AM1 and PM3 studies of some thio analogues of pyrimidine bases in the gas and aqueous phases

ÜNAL CİVCİR P.

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, cilt.14, sa.3, ss.171-179, 2001 (SCI-Expanded) identifier identifier

  • Yayın Türü: Makale / Tam Makale
  • Cilt numarası: 14 Sayı: 3
  • Basım Tarihi: 2001
  • Doi Numarası: 10.1002/poc.349
  • Dergi Adı: JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
  • Derginin Tarandığı İndeksler: Science Citation Index Expanded (SCI-EXPANDED), Scopus
  • Sayfa Sayıları: ss.171-179
  • Anahtar Kelimeler: 2-thiocytosine, 2,4-dithiouracil, 2,4-dithiothymine, tautomerism, dipole moment, ionization potential, AM1, PM3 semiempirical calculations, NUCLEIC-ACID BASES, TAUTOMERIC EQUILIBRIA, RELATIVE STABILITIES, INFRARED-SPECTRA, MATRIX-ISOLATION, AB-INITIO, 4-THIOURIDINE, DERIVATIVES, CYTOSINE, ABINITIO
  • Ankara Üniversitesi Adresli: Evet

Özet

Heats of formation, entropies, Gibbs free energies, relative tautomerization energies, dipole moments, and ionization potentials for the tautomers of 2-thiocytosine, 2,4-dithiouracil and 2,3-dithiothymine and their 1-methyl analogues were studied by using semiempirical AM1 and PM3 quantum-chemical calculations at the SCF level both in the gas phase and in aqueous solution, with full geometry optimization. The COSMO solvation model was employed for aqueous solution calculations. The calculations show that the thiol-amino tautomer of 2-thiocytosine is the most stable in the gas phase whereas the thione-amino tautomer is the most stable in the aqueous phase. For 2,4-dithiouracil and 2,3-dithiothymine, dithione tautomers are the most stable in both phases. The results are in good agreement with available experimental and theoretical results. The entropy effect on the Gibbs free energy of the thiopyrimidine bases is very small and has practically no significance for the tautomeric equilibria of thiopyrimidine bases. The enthalpic term is dominant in the determination of the equilibrium constant. Copyright (C) 2001 John Wiley & Sons, Ltd.