Calorimetric determination of the heat capacity changes associated with the conformational transitions of polyriboadenylic acid and polyribouridylic acid

The heat capacities of the single‐stranded and double‐stranded forms of polyadenylic acid, polyuridylic acid, and poly(uridylic and adenylic acid) were determined with a drop heat capacity calorimeter. In addition, the temperature dependence of the apparent partial heat capacity (ϕ Cp ) was measured with a newly developed differential scanning calorimeter. The calculated Δ Cp at 28°C for the transition poly(A)·poly(A) ⇄ 2 poly(A) was found to be 165 ± 24 cal/Kmol‐base pair, compared with a value of 140 ± 28 for the transition poly(A)·poly(U) ⇄ poly(A) + poly(U). The temperature dependence of ϕ Cp of single‐stranded poly(U) was consistent with the conclusion that it is totally unstacked at temperatures above 15°C. The temperature dependence of ϕ Cp of single‐stranded poly(A) was used to determine the base‐stacking parameters for poly(A). The experimental results are consistent with a stacking enthalpy change of −8.5 ± 0.1 kcal/mol bases and a cooperativity parameter σ of 0.57 ± 0.03 for the stacking of adenine bases. These results demonstrate that the heat capacity of single‐stranded polynucleotides is greater than that of the double‐stranded forms. This increased heat capacity is mainly the result of the temperature dependence of the base‐stacking interactions in the single‐stranded form.