Biopolym. Cell. 1994; 10(6):52-60.
Prototropic molecular-zwitterion tautomerism of xanthine: AMI calculation
1Kondratyuk I. V., 1Govorun D. M., 1Zheltovsky N. V.
  1. Institute of Molecular Biology and Genetics, NAS of Ukraine
    150, Akademika Zabolotnoho Str., Kyiv, Ukraine, 03680

Abstract

The prototropic molecular-zwitierion tautomerism of xanthine (Xan) was investigated by means of semiempirical quantum mechanical AMI method with full parameters optimization in vacuum. It was established that molecular-Zvitterion family of Xan occupies the energetical range about 82 ccal/mol and it consists of 32 structural isomers – 24 molecular and 8 zwitlerion; most of them are planar dipole-stable structures. The diketo-configuration N7H is the main and practically the only tautomeric form of Xan in gase phase; there is zero concentration of the main zwitterion (with protons localization at Nl, N7 and N9) at the same conditions. Transition to solvent with universal solvatation mechanism does not violate the character of equilibrium, that is evidenced from guantitative estimation in the frame of Onzager's classical model. We construct rows of donor and acceptor properties for two energetically prevalent molecular tautomers of N7H(I) ans N9H(II) and the main tautomer-zwitterion (Z) based on their calculated fundamental vibrational frequencies: N7H(Z) >N9H(Z);>N1H(Z); N9H(II) >N7H(I) >N3H(II)>N3H(I) > N1H(I) > NIH(II); C60(Z) >C20(Z); C60(I) >C60(II)> >C2O(I)>C2O(II); C8H(I)>C8H(II). The ilid mechanism of hydrogen-deuterium-tritium exchange of C8H group of Xan in water at the acidic and neutral pH is confirmed from the analysis of calculated in vacuum physico-chemical properties (the heat of formation, proton affinity and creative energy) of the main Xan's ilid form (with iminoprotons localization at Nl, N3, N7, N9 and without proton at C8).

References

[1] Danilov VI, Kventsel GF. Electronic submission to the theory of point mutations. Kiev: Naukova Dumka, 1971; 83 p.
[2] Poltev VI, Bruskov VI, Shuliupina NV, Rein R, Shibata M, Ornstein R, Miller J. Genotoxic modification of nucleic acid bases and biological consequences of it. Review and prospects of experimental and computational investigations. Mol Biol (Mosk). 1993;27(4):734-57.
[3] Zheltovsky NV, Samiylenko SP, Kolomiets IN, Kondratyuk IV, Stepanyugin AV. The investigation of interactions of hypoxanthine, xanthine and their methyl and glycosyl derivatives with amino acid carboxylic group by spectroscopic methods. Biopolym Cell. 1993; 9(3):17-22.
[4] Sukhorukov BI, Poltev VI. Theoretical analysis of tautometric conversions in various components of nucleic acids and their role in mutagenesis. Biofizika. 1964;9:148-59.
[5] Kondratyuk IV, Govorun DM, Zheltovskyy MV. Prototropic tautomerism of molecular xanthine. Dopovidi Nats Akad Nauk Ukrainy. 1995; (4):109-12.
[6] Burstein KYa, Shorygin P. Quantum-chemical calculations in organic chemistry and molecular spectroscopy. M. Nauka, 1989. 104 p.
[7] Norinder U. A theoretical reinvestigation of the nucleic bases adenine, guanine, cytosine, thymine and uracil using AM1. J Mol Struct: THEOCHEM. 1987; 151(C):259-69.
[8] Sabio M, Topiol S Lumma Jr WC. An investigation of tautomerism in adenine and guanine. J Phys Chem. 1990; 94 (4):1366-1372.
[9] Govorun DN, Danchuk VD, Mishchuk YR, Kondratyuk IV, Radomsky NF, Zheltovsky NV. AM1 calculation of the nucleic acid bases structure and vibrational spectra. J Mol Struct. 1992;267:99–103.
[10] Govorun DN, Danchuk VD, Mishchuk YaR, Kondratyuk IV, Radomsky NF, Zheltovsky NV. Mirror symmetrical conformational states of canonical nucleic acid bases. Doklady Akad Nauk Ukrainy. 1992; (2):66-9.
[11] Rambidi NG. Molecular structure in terms of experimenter-evolution of concepts and problems. Zh Strukt Khim. 1982. 232(6):113-33.
[12] Urazovsky S. Molecular polymorphism. Kiev: Academy of Sciences of the USSR, 1956; 336 p.
[13] Samoshin VV, Zefirov NS. Conformational transformations of organic molecules in solution. Zh Vsesoyuz khim obshchestva im. D. I. Mendeleyeva. 1984; 29(5):521-30.
[14] Morozov YuV, Vazhulina NP. Electronic structure, spectroscopy and reactivity of molecules: nucleobases, vitamins B6 and their analogs. Moscow: Nauka, 1989. 288 p.
[15] Bilobrov VM. Hydrogen bond. Intermolecular interactions Kiev. Nauk. dumka, 1993 520 p.
[16] Minkin VI, Osipov OA, Zhdanov YuA. Dipole moments in organic chemistry. M.: Khimiya, 1968. 245 p.
[17] Zefirov NS, Trach SS. Rearrangement and cyclization. XV. Tautomerism: common problems, classification, search for new topological and reaction types. Zh Org Khim. 1976; 12(4):697-718.
[18] Maslova RN, Lesnik EA, Varshavskiĭ IaM. Kinetics and mechanism of the 3H to 1H in C(8)H groups of purine derivatives. Mol Biol (Mosk). 1975;9(2):310-20.
[19] Agranovich IM. Conformational features of DNA in solution and in biological objects, revealed by the slow H1-H3 - exchange: Author. dis. ... kand. khim. nauk, Moscow: USSR Academy of Sciences Institute of Molecular Biology, 1985 21 p.
[20] Govorun DM, Kondratyuk IV, Zheltovsky NV. Acidic-basic properties of molecular xanthine and its complex formation ability. Biopolym Cell. 1994; 10(6):61-4.