Melting temperature and heat of fusion of cytosine revealed from fast scanning calorimetry

Abstract

© 2017 Elsevier B.V. Thermophysical properties in the melting range of cytosine, one of the five nucleobases of DNA and RNA, are hard to determine because of the low thermal stability of the compound and the high vapor pressure. As for other biomolecules fast heating rates allow melting of cytosine without detectable decomposition. By applying fast scanning calorimetry with the heating rate at 6000 K s −1 we succeeded to avoid decomposition and determine the melting temperature of cytosine (extrapolated to zero heating rate), as T fus = (606 ± 4) K, the glass transition temperature of the supercooled liquid state as T g = (388 ± 3) K, cold-crystallization temperature as T cryst = (448 ± 8) K, and the liquid state molar heat capacity C p,m ° (l) = (272 ± 2) J mol −1 K −1 at 423 K. Taking into account the temperature dependent mass loss of the nanogram sized sample (up to 25% during the melting scan) we obtained the molar enthalpy of fusion of cytosine as Δ cr l H(T fus ) = (35 ± 4) kJ mol −1 in good agreement with the adjusted molar enthalpy of crystallization Δ l cr H(T fus ) = (34 ± 2) kJ mol −1 .