Enthalpies of Solution of Nucleic Acid Bases. 1. Adenine in Water

An adiabatic solution calorimeter was used to measure the enthalpy of solution in water of various adenine samples for which a large amount of analytical information is reported. The experimental imprecision of the measurements was 1.1 percent. However, it was necessary to assign an overall uncertainty of 3 percent because of impurity uncertainties. Thus, the best value for the enthalpy of solution is ΔC = (78.7 ± 10.4) J·mol^-1·K^-1 in the range 298 to 328 K at 5 mmol·kg^-1, and the enthalpy of dilution is -(316 ± 208) kJ·mol^-1 (mol·kg^-1)^-1 in the range 1 to 7 mmol·kg^-1. The entropy of solution for adenine was calculated to be ΔS° (298.15 K) = (72.1 ± 3.9) J·mol^-1·K^-1, and the partial molar heat capacity at infinite dilution, C° = (226 ± 11) J·mol^-1·K^-1. The density of adenine was measured by displacement as 1.47 g·mL^-1 with an estimated uncertainty of 1 percent.