A theoretical study of tautomerism of 2,6-dithioxanthine in the gas and aqueous phases using AM1 and PM3 methods

ÜNAL CİVCİR P.

JOURNAL OF MOLECULAR STRUCTURE-THEOCHEM, cilt.572, ss.5-13, 2001 (SCI-Expanded) identifier identifier

  • Yayın Türü: Makale / Tam Makale
  • Cilt numarası: 572
  • Basım Tarihi: 2001
  • Doi Numarası: 10.1016/s0166-1280(01)00572-3
  • Dergi Adı: JOURNAL OF MOLECULAR STRUCTURE-THEOCHEM
  • Derginin Tarandığı İndeksler: Science Citation Index Expanded (SCI-EXPANDED), Scopus
  • Sayfa Sayıları: ss.5-13
  • Anahtar Kelimeler: 2,6-dithioxanthine, tautomerism, tautomeric equilibrium constant, dipole moment, ionisation potential, AM1, PM3 semiempirical calculation, NUCLEIC-ACID BASES, SEMIEMPIRICAL METHODS, METHYL-DERIVATIVES, ESCHERICHIA COLI, THIO ANALOGS, AB-INITIO, HYPOXANTHINE, XANTHINE, 2-THIOURACIL, PURINES
  • Ankara Üniversitesi Adresli: Evet

Özet

Heats of formation, entropies, Gibbs free energies, relative tautomerisation energies, tautomeric equilibrium constants, dipole moments and ionisation potentials for the 14 possible tautomers of 2,6-dithioxanthine have been studied using semi-empirical AM1 and PM3 quantum-chemical calculations at the SCF level, both in the gas and aqueous phases, with full geometry optimisation. The COSMO solvation model was employed for aqueous solution calculations. The calculations show that 2,6-dithioxanthine exists as the dithiol forms, but DTX (9, 10,11) with N9H of imidazolic ring is more stable than the dithiol-N7H tautomer, DTX (7, 10,11) in the gas phase. The predominant tautomer in the aqueous solution exists in an dithione form with the imidazole proton on N9, that is DTX(1,3,9). The results are in good agreement with LTV experimental results. The entropy effect on the Gibbs free energy of the 2,6-dithioxanthine base is very small and practically has no significance for the tautomeric equilibria of the base. The enthalpic term is dominant in the determination of the equilibrium constant. (C) 2001 Elsevier Science B.V. All rights reserved.