Thermodynamics of aqueous adenine: Standard partial molar volumes and heat capacities of adenine, adeninium chloride, and sodium adeninate from T = 283.15 K to 363.15 K

Abstract

Apparent molar volumes, Vϕ, and apparent molar heat capacities, Cp,ϕ, were measured for aqueous solutions of neutral adenine (AdH) and adeninium chloride (AdH2Cl) at temperatures from 288.15K to 363.15K, and for sodium adeninate (NaAd) at temperatures from 283.15K to 313.15K. Density measurements used in the determination of the standard partial molar volume, Vo, were made with a vibrating-tube densimeter at ambient pressure. Heat capacity measurements used in the determination of the standard partial molar heat capacity, Cpo, were made with a twin fixed-cell, differential output, power compensating, temperature-scanning calorimeter at a pressure of 0.4MPa. The contributions from minor equilibrium species were removed using Young’s Rule to yield values for the standard partial molar volume and heat capacities of the ionic species AdH2+(aq) and Ad−(aq), and the equilibrium mixture of neutral species AdH∗(aq). Semi-empirical equations describing the temperature dependence of Vo and Cpo for each species were fitted by regression to the experimental results. From these equations, the standard partial molar volume change ΔrVom and heat capacity change ΔrCpom for the ionization reactions were calculated. The standard partial molar volumes presented in this study for neutral adenine are the first to be experimentally determined at temperatures above 330K, while the standard partial molar heat capacities are the first to be reported at temperatures above 298.15K. The experimental values for the adeninium cation and adeninate anion are the first to be reported in the literature.