ab initio techniques, which are briefly characterized in the following sections. Structure Optimization. All optimizations of molecular geometries utilized in this study were performed with the GAUSSIAN86 program60 at the S C F level with 3-21GC basis set by means of the first and second analytical derivatives of the total energy with respect to the nuclear displacements. The ring geometries were assumed planar. The optimal SCF 3-21G* geometries of 2thio~racil,~' and (4)- hydr~xypyrimidine,~"~~ cytosine,64 2-(thiomethyl)uracilM were already published. The optimized SCF/3-21G* geometries for isocytosine and 2- and I-(thiomethy1)uracil tautomers can be found in the Appendix. IR Frequencies and the Zero-Point Energy (ZPE). The IR vibrational frequencies were calculated in the harmonic approximation via the diagonalization of the mass-weighted second-order analytical derivatives of the SCF/3-21G* energy (option available in GAUSSIAN~~). The sum of the zero-point energies for all normal-mode vibrations produced an estimation of the total nuclear energy contribution. The final result for ZPE was obtained by scaling this value by a factor of 0.91, which is a commonly accepted correction.I6 Total Energy Calculation. The calculation of total electronic energy was performed for each molecule using the SCF+MBPT(2) method with the double-{ basis set of Dunning,65 augmented by one set of polarization functions for each secondand third-row atom (d-shell with six functions) and hydrogen atom (p-shell). The results are presented in Tables I-V. Exponents The calof these functions were taken from Redmon et (60) Binkley, J. S.;Frisch, M.; Raghavachari, K.; DeFrees, D.; Schlegel, H. B.;Whiteside, R.; Fluder, E.; Seeger, R.; Fox, D. J.; Head-Gordon, M.; POpk J. A. GAUSSIAN86, release C, January 26, 1988, Carnegie Mellon University. (61) La Manna, G. J . Mol. Struct. 1984, 110, 183; Ibid. 1987, 152, 83; Ibid. 1989, 188, 199. (62) Cieplak, P.; Bash, P.; Chandra, U. C.; Kollman, P. J . Am. Chem. Soc. 1987, 109. 6283. (63) Scanlan, M. J.; Hillier, I. H. J . Am. Chem. Sac. 1984, 106, 3737. (64) Katritzky, A. R.;Szafran, M.; Stevens, J. J . Chem. Sac., Perkin Trans. I1 1989, 1499, 1501. (65) Dunning, Th. H., Jr. J . Chem. Phys. 1970, 53, 2823. (66) Redmon, L. T.; Puwis, G. G.,111; Bartlett, R. J. J. Am. Chem. SOC. 1979, 101, 2856.
TABLE IV: Total Energy (and Its Components) of the Oxo (N3H, N,H) and Hydroxy Tautomeric Forms of S-Methylated 2-Thiouracil (hartrees) N3H-oxo N,H-oxo hydroxy total energy components -770.162 525 -770.1 56 214 SCF/3-21 GI" -770. I63 156 -770.148 454 -770.156 239 SCF/3-21 G* -774.1 17 227 -774.114618 SCF/6-3 1G**"vb -774.1 17 759 -774.114677 SCF/6-31G* -774.129554 SCF/6-31G** -774.130043 -774.189927 -774.175 251 -774.188 578 SCF/DZP -1.294 958 -1.296 747 -1.295 025 MBPT(Z)/DZP 0. IO9 3 I6 0.108994 0.108297 0.91 *ZPE/3-21G* total energyC -775.375 635 -775.361 214 -775.377028 "Reference 64. *Probably the SCF/6-31G* method was used;6' see text. CSeefootnotes to Table I.
TABLE V: Total Energy (and Its Components) of the Oxo (N,H) and Hydroxy Tautomeric Formsovbof S-Methylated 4-Thiouracil (hartrees) OXO (NIH) hydroxy total energy components SCF/3-2 1G* -770.161 563 -770.155814 SCF/DZP -774.187 708 -774.189291 -1.295 940 -1.296 493 MBPT(Z)/DZP 0.91 *ZPE/3-21G1 0.109 335 0.108435 total energy -775.374 31 3 -775.377 349 We disregarded the oxo (N,H) tautomeric form because its SCF/3-21GZ energy (-770.148 751 hartrees) is 34 kJ mol-' above the reference oxo ( N , H ) form; cf. Table 1V. bSee footnote c to Table I.
TABLE VI: Beyond-Hartree-Fock Calculations of the Total Energy Components of the Oxo and Hydroxy Tautomeric Forms of 2-Hydroxypyrimidine, with the S V P Basis Set and with the 3-21G Optimized Geometry (Energies in hartrees) oxo hydroxy S CF -337.612 53 1 -337.619 703 calcns with the first-order corrn orbitals -0.744 170 -0.742 382 MBPT(2)' -0.01 3 422 -0.01 3 969 MBPT(3)' -0.005 415 -0.004 374 MBPT(4); -0.017 163 -0.016750 MBPT(4)d' MBPT(4),' 0.012 075 0.012 876 -0.768 095 -0.764 599 MBPT(2+3+4,,) -0.768 550 -0.766 246 CCSDC -0.784 894 CCSD+T(CCSD)' -0.787 010 ( . = MBPT(2)'/MBPT(2), % 75.75 75.49 A = CCSD+T(CCSD)' - MBPT(2)' -0.042 840 -0.042 5 12 A .( (higher order corrn) -0.056 554 -0.056 313 t" ,j(max) -0.0643 -0.082 1 -0.02 16 -0.01 92 tnb(max) (WP) 0.264 0.266
6
"SVP (split-valence plus polarization) basis set (1 25 basis functions) of Dunning6s composed of 9s5p primitive Gaussians contracted to 3s2p and augmented by one d-function (6d) on carbon, nitrogen, and oxygen atoms; 4s contracted to 2s and augmented by one p-function on hydrogen atoms. bThe second-order correlation energy calculated with the SVP basis set (107 virtual and 18 occupied orbitals). The frozencore approximation assumed (seven core orbitals not correlated). 'The MBPT and coupled cluster calculations have been performed with the 36 FOCOs. The frozen-core approximation assumed. MBPT(4x), x = s, d, q, denote the fourth-order contributions to the correlation energy calculated with the single, double, and quadruple excitations, respectively. MBPT(2+3+4,,) is the sum of the second-, third-, and fourth-order terms, which do not take into account the triple excitations. The latter are noniteratively incorporated within the CCSD+T(CCSD) method.
culation of the second-order correlation energy was carried out with our computer code involving a fast partial transformation from atomic to molecular integrals. Additionally, for the two parent molecules, 2-hydroxypyrimidine and 4-hydroxypyrimidine, we calculated higher order correlation corrections using our
Protomeric Tautomerism of Pyrimidines TABLE VII: Beyond-Hartree-Fock Calculations of the Total Energy Components of the Oxo and Hydroxy Tautomeric Forms of 4-Hydroxypyrimidine, with the SVP Basis Set and with the 3-21G Optimized Geometry (Energies in hartrees) oxo hydroxy -337.622 107 -337.622 381 SCF -0.983 532 -0.985 362 MBPT(2)d calcns with the first-order corrn orbitals ( F O C O S ) ~ -0.740 759 -0.745 258 MBPT(2)C -0.010 592 M BPT( 3)c -0.01 3 7 14 -0.006 666 MBPT(4,)c -0.004 377 -0.01 7 722 MBPT(4d)' -0.01 7 048 0.01 1 349 0.012 691 MBPT(4J -0.764 390 -0.767 706 M BPT( 2k3+4dq) -0.764454 -0.769 365 CCSD' -0.784354 -0.787914 CCSD+T(CCSD)c 75.32 { = MBPT(2)C/MBPT(2), % 75.63 -0.043 595 -0.042 656 A CCSD+T(CCSDY - MBPT(2)' -0.057 883 -0.056 399 -0.0646 -0.0784 -0.0313 -0.01 49 0.269 0.267 ($(')I$(')) "See footnotes to Table VI.
The Journal of Physical Chemistry, Vol. 94, No. 18, 1990 7025 R=H
2-HYDROXYPYRIMIDINES R=NH2 R=OH R=SH R=SCH3
Figure 3. 2-Hydroxypyrimidines. The contributions to the relative tautomerization energy, in kJ mol-I: (solid bars) SCF/DZP/3-21G1, (dotted bars) MBPT(2)/DZP/3-21G*, (open bars) scaled ZPE/3-21GS, (circles) cumulative relative energy, including the estimated higher order electron correlation effects. R=H
4-HYDROXYPYRIMIDINES R=NHP R=OH R=SH R=SCH3
first-order correlation orbital (FOCO) t e c h n i q ~ e ~ and ' . ~ ~the coupled cluster method with single and double excitations (CCSD) and with a noniterative inclusion of triple excitation^^^ (Tables VI and VII). These higher order calculations were done with Dunning's split-valence basis set augmented with the same polarization functions as used with the DZP basis. The FOCO method was used in our previous s t ~ d i e s . ~ ~ The v ~ ~method *~~*~~ is based on the second-order Hylleraas functional and variational determination of the first-order correlation orbitals, which are linear combinations of all virtual S C F orbitals. The major advantage of this method lies in the substantial reduction of the number of correlation orbitals, enabling high-level calculations of the correlation energy. In the present calculations the number of FOCOs used was equal to twice the number of occupied valence orbitals (core electrons were left uncorrelated). Thus, for both 2-hydroxypyrimidine and 4-hydroxypyrimidine, we generated 36 FOCOs. The second-order energy calculated with this small set of orbitals was above 75% of the result obtained with all SCF virtual orbitals. To get a more precise estimation of the higher order correlation correction, its value obtained with FOCOs was scaled by the ratio I -10 of E2(all virtual orbitals)/E2(FOCOs). A similar procedure was Figure 4. 4-Hydroxypyrimidines. The contributions to the relative tauused before.'O tomerization energy, in kJ mol-': (solid bars) SCF/DZP/3-21G*, The total energy of a particular molecule was calculated with (dotted bars) MBPT(2)/DZP/3-21G*, (open bars) scaled ZPE/3-21GS, the expression (circles) cumulative relative energy, including the estimated higher order
'
electron correlation effects.
which comprises the S C F energy, the second-order correlation energy calculated with all S C F virtual orbitals, the higher order correlation correction taken from the parent compound (2- or 4-hydroxypyrimidine), and the zero-point nuclear energy. Obviously, one may question the transferability of the higher order correlation energies from the parent compounds to the derivatives. At present, however, coupled cluster calculations for all the molecules considered in this article would be prohibitively expensive. The approximation we use can be justified by two observations. First, the tautomerization process involves a very similar bond alternation in the parent compounds and in the derivatives. Second, the changes in the second-order energy between different tautomeric forms of the parent compounds and the derivatives appear to be very similar. All calculations reported here were performed on a VAX station 3100 and an SCS-40 computer. (67) Adamowicz, L.; Bartlett, R. J . J. Chem. Phys. 1987,86,6314. (68) Adamowicz, L. J. Comput. Chem. 1989, 10, 928. (69) Lee, Y.S.: Kucharski, S.A.; Bartlett, R.J. J . Chem. Phys. 1984,81, 5906.
(70) Adamowicz, L. Chem. Phys. Lett. 1989, 161, 73.
How Important Are the Higher Order Electron Correlation Effects? Recently, a few attempts toward the estimation of the higher order electron correlation effects on the position of the tautomeric equilibrium were made for 2-hydro~ypyridine'~ and for cytosine.20 For both cases, the higher order effects were not negligible and they shifted the tautomeric equilibrium noticeably toward the oxo form. In this paper we estimate higher order correlation effects by carrying the coupled cluster calculations with FOCOs for the 2- and 4-hydroxypyrimidine tautomers, which can be considered the parent compounds for the series of molecules being studied in this paper. The higher order correlation energy for the parent molecules are subsequently added t o the energies of the derivatives. As mentioned above, this extrapolation procedure can be justified by the observation that the second-order energy is nearly constant for the series of derivatives of the same parent compound, and one may expect that the higher order corrections will also follow this trend.
Results The relative energy of the oxo and hydroxy tautomeric forms of substituted 2(4)-hydroxypyrimidines are presented in Table
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Leg and Adamowicz
TABLE VIII: Stability (and Its Components) of the Hydroxy Tautomeric Form Relative to the Oxo Form of Substituted 2(4)-Hydroxypyrimidines (in kJ mol-') 2-h ydroxypyrimidine R = H R = NH2' R = OHb R = SHC R = SCHj components of the re1 stability -14.35 -2.30 -1.22 -7.74 -4.16 SCF/DZP MBPT(Z)/DZP -1.75 -0.71 -0.92 -1 .oo -1.45 0.63 1.08 0.63 0.63 0.63 hoec' 0.91 *ZPE/3-2IG -2.23 -2.25 -2.5 I -2.56 -2.36 cumulative, up to 2nd-order el corr -18.34 -5.26 -4.66 -1 1.30 -7.97 with hoec -17.71 -4.03 -10.67 -7.34 -4.18 hydroxy" only hydroxye coexists hydroxy' not found expt thiolf not found -1.58 with oxo 4-hydroxypyrimidine R = H R = NH, R = OHb R = SHC R = SCHi SCF/DZP 2.96 -6.66 8.96 -4.62 3.54 -4.89 MBPT(Z)/DZP -3.71 -3.37 -2.25 -4.52 3.90 how' 3.90 3.90 3.90 3.90 -2.26 -2.42 -2.68 -1.61 -2.68 0.91 *ZPE/3-21G cumulative, up to 2nd-order el corr -3.43 -1 1.64 1.65 -9.13 -3.65 with hoec 0.47 -7.74 5.55 -5.23 0.25 expt hydroxyh coexist, hydroxy' dominates hydroxy' not found thiolf not found 2.7) with oxo Reference 20. Reference 43. The values presented in this column correspond to two highly unfavorable tautomers of uracil. Uracil appears in biological environment exclusively in the dioxo form. cReference 47; the S C F and MBPT(2) calculations were performed with the 6-31G** basis set; cf. footnote b. 2(4)-Thiouracil appears in biological environments exclusively in the oxo-thione form. In argon and nitrogen matrices; the contribution from the oxo tautomeric form estimated to be less than 1/300 part of the total amount of the compound inserted into a matrix.'I The present theoretical equilibrium constant is K(oxo/hydroxy) = 0.01 at T = 500 K. eThe equilibrium constant K(oxo/hydroxy) = 0.5 in argon and nitrogen matrices at 15 K.16 The corresponding theoretical value is K = 0.37 at T = 500 K. 'No rare tautomeric forms have been firmly determined up to date; the theoretical values correspond to the 'frozen" highly unfavorable tautomers. 84-(Thiomethyl)-6-methyluracilfree energy difference (estimated standard deviation 1.2 kJ mol-'); the equilibrium constant K(oxo/hydroxy) = 0.55 f 0.3.72 Our theoretical value is K = 0.17 at T = 500 K. h T h e equilibrium constant K(oxo/hydroxy) about 1 in the inert matrices." Our theoretical value is K = 1.06 at T = 500 K. '6-Methylisocytosine exist in nitrogen matrix in the hydroxy tautomeric form; however, a small amount of the oxo form and N(2)-(monomethylamino)-6-methylisocytosine can not be excluded.33 Our theoretical equilibrium constant is K(oxo/hydroxy) = 0.1 5 at T = 500 K. I Free energy difference of 2-(thiomethyl)uracil (estimated standard deviation 0.6 kJ mol-I); the experimental equilibrium constant is K(oxo/hydroxy) = 1.5 f 0.4.72 Our theoretical value is K = 1.08 at T = 500 K. kThe higher order electron correlation effects correspond to the parent compound, 2-hydroxypyrimidine, except cytosine (R = NH2), where these effects were explicitly calculated.20 'The higher order electron correlation effects correspond to the parent compound, 4hydroxypyrimidine.
VI11 and visualized in Figures 3 and 4. It is seen that the theoretically predicted relative stability is mainly driven by the S C F contribution. The reason for such a behavior is a nearly constant contribution to the relative stability resulting from the electron correlation effects and from the nuclear zero-point vibrations, which almost cancel themselves. However, such a cancelation occurs only if the electron correlation energy includes the higher order corrections. It is worth mentioning that the sum of the second-order electron correlation term slid the nuclear zero-point vibration generally stabilize the hydroxy tautomer, while the higher order electron correlation terms favor the oxo tautomer. We may then conclude that a theoretical quantitative prediction of the relative tautomerization energy should be performed on the basis of the highest level calculations. The conclusion is substantiated by the size of the norm of the first-order correlation correction to the wave function, which for both 2- and 4hydroxypyrimidines exceeds 0.26 (see Tables VI and VII). This indicates that one may expect sizable renormalization contributions in higher orders, as well as a quite large magnitude of the direct MBPT-linked terms. The most important parts of these contributions are incorporated up to the infinite order in the coupled cluster calculations. Moderate sizes of the largest CC amplitudes of single and double excitations shown in Tables VI and VI1 justify the single-reference approach taken in these calculations. How Does a Ring Substituent Influence the Hydroxy-Oxo Tautomerism of 2- and 4-Hydroxypyrimidines? We found that (71) Lapinski, L.; Czerminski, R.; Nowak, M. J.; Fulara, J. Ab initio, CNDO/2 and matrix isolation studies of 2-hydroxypyrimidine infrared absorption spectra, to be published. (72) Rostkowska, H.;Szczepaniak, K.; Nowak, M.J.; Leszczynski, J.; Ku Bulat, K.;Person, W.B., submitted to J . Am. Chem. SOC.
the amino group NHz strongly influences the tautomerization for the unsubstituted hydroxypyrimidines. In 2-hydroxypyrimidine, the NHz substituent at C4 causes a definite shift toward the oxo form, a behavior completley opposite from that observed in 4hydroxypyrimidine, where amino group substitution at the C(2) position stabilizes the hydroxy tautomeric form. The sulfhvdryl (SH) and thiomethyl (SCH3) groups exhibit modest influence on tautomeric equilibrium, with the SH substituent acting stronger than SCH3. Both substituents stabilize the oxo form in 2hydroxypyrimidine and the hydroxy form in 4hydroxypyrimidine. The hydroxyl group (OH) substitution in the Cz and C4positions strongly favors the oxo forms. One can suggest a qualitative and simplified interpretation of the above results. The more electronegative (electron withdrawing) the substituent at C(4), the more positive the O(2) oxygen atom becomes. Consequently, O(2) more easily transfers the proton to N( I ) in the ring (oxo form). The effect of a substitution at C(2) on the 4-hydroxypyrimidine tautomerism is more difficult to explain. In this case both N(3) and O(4) atoms are involved in the protomeric tautomerism. It would be difficult to predict which of the atoms loses and which gains the electron charge and to determine the most stable position of the proton without performing quantum chemical calculations. Unlike the charge at N(3) and 0(4), the local electron density at N ( l ) in 2hydroxypyrimidine is likely to be undisturbed by a C(4) substituent. One notices that the N ( 1) atom is located outside the region where the major charge flow is expected to take place. Why Does Isomerization of Cytosine Not Occur? It is now well documented by both the matrix-isolation experiments and the theoretical calculations that cytosine and isocytosine occur in the gas phase in the hydroxy-amino forms. An interesting conclusion can be drawn from the comparison of the total energies
Protomeric Tautomerism of Pyrimidines TABLE I X IR Freqwncies (u, em-'), IR Intensities ( I , km mol-'), Raman Activities (A, A' amu-I), and Raman Depolarization Ratios ( d ) of Selected Normal Modes of 2-Hydroxypyrimidines Calculated with the SCF/I21C (or 3-21C*) Method (The IR Frequencies Are Uniformly b l e d by a Factor of 0.91) main contribution to the normal mode ring C6H C N oxo form NIH C5H 1776 1663 3130 3090 R = H Y 3452 125 3 748 I 106 1 20 4 104 68 A 82 0.38 0.15 0.18 0.46 d 0.30 R = NH2 u 3465 3121 3085 1773 1663 678 622 2 3 I 104 16 9 69 A 96 87 0.20 0.41 0.19 0.47 d 0.28 expt' Y 3471 1720 1656 R = SCH, Y 3458 3129 3091 1774 1649 750 405 I 128 2 3 21 8 75 88 A 105 0.32 0.41 0.22 d 0.30 0.19 exDtb Y 1720 main contribution to the normal mode hydroxy form OH C5H C6H ring R = H u 3552 3119 3081 1591 I 123 3 3 210 109 12 A 106 122 0.34 0.46 d 0.31 0.14 1591 3056 3016 expf u 3591 3015 1592 Y 3575 3056 exptd R =NH2 Y 3552 3109 3078 1602 480 5 15 I 119 16 98 81 A 111 0.47 0.74 d 0.32 0.17 expt' u 3591 1623 R = SCH, Y 3550 3115 3085 1585 15 387 I 138 2 21 89 103 A 130 0.42 0.74 d 0.32 0.17 'Argon matrix, I5 K.I6 bVapor phase.74 CArgon matrix, IO K.71
of these molecules, equal to -393.769 026 and -393.775 012 au for cytosine and isocytosine, respectively (SCF+MBPT(2)/ DZP/3-21G scaled ZPE/3-21G level). It appears that isocytosine has larger internal energy than cytosine, and therefore one may infer that the gas-phase isomerization process cytosine isocytosine
+
-
should be exothermic, with an enthalpy of about -15.7 kJ mol-'. It is than not clear why isocytosine is not synthesized during biochemical transformations (related to the genetic material) in amounts comparable to cytosine. An explanation of this fact may involve specific reaction mechanisms in the synthetic process that are not driven by the global change of the enthalpy. There exists another stabilizing factor related to the intermolecular interactions involving isocytosine that should be elucidated. It is well documented, both t h e ~ r e t i c a l l yand ~ ~ e ~ p e r i m e n t a l l y , ~that . ~ ' isocytosine can form complementary pairs with some other nucleic bases, especially with isoguanine. This implies that isocytosine will be stabilized by a set of hydrogen bonds. Thus, the reason isocytosine is not included in the genetic code still. remains obsc~re.~~ Vibrational Specrra. The vibrational spectra belong to the most important properties that can be used for identification of certain molecular fragments occurring in more complex biopolymers. In the present work we calculated the harmonic frequencies of the nuclear vibrations using the SCF/3-21G (or SCF/3-21G* for sulfur-containing molecules) force field. Although such an approach suffers from several drawbacks (e&, not accurate enough IR/Raman intensities, overestimated frequencies due to the unsaturated basis set, and lack of electron correlation effect^'^), one (73) Hes, B. A., Jr.; Schaad, L. J.; CBrsky, P.;Zahradniik, R. Chem. Reo. 1986, 86, 709.
The Journal of Physical Chemistry, Vol. 94, No. 18, 1990 7027 TABLE X: IR Frequencies (v, cm-I), IR Intensities ( I , km mol-'), Raman Activities ( A , A' am&), and Raman Depolarization Ratios ( d ) of Selected Normal Modes of 4-Hydroxypyrimidines Calculated with the SCF/3-21C (or 121G*) Method (The IR Frequencies Are Uniformly Scaled by a Factor of 0.91) main contribution to the normal mode C=O ring oxo form N,H C6H C5H 1753 1635 3125 3098 R = H Y 3443 600 119 I 94 2 8 A 79 111 72 24 1 d 0.32 0.17 0.39 0.35 0.19 3049 3031 1727 1611 expt' Y 3428 1615 3037 1739 3062 exptb Y 3438 1748 1612 3084 Y 3442 3130 R = NH2 18 597 300 I 47 1 ' A 30 101 90 28 1 0.34 0.60 0.21 0.44 d 0.70 R=SCH, Y 3431 3128 3095 1754 1610 13 663 75 I 88 2 33 7 A 45 124 81 0.31 0.24 0.20 0.52 d 0.34 exptc Y 3393 exDtd Y 3416 main contribution to the normal mode hydroxy form C6H ring OH C5H 3088 1597 3131 R = H Y 3535 I 122 0 15 207 85 4 A 113 105 0.48 0.60 d 0.31 0.21 3049 3031 1607 expt' Y 3563 1611 3062 3037 exptb Y 3589 3077 1599 Y 3538 3141 R =NH2 20 370 I 104 0 21 92 98 A 108 0.36 0.49 d 0.31 0.23 R=SCH3 Y 3532 3136 3089 1580 14 23 1 I 122 0 A 109 114 91 IO 0.49 0.23 0.39 d 0.32 exptc u 3550 exptd Y 3582 'Argon matrix, 5 K.75 bNeon matrix, 5 K.'I CN2matrix, 15 K.76 dVapor phase, 500 K.72+76
may use them in a qualitative analysis of the vibrational spectra. In Tables 1X and X we gathered data for certain characteristic modes of 2(4)-hydroxypyrimidines and compare them with the available experimental values.13J6~33~72 More details concerning the calculated geometries, rotational constants, force constant matrix, dipole, and polarizability derivatives are available from the authors upon request.77 From the comparison of the data presented in Tables IX and X, one sees that indeed some selected bands are not influenced by substituent and, therefore, can be used for the analytical identification of oxo-hydroxy tautomeric forms by IR/Raman spectroscopy. Discussion
Do the Matrix-Isolation IR Spectra Correspond to the Gas Phase? In our theoretical approach we cannot think of any obvious improvements that could dramatically change the results and possibly bring the theory closer to the experimental estimations of the relative populations of various tautomeric forms. However, there are still several limitations and approximations in our calculations (e.g., incompleteness of the basis set, planar geometries assumed, approximate solution of the Schriidinger equation, the (74) Szczepaniak, K.;Barski, A.; Baranska, H.; Shugar, D. C.R.- Conf. Inr. Specrrosc. Raman, 7rh, 1980, Ottawa; Murphy, W. F., Ed.; NorthHolland: Amsterdam, 1980; pp 590-591. (75) Nowak, M. J.; Fulara, J.; Lapinski, L. J . Mol. Srrucr. 1988, 175, 91. (76) Rostowska, H.; Barski, A.; Szczepaniak, K.; Szczesniak, M.; Person, W. B. J. Mol. Srruct. 1988, 176, 137. (77) BITNET Address:
[email protected] or
[email protected]. edu.
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The Journal of Physical Chemistry, Vol. 94, No. 18. 1990
Born-Oppenheimer approximation) that still impairs the results somewhat. On the other hand, one may also raise some questions about the interpretation of the results of the matrix-isolation experiment. One may, for example, suggest a modification of the experimental conditions in a way that ensures that the gas-phase equilibrium has indeed been achieved for the sublimated compound before its matrix deposition. On the basis of the results obtained in the present work, we would like to comment on the very recent64 estimation of the relative total energy of S-methylated 2-thiouracil tautomers. In the quoted paper, the semiempirical AMI, ab initio SCF/321G*//3-2 I G*, and SCF/6-3 I G**//3-21 G* (probably the SCF/6-31G*//3-21G*; see below) methods were used. The authors argued that their prediction (favoring the oxo tautomeric form by 12.3, 16.6, and 6.9 kJ mol-], respectively) agrees well with the 1R spectral data76from which a slight dominance of the oxo form can be deduced. In the IR spectra one can also identify certain absorption peaks corresponding to the hydroxy form, and by comparison of the relative intensities of respective peaks one gets the equilibrium constant of the two forms K(oxo:hydroxy) of 1.1-1.5.72 This result, however, is not consistent with the theoretically estimated large energy difference of at least 7 kJ mol-' between the oxo and hydroxy tautomers. In the course of our studies on 2 - t h i o ~ r a c i 1we , ~ ~performed the geometry optimization with the SCF/3-21G* method and obtained structures essentially identical with those presented by Katritzky et aI.@ For S-methylated 2-thiouracil, however, the total SCF/3-21 G * / / 3 21G* and SCF/DZP//3-21G* energies were substantially different from those reported by Katritzky et al., as explained in Table IV. To elucidate this problem, we performed additional calculations of the tautomers total energy with the SCF method using 6-3lG" and 6-31G** basis sets. The results revealed that Katritzky et al. used, in fact, the SCF/6-31G*//3-21G* method, Le., they did not include the hydrogen p-type functions in the basis set. If these essential functions were included in the S C F calculations, the predicted relative energy would be reduced down to 1.3 kJ mol-'. The data presented in Table IV enabled an analysis of how the relative energy changes with the improvement of the theoretical approach. The extension of the basis set from 6-31G** to DZP shifts the relative energy up to 3.5 kJ mol-], and then the zero-point nuclear vibrations shift it down by -2.7 kJ mol-'; also the second-order electron correlation effect is negative, -4.5 kJ mol-], and finally the higher order electron correlation effects (extrapolated from 4-hydroxypyrimidine) shift the value up by 3.9 kJ mol-'. After all the contributions to the relative total energy are added, we obtain 0.2 kJ mol-' in favor of the oxo tautomeric form, i.e., to the value that is considerably different from that predicted from the S C F calculation alone. The equilibrium constant K(oxo:hydroxy) calculated from the exponential formula K = exp(0.25/kT), k = 0.008 31 kJ mol-' and T = 500 K, now becomes 1.06, which is close to the experimental estimate: I .2 f 0.1 in N2 matrix,76 1.1 f 0.1 in vapor phase,76and 1.5 f 0.4 in a mixed experimental-theoretical e ~ t i m a t i o n .This ~ ~ example shows that the prediction of the relative tautomerization energy in the molecules related to 2(4)-hydroxypyrimidine may be completely unrealistic when one stops the calculation at the S C F level. Can Isocytosine Tautomerism Help Understand the Guanine Tautomerism? One notices that the isocytosine moiety occurs in the guanine molecule. Formally, the guanine ring can be regarded as a composition of two fused rings, i.e., the rings of isocytosine and imidazole. There are several indications that guanine may appear in the hydroxy tautomeric form in nonpolar ~ n e d i a . ~Although ~ . ~ ~ this tautomer has not been detected in the biological environment, its 0(6)-methylated derivative is a well-known compound exhibiting a strong mutagenic effect.80 It (78) Szczepaniak, K.; Szczesniak, M.; Person, W. B. Chem. Phys. Lett. 1988.153. 39. (79) Latajka, Z.; Person, W. B.: Morokuma, K . J . Mol. Strucf. (THEOC H E W 1986,135, 253. (80) Chemical Carcinogens:Searle. C. E.. Ed.;America1 Chemical Society: Washington, D.C.,1984.
Lei and Adamowicz
hydroxy-amino
oxo-amino (NIH)
oxo-amino (N3H)
oxo-imino
Figure 5. SCF/3-21G* optimized geometries of four tautomeric forms of isocytosine (2-amino-4-hydroxypyrimidine): hydroxy-amino (A), oxo-amino ( N , H ) (B), oxo-amino (N,H) (C), and oxo-imino (D).
was also found that the O(6) position can be efficiently protected (from the undesired side reactions) by the (buty1thio)carbonyl ~ ~ O U ~ C O S ( C H ~ ) The C H lack ~ . ~ of ' the hydroxy tautomeric form of guanine in the polar environment can be explained by a strong stabilization of the oxo form due to its relatively high dipole moment (5.2 DS2)as compared to the low dipole moment of the hydroxy form (1.7 DS2). The guanine tautomerism can be studied by investigating the influence of the imidazole ring on the tautomerism for the isocytosine moiety. A very recent ab initio study of Gould and Hillier2' suggests that guanine should appear in the gas phase as a nearly equimolar mixture of hydroxy and oxo tautomeric forms, in agreement with the conclusion derived from the matrix-isolation experiment.78 Thus, taking into account strong stability of the hydroxy tautomeric form of isocytosine that emerges both from our calculations and from matrix-isolation experiment^,)^ it appears that the imidazole ring fused to the isocytosine ring at the C(5) and C(6) positions considerably stabilizes the oxo tautomeric form. Such a qualitative conclusion should be, however, verified more carefully, because one must consider the following observation. Upon comparing our results for isocytosine with Hillier's results for guanine, one notices that all the components of the relative energy of guanine tautomers have a sign opposite those of isocytosine. At present, it is not clear if such a behavior is crucially geometry dependent. Gould and Hillier calculated the electronic energy at the SCF+MBPT(2) level with the 6-31G** basis set and with the geometry optimized at the SCF/6-31G** level. It is well-known, however, that the near-limit Hartree-Fock geometries deviate considerably from the experimental gas-phase values,s3 and it is then necessary to perform a supplementary beyond-Hartree-Fock optimization of the geometry to obtain a satisfactory agreement with the experimental structures. On the other hand, the 3-21G geometries usually agree quite well with experiment due to fortuitous cancellation of errors. The reason we think the geometry could (81) Yamakage, S.;Fujii, M.; Harinouchi, Y.; Takaku, H. Nucleic Acids Symp. 1988, Ser. 19, 9.
(82) Sygula, A.; Buda, A. J . Mol. Struct. (THEOCHEM)1983,92,267. (83) Handy, N.C. In Supercomputer Algorithms for Reactivity, Dynamics and Kinetics of Small Molecules; Ladana, A,, Ed.; Kluwer Academic Publishers: Dordrecht, 1989; pp 23-36
Protomeric Tautomerism of Pyrimidines
The Journal of Physical Chemistry, Vol. 94, No. 18, 1990 1029
TABLE XI: Molecular Dipole Moments Calculated with the SCF/121C Method of the Oxo and Hydroxy Tautomeric Forms of 2(4)-Hvdroxv~vrimidines(in debves)"
R SH
SCH3
oxo hydroxy hydration effect
H NH2 OH 2-H ydroxypyrimidines 6.6 7.1 5.1 3.4 3.7 1.5 26-38 29-44 19-28
5.6 2.3 20-3 1
4.8 1.5 16-25
oxo hydroxy hydration effect
4-Hydroxypyrimidine 2.5 4.6 3.4 1.0 1.5 1.5 4-6 15-22 7-11
2.9 1.4 5-8
3.7 1.4 9-14
"The hydration effect (he.) estimated from the formula he. = -kk = 0.81-1.2 kJ mol-'.
(phydrox;-pox:),
potentially influence the estimation of the relative tautomerization energy emerges from the Gould and Hillier's calculation of the ZPE contribution in cytosine. They obtained the value 0.9 kJ mol-' (SCF/6-31G**) in favor of the oxo form, while our estimation is -2.5 kJ mol-' (based on a SCF/3-21G calculation) in favor of the hydroxy form. The ZPE energy can be estimated directly from the experimental IR frequencies of cytosine reported by Szczesniak et a1.I6 The value one gets from such an estimation, equal to -1.5
kJ mol-', agrees better with our result than with that of Hillier's. Tautomers in the Mutation Theory. The theoretically predicted "rare" tautomeric forms (in weakly polar solvents, hydroxy forms for all 2(4)-hydroxypyrimidines; in polar solvents, oxo forms with the protonated ring nitrogen N( 3) atom for 2-hydroxypyrimidines and with the protonated N( 1) atom for 4-hydroxypyrimidines) may contribute to the formation of mismatches that can subsequently lead to the point mutations. We should, however, remember that a single mismatch does not necessarily lead to the macroscopically observed mutation, cf. ref 41. At least two arguments are worth mentioning. First, the existence of several repair mechanisms may considerably reduce the level of the spontaneous or induced lesions due to mismatch Second, there exists a redundancy of the genetic code that allows for alternative codings of an amino acid by different triplets of nucleosides. How Does Environment Influence the Tautomeric Equilibria? Probably one of the most crucial factors determining the tautomer distribution in the biological material is the environment (solvent). Some of the nucleotides, such as those related to cytosine, guanine and their is0 congeners, are particularly sensitive to the environmental effects; others, for example, uracil, adenine, and xanthine nucleotides, are less so. This phenomenon has been investigated in a series of model compounds related to nucleic The results clearly show
TABLE XII: Geometry' (Bond Lengths (A), Bond Angles (deg)), Rotational Contants (MHz), and Dipole Moments (D) of the Oxo-Amino (H at N3),Oxo-Amino (H at N f ) , Oxo-Imino, and Hydroxy-Amino Tautomeric Forms of Isocytosine Obtained with the SCF/3-21G Method tautomer cryst datab oxo oxo oxo oxo oxo( (N1H) (N,H) imino hydroxy N,H NiH NnH Bond Lengths 1.297 1.373 1.330 1.357 1.387 1.332 1.361 1.280 1.386 1.340 1.360 1.369 1.333 1.33 1 1.409 1.394 1.396 1.313 1.375 1.363 1.360 1.478 1.441 1.422 1.438 1.454 1.390 1.440 1.324 1.329 1.373 1.342 1.356 1.331 1.351 1.387 1.371 1.350 1.358 1.366 1.334 1.352 1.324 1.323 1.257 1.342 1.346 1.327 1.213 1.214 1.347 1.216 1.246 1.248 1.256 0.996 0.997 0.96 1 .oo 1.000 1.ooo 0.95 0.967 1.066 1.067 0.92 0.92 1.065 1.065 1.070 1.069 1.070 1.070 0.98 1.oo 0.996 0.994 1.02 0.995 0.89 0.98 0.994 0.996 1.007 0.994 0.89 0.95 0.95 122.2 123.9 1 12.4 119.8 124.6 117.1 120.0 119.3 121.2
123.2 122.1 115.4 120.1 120.6 118.6 116.4 122.4 121.3
117.7 120.8 117.3 123.3 I 19.4
118.2 123.7 123.8 116.7 119.5
3958 2015 1335
3902 2026 1333
4.6
9.0
Bond Angles 112.9 127.6 114.0 119.5 122.6 123.2 119.0 120.1 117.8 11 5.0
123.9 118.0 122.7 115.2 123.2 117.0 118.3 117.8
110.6 118.1 121.2 122.0 120.6 119.3 116.6 119.8 120.9 Rotational Constants 3915 3958 2029 2029 1341 1336
121.9 123.3 114.8 118.2 125.9 115.9 120.3 118.8 1 I9 120 117 1 I6 1 I9
124
121.8 119.7 118.7 119.1 120.5 120.2 118.9 119.3
123.0 118.5 119.8 119.5 118.8
125
120
121 126 121 1 I9 120
123 113 124
117.3 118.5
Dipole Momentd 1.5 3.1
"See Figure 5. bReference 30: estimated standard deviations for the bond distances and angles involving heavy atoms are about 0.003 A and 0.1"; for those involving hydrogen atoms, about 0.05 A and 1.0". C6-Methylcytosine crystal^.'^ dThe SCF/DZP dipole moments (debyes): 4.8 (oxoamino, H at N3); 3.3 (oxo-imino): 1.4 (hydroxy-imino).
7030 The Journal of Physical Chemistry, Vol. 94, No. 18, 1990
Lei and Adamowicz
TABLE XIII: Geometrya (Bond Lengths (A), Bond Angles (deg)), Rotational Constants (MHz), and Dipole Moments (D) of Oxo and Hydroxy Tautomeric Forms of S-Methvlated 4-Thiouracil (R = SCH,). the Thiol Form of 2-Thiouracil (R = SH). and 2-Hvdroxv~vrimidine'(R = H)
tautomer R = SCH]
OXO(N t H) R = SH 1.404 1.389 1.283 1.436 1.339 1.358 1.758 1.324'j
hydroxy
R=H Bond Lengths 1.404 1.394 1.284 1.431 1.340 1.359 1.072'
1.403 1.388 1.285 1.439 1.338 1.360 1.751 1.810 1.08 I 1.078 1.208 0.999
1.207 0.999
1.208 1.000
CS-H C6-H
1.066 1.070
1.066 1.070
1.066 1.070
Nl-CZ-Nj Cz-N3-C4 NJ-Cd-Cs
114.8 121.9 123.4 116.0 120.5 120.3 100.6 106.7 110.2 110.4 109.0 119.7 1 1 5.7
114.7 121.7 123.8 115.8 120.5 120.3 95.0d
119.1 115.6
116.8' 1 1 5.4
121.4 122.9
121.6 122.8
121.6 123.5
NrC2 C,-N, NrC4
c4-cs c5-c6
N
c4-s
S-C(Me) C( Me)-Hb C( Me)-HC
c2-0 N ,-H 0- H
c4-cs-c6
C5-C6-N I N,-C2-0 C,-S-C( Me) S-C( Me)-Hb S-C( Me)-HC H"-C( Me)-HE Hb-C( Me)-H' N3-C4-S CZ-NI-H C2-0-H C4-C5-H C5 4 6 - H
Bond Angles 114.8 120.9 124.4 116.0 120.0 120.3
R = SCH]
R = SH
R=H
1.318 1.325 1.323 1.397 1.371 1.337 1.752 1.812 1.081 1.078 1.341
1.321 1.323 1.324 1.392 1.375 1.334 1.756 1 .324'j
1.324 1.322 1.327 1.385 1.380 1.331 1 .O7Oc
1.340
1.342
0.966 1.068 1.069
0.967 1.067 1.069
0.966 1.068 1.070
125.2 117.9 121.3 116.3 122.1 118.2 101.0 106.4 110.4 110.2 109.I 119.0
125.1 117.8 121.6 116.2 122.1 118.2 94.9d
125.0 117.2 122.2 116.2 121.6 117.9
118.1
116.6
111.7 121.5 121.3
110.9 121.8 121.2
110.8 121.9 121.7
2.L
3.4k
Rotational Constants A
B C
2663 1056 759
2664 1053 758
Dipole Moment 4.88
5.6h
6.6'
1.5
'See Figure 6. bln-plane. cOut-of-plane. dS-H bond in place of %CHI bond. C Hatom bound to C,-ring carbon atom. /N,-C,-H angle. #Some structural data of the oxo (H at N,) form: rotational constants 2698, 1026, 747 MHz; dipole moment 6.4 D (SCF/3-21G*). *S.6 D (SCF/6-31G**). '6.7 D (SCF/DZP). '1.9 D (SCF/6-31G**). k3.0 D (SCF/DZP). 'SCF/3-21G calculations for 2-hydroxypyrimidine (R = H). that by varying the solvent polarity, one can dramatically shift the position of the tautomeric equilibrium. Katritzky et al.1346 have shown that the relative concentration of tautomers in different phases can be related to the proton donor/acceptor properties. In particular, they argued that the tautomer with the lowest proton affinity will predominate in the gas phase. The present state-of-the-art theoretical and computational methods do not seem to be sufficiently developed to predict quantitatively the environment's effect on the tautomeric phenomenon for a general case,84 although some efforts were undertaken@using a combined quantum mechanical and statistical mechanics approach. The solvent effect can be estimated, in a crude way, by means of the Kirkwood-Onsager theory.8s In this theory, the interaction between solute and solvent is represented by an electrostatic term, e , that is proportional to the square of the dipole moment of the solute molecule: e = -kp2
The constant k is related to the molecular polarizabilities as well (84)Person, W. B.; Szczepaniak, K.; Szczesniak, M.; Kwiatkowski, J. S.; Hernandez, L.; Czermtnski, R. J . Mol. Struct. 1989, 194, 239. (85) Kirkwood, J. G.J . Chem. Phys. 1934. 2, 351. Westheimer, F. H.; Kirkwood, J. G. J . Chem. Phys. 1938,6, 513. Onsager, L. J . Am. Chem. Soc. 1936, 58, 1486.
as to the solvent dielectric constant and to the cavity radius. One may treat k as an adjustable parameter, k = 0.8-1.2 kJ mol-' D-2,86and estimate the solvent effect by simply scaling the difference between squared dipole moments of the species involved in the tautomeric rearrangement. Taking into account the dipole moment values gathered in Table XI, one may expect that in aqueous solutions the equilibria will be strongly shifted toward the oxo form. The cumulative relative stability (gas phase plus the solvent effect) indicates the predominance of the oxo tautomeric form. The solvent stabilization effect of a less polar environment can be estimated by reducing the values of k . The general conclusion that can be drawn from this approximate analysis is that variations of the environment polarity may shift the tautomeric equilibrium from the predominance of the hydroxy tautomer (nonpolar, k = 0) to the predominance of the oxo tautomer (polar, k about 1). Our conclusions with respect to the environment stabilizations of particular tautomeric forms may have some practical implications. Recently, Undheim's group8' performed an alkylation of 2-pyrimidones. The reaction yielded two isomers, 0-alkylated (formal derivatives of the hydroxy tautomeric form) and N,-al(86) Lei, A.; Kukawska-Tarnawska, B. THEOCHEM 1986, 33, 45. (87) Benneche, T.;Strande, P.; Undheim, K.Acra Chem. Scand. 1987, 841. 448.
Protomeric Tautomerism of Pyrimidines
The Journal of Physical Chemistry, Vol. 94, No. 18, 1990 7031
TABLE XIV: Geometry' (Bond Lengths (A), Bond Angles (deg)), Rotational Constants (MHz), and Dipole Moments (D) of the Oxo and Hydroxy Tautomeric Forms of S-Methylated 2-Thiouracil (R = SCH,), Thiol Form of 2-Thiouracil (R = SH), and 4-Hydroxypyrimidine (R = H): SCF/3-21C* Calculationsm
tautomer hydroxy
OXO(N 1H)
R = SCH,
R = SH
1.282 1.361 1.403 1.450 1.336 1.388 1.754 1.811 1.081 1.077 1.214 1.002
1.280 1.360 1.404 1.450 1.335 1.389 1.759 1 .323d
1.278 1.60 1.398 1.452 1.336 1.392 1.069?
1.214 1.002
1.067 1.069
1.067 1.069
123.7 123.8 112.3 120.2 123.6 121.8 99.9 106.5 110.1 110.4 109.4 120.4 120.7
123.0 123.6 112.2 120.3 123.5 121.2 94.3d
123.5 123.6 11 2.0 120.6 123.4 120.2c
120.3 121.0
120.8 120.4
117.3 121.7
117.3 121.8
117.1 121.m
R=H Bond Lengths
R = SCH,
R = SH
R = H
1.319 1.334 1.314 1.392 1.369 1.344 1.752 1.324d
1.319 1.334 1.315 1.391 1.370 1.344 1 .067c
1.215
1.319 1.338 1.311 1.394 1.368 1.345 1.748 1.810 1.082 1.078 1.345
1.344
1.347
1.067 1.069
0.968 1.066 1.069
0.968 1.066 1.069
0.968 1.066 1.069
124.1 118.4 122.0 1 1 5.8 122.5 120.8 100.7 106.5 110.6 109.9 109.1 118.5
124.4 118.2 121.9 115.9 122.5 120.0 95.2d
124.6 118.1 121.8 116.1 122.3 1 l8.2f
118.4
118.6
111.0 120.9 121.2
111.1 120.9 121.2
110.9 120.7 121.1
1.4k
1.O'
1.oo 1
Bond Angles
Rotational Constants 3298 936 732
3277 944 736
3.78
2.9'
Dipole Moment 2.5'
1.4*
"See Figure 7. bln-plane. 'Out-of-plane. dS-H bond in place of S-CH, bond. c Hatom bound to C{,)-ring carbon atom. f N , C z - H angle. 84.0 D (SCF/6-31G*), 4.1 D (SCF/6-31G**).
' 1.1 D (SCF/DZP).
(SCF/6-31G**).
*
1.4 D (SCF/6-31G*), 1.4 D (SCF/6-31G**). '3.1 D (SCF/6-31GS*). '2.8 D (SCF/DZP). '"SCF/3-2IG calculations for 4-hydroxypyrimidine(R = H).
p.
s7r CIZ
hydroxy
OXO
(NIH)
Figure 6. The SCF/3-21G* optimized geometries of two tautomeric forms of S-methylated 4-thiouracil: hydroxy (A) and N,H-oxo (B).
kylated (formal derivatives of the oxo tautomeric form) compounds. The separation of these compounds was achieved in a mixture of water with an organic solvent. The 0-alkylated derivative then appeared in the organic phase, while the N-alkylated isomer appeared in the aqueous phase. Such a result can be
1.5 D
understood on the basis of our theoretical calculations that predict that a weakly polar solvent should stabilize the hydroxy form (0-alkylated), while the polar aqueous solvent should stabilize the oxo form (N-alkylated).
Conclusions A reasonable estimation of the tautomeric equilibrium in the gas phase or in the weakly polar environment mandates goodquality SCF and post-SCF ab initio calculations. We have shown that the commonly applied MBPT(2) approach for the evaluation of the electron correlation effects should be supplemented by an estimation of the higher order electron correlation effects. In all cases studied we achieved a qualitative agreement with the conclusions based on the recent low-temperature matrix-isolation IR spectra. We also noticed a small systematic difference between experimental and theoretical predictions of the gas-phase relative concentration of the hydroxy and oxo tautomeric forms. We suggested a possible modification of the experimental conditions to clarify the source of the observed discrepancy. We lend support to the experimentally derived conclusiosn that the 2-hydroxypyrimidine should exist in the gas phase (or in the weakly polar environment) almost exclusively in the hydroxy tautomeric form. A coexistence of the hydroxy and oxo forms, with a clear predominance of the hydroxy form, should charac-
J . Phys. Chem. 1990, 94, 1032-1037
7032
hydroxy
N1 H-oXO
N~H-oxo
Figure 7. SCF/3-21 G* optimized geometries of three tautomeric forms of S-methylated 2-thiouracil: hydroxy (A), N,H-oxo (B), and N3H-oxo (C).
terize the cytosine, isocytosine, and S-methylated 4-thiouracil vapor. A nearly equimolar hydroxy:oxo mixture should exist in the gas-phase 4-hydroxypyrimidine and S-methylated 2-thiouracil. In the polar environment, however, the hydroxy-oxo tautomeric equilibrium should be strongly shifted toward the oxo form. We do not expect the hydroxy tautomeric form to be detectable in the polar (e.g., aqueous) solutions. Another tautomeric rearrangement corresponding to the proton transfer between the endocylic nitrogen atoms (N( 1)-H to N(3)-H and vice versa) appears to be highly unfavorable in the gas phase. On the other hand, such a process can easily occur in the polar solvents due to an additional strong (mostly electrostatic) solvent stabilization of the energetically unfavored tautomeric form. We may expect that by means of the variation of the environment polarity one may enforce the presence (and the pre-
dominance) of a particular tautomeric form. This, in consequence, may influence the rate of the chemical exchange of the mobile proton on the electrophilic reagent (e.g., the methyl group). Acknowledgment. This study was supported by an institutional grant from the National Cancer Institute and by a Biomedical Research Support grant provided by The University of Arizona. We are greatly indebted to Drs. H. Rostkowska, K. Szczepaniak, M. J. Nowak, J. Leszczynski, K. KuBulat, and W. B. Person for sending us their paper (ref 72) prior to its publication.
Appendix The optimized SCF/3-21G* geometries for isocytosine and 2and 4-(thiomethyl)uracil are found in Figures 5-7 and Tables XII-XIV.
Comparative SO2 Infrared Spectra: Type I and I 1 Clathrate Hydrate Films, Large Gas-Phase Clusters, and Anhydrous Crystalline Films Fouad Fleyfel, Hugh H. Richardson,+ and J. Paul Devlin* Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078 (Received: April 6, 1990)
The mechanism by which SO2is incorporated into microparticles of ice in the vapor phase is receiving special interest because of the unexpectedly high efficiency with which SO2 is scavenged by ice crystals. A possible explanation of this efficiency might be found in the tendency for small polar molecules, such as the small ring ethers, to form clathrate hydrates at low temperatures and low partial pressures. This possibility has been examined by spectroscopic studies at 120 K of large gas-phase clusters formed from anhydrous SO2and H20-S02 mixtures with a ratio appropriate for clathrate hydrate formation. On the basis of a comparison with new thin-film infrared data for the simple type I SO2hydrate, the mixed SO,-ethylene oxide type 1 hydrate and the type I1 double hydrate with tetrahydrofuran, it is apparent that SO2is not enclathrated under the conditions that are known to cause formation of clathrate hydrate crystalline clusters of ethylene oxide, trimethylene oxide, and tetrahydrofuran. This indication, that the SO2clathrate hydrate, although stable once formed, grows relatively slowly, reduces the likelihood that SO2enclathration is the basis for its large uptake in ice crystallites. The nature of the anhydrous-SO, cluster spectra, together with existing data for anhydrous-C02clusters, prompted an examination of large cluster spectra from a macroscopic dielectric approach. A remarkable similarity of cluster spectra with spectra of thin films, at off-normal incidence, has revealed a close relationship between the cluster and thin film spectra.
-
I. Introduction Sulfur dioxide is an important atmospheric species of natural and anthropogenic origin that is generally believed to be implicated in the formation of acid rain. As such, the form and mechanism by which SO2is incorporated into icy crystals and water droplets is of particular interest. A recent study has revealed that a surprisingly high concentration of SO2is captured and deposited 'Present address: Department of Chemistry, Ohio University, Athens, OH.
in ice particles formed within cold chambers containing a few parts per million of SO2.' Since the scavenging of SO2exceeded that anticipated, in view of the tendency of solutes to be excluded from ice grown in solutions, a model was offered on the basis of the enrichment of SO2in an aqueous surface film of the growing ice particles. The existence of such a liquid film a t the surface of ( 1 ) Valdez, M. P.; Dawson, G . A.; Bales, R. C. J . Geophys. Res. 1987.92,
9789.
0022-3654 l9012094-7032%02.50/0 0 1990 American Chemical Societv
