Thermochemistry of uracil and thymine revisited

https://doi.org/10.1016/j.jct.2015.03.015Get rights and content

Highlights

Abstract

Thermochemical properties of uracil and thymine have been evaluated using additional experiments. Standard (p0 = 0.1 MPa) molar enthalpies of formation in the gas phase at T = 298.15 K for uracil −(298.1 ± 0.6) and for thymine −(337.6 ± 0.9) kJ · mol−1 have been derived from energies of combustion measured by static bomb combustion calorimetry and molar enthalpies of sublimation determined using the transpiration method. The G3 and G4 quantum-chemical methods were used for calculations of theoretical gaseous enthalpies of formation being in very good agreement with the re-measured experimental values.

Introduction

Uracil and its derivative thymine (5-methyluracil) are common and naturally occurring pyrimidine derivatives. They are well known for their importance in biochemistry, molecular biology and medicine. They are two of the primary nucleobases. Uracil or pyrimidine-2,4(1H,3H)-dione (see Fig. 1a) is one of the four nucleobases in the nucleic acid of the RNA. In the DNA, the uracil is replaced by the thymine or 5-methylpyrimidine-2,4(1H,3H)-dione, (see Fig. 1b). Recently, we published [1] an experimental and computational study on the thermochemistry of three derivatives of the uracil: 5,6-dimethyluracil, 1,3,5-trimethyluracil, and 1,3,5,6-tetramethyluracil. Good agreement between experimental and theoretical (with the composite G3 and G4 methods) gas phase enthalpies of formation was achieved [1]. Surprisingly, for thymine a significant disagreement between the experimental and theoretical values was observed. The experimental enthalpy of formation reported by Sabbah et al. [2] deviates by 9 kJ · mol−1 and the more recent value reported by Ribeiro da Silva et al. [3] deviates by 17 kJ · mol−1 in comparison with the G4 result. This discrepancy has motivated a re-determination of the enthalpies of formation and sublimation for thymine in order to ascertain thermochemical information for this compound. In order to reveal possible experimental shortages, some additional experiments on similarly shaped uracil have been also performed.

Section snippets

Materials and purity control

All samples used for this work were of commercial origin (see table 1). Prior to experiments the samples were purified twice by re-crystallisation from water and further purified by the repeated vacuum fractional sublimation. No impurities (greater than mass fraction 0.001) could be detected by DSC [4] in the samples used for the thermochemical measurements. DSC curves are given on figures S1 an S2 in the Supporting Information. Samples were additionally analysed with a Hewlett Packard gas

Vapour pressures of uracil and thymine

Experimental absolute vapour pressures on uracil and thymine at different temperatures were carefully collected from the literature. They are compared graphically (see FIGURE 2, FIGURE 3). Unfortunately, most of original sources provide only linear approximated p–T results. Absence of the primary experimental points has thwarted fair comparison of the data sets. Vapour pressure studies on uracil have been a popular endeavour over last 40 years (see Fig. 2). However, the spread of the available

Acknowledgements

The work has been supported by the Russian Government Program of Competitive Growth of Kazan Federal University. The support of the Spanish Ministerio de Economía y Competitividad under Project CTQ2010-16402 is gratefully acknowledged.

References (32)

  • M.A.V. Ribeiro da Silva et al.

    J. Chem. Thermodyn.

    (2011)
  • Y.B. Tewari et al.

    J. Chem. Thermodyn.

    (2004)
  • S.P. Verevkin et al.

    Fluid Phase Equilib.

    (2008)
  • G. Bardi et al.

    Thermochim. Acta

    (1980)
  • P. Szterner et al.

    J. Chem. Thermodyn.

    (2002)
  • D. Ferro et al.

    Thermochim. Acta

    (1980)
  • A.L.F. de Barros et al.

    Nucl. Instrum. Methods Phys. Res., Sect. A

    (2006)
  • A.B. Teplitsky et al.

    Biophys. Chem.

    (1980)
  • S.R. Wilson et al.

    J. Chem. Thermodyn.

    (1979)
  • R. Notario et al.

    J. Phys. Chem. A

    (2013)
  • P.M. Nabavian et al.

    J. Chim. Phys.

    (1977)
  • E.E. Marti

    Thermochim. Acta

    (1973)
  • V.N. Emel’yanenko et al.

    J. Am. Chem. Soc.

    (2007)
  • M.E. Wieser et al.

    Pure Appl. Chem.

    (2013)
  • G. Olofsson
  • G.S. Parry

    Acta Cryst.

    (1954)
  • Cited by (15)

    • Gas phase proton affinity and basicity of methylated uracil-derivatives

      2021, International Journal of Mass Spectrometry
    • Energetic characterization of a bioactive compound: Uridine

      2018, Journal of Chemical Thermodynamics
    • Experimental and computational thermochemical studies of 6-azauracil derivatives

      2016, Journal of Chemical Thermodynamics
      Citation Excerpt :

      The theoretical calculations were extended for 6-azathiouracil, whose working reactions and results are shown in table 12. Considering the literature values for the standard molar enthalpies of formation for uracil [62], thymine [62], 2-thiouracil [14] and 2-thiothymine [14], one can calculate the enthalpic increment for the methylation of uracil and 2-thiouracil as shown in the scheme below (see scheme 1). It can be seen, from the scheme presented above, that the enthalpic effects for the methylation of uracil and 2-thiouracil, are very similar, within the experimental uncertainties.

    • 5-Isopropylbarbituric and 2-thiobarbituric acids: An experimental and computational study

      2016, Thermochimica Acta
      Citation Excerpt :

      According to the experimental and computational enthalpies of reaction it is possible to notice a slight enthalpic instability of 2-thiobarbituric acid when compared with barbituric acid considering this reaction. Using this type of reaction, which allows for the cancelation of errors between the reactants and products, it is also possible to verify that the calculated values using the G3 and G4 computational methods are in agreement with the value calculated experimentally, within the experimental uncertainty and chemical accuracy of 4 kJ mol−1 [89,90]. Another interesting aspect is related with the effect of the isopropyl substituent from isopropylbenzene or isopropylcyclohexane comparatively with its effect into barbituric acid (reaction I or II, respectively), as shown in Fig. 5.

    View all citing articles on Scopus
    View full text