Article pubs.acs.org/JPCA
Benchmark Thermochemistry for Biologically Relevant Adenine and Cytosine. A Combined Experimental and Theoretical Study Vladimir N. Emel’yanenko,*,†,‡ Dzmitry H. Zaitsau,‡ Evgeni Shoifet,§ Florian Meurer,∥ Sergey P. Verevkin,†,‡ Christoph Schick,§ and Christoph Held*,∥ †
Department of Physical Chemistry and Department, Science and Technology of Life, Light and Matter, University of Rostock, Dr-Lorenz-Weg 1, D-18059, Rostock, Germany ‡ Department of Physical Chemistry, Kazan Federal University, Kremlevskaya str. 18, 420008 Kazan, Russia § Institut für Physik, Universität Rostock, Wismarsche Strasse 43-45, 18051 Rostock, Germany ∥ Department BCI, Laboratory of Thermodynamics, Technische Universität Dortmund, D-44227 Dortmund, Germany S Supporting Information *
ABSTRACT: The thermochemical properties available in the literature for adenine and cytosine are in disarray. A new condensed phase standard (p° = 0.1 MPa) molar enthalpy of formation at T = 298.15 K was measured by using combustion calorimetry. New molar enthalpies of sublimation were derived from the temperature dependence of vapor pressure measured by transpiration and by the quarz-crystal microbalance technique. The heat capacities of crystalline adenine and cytosine were measured by temperature-modulated DSC. Thermodynamic data on adenine and cytosine available in the literature were collected, evaluated, and combined with our experimental results. Thus, the evaluated collection of data together with the new experimental results reported here has helped to resolve contradictions in the available enthalpies of formation. A set of reliable thermochemical data is recommended for adenine and cytosine for further thermochemical calculations. Quantum-chemical calculations of the gas phase molar enthalpies of formation of adenine and cytosine have been performed by using the G4 method and results were in excellent agreement with the recommended experimental data. The standard molar entropies of formation and the standard molar Gibbs functions of formation in crystal and gas state have been calculated. Experimental vaporpressure data measured in this work were used to estimate pure-component PC-SAFT parameters. This allowed modeling solubility of adenine and cytosine in water over the temperature interval 278−310 K. elevated temperatures.1 Moreover, adenine plays a major role in the energy metabolism of biological cells because it is an
1. INTRODUCTION Adenine and cytosine (Figure 1) are weak organic bases and structural components of nucleic acids which make up DNA and RNA, the caretakers of the biological building plan of life. Their thermodynamic properties will therefore allow for assessment of the stability of life at ambient as well as at © 2015 American Chemical Society
Received: May 19, 2015 Revised: August 20, 2015 Published: August 28, 2015 9680
DOI: 10.1021/acs.jpca.5b04753 J. Phys. Chem. A 2015, 119, 9680−9691
Article
The Journal of Physical Chemistry A
by a heating rate of 0.42 K·s−1 to T = 473 K. No impurities (greater than mass fraction 0.001) could be detected by GC. 2.2. Temperature-Modulated Differential Scanning Calorimetry (TMDSC). The specific heat capacities of the nucleobases were measured by using PerkinElmer DSC Pyris 1 equipped with an intracooler. We used the temperaturemodulated differential scanning calorimetry (TMDSC) method.7,8 This method has an advantage over the standard DSC procedure because the high- and the low-frequency noise, particular signal drifts, are excluded due to the frequency selective data treatment. Application of the temperature modulation mode allowed increasing accuracy of heat capacity measurements (maximum deviations
