Low-Temperature Thermodynamic Properties of Purine Bases: Adenine and Guanine

The work presents precision data on the heat capacity of adenine (C5H5N5; IUPAC name: 9H-purin-6-amine; CAS Number: 73–24–5) and guanine (C5H5N5O; IUPAC name: 2-amino-1,9-dihydro-6H-purin-6-one; CAS Number: 73–40–5) in the range 6–330 K obtained by adiabatic calorimetry. The data were used to calculate the thermodynamic functions (entropy, enthalpy increment, and Gibbs reduced energy) between 0 and 330 K. For the first time, a λ-type anomaly was revealed in the functional behavior of the heat capacity of guanine in the range of 225–250 K, which indicates the presence of a second-order phase transition in this temperature range. The anomalous component was segregated from the experimental data. Approximation of the anomalous component was done to find its thermodynamic parameters (entropy and enthalpy increment) and the critical temperature of phase transition: Ttr = 242.7 ± 0.1 K, ΔSan = 0.042 ± 0.004 J·mol–1K–1, ΔHan = 10.2 ± 1.0 J·mol–1.