Cooperative binding of adenosine by polyuridylic acid: A further analysis

The dialysis data of Huang and Ts'o for the cooperative binding of adensine to polyuridylic acid are analyzed here using a grand‐partition function Ising model method similar to that originally employed for polyelectrolytes by Rice and Nagasawa. An appropriate modification permitting the treatment of the sliding degeneracy of the two polyuridylic acid strands is also included. In addition to the previously estimated stacking energy of about −6 kcal/mole one also obtains the free energy change F̄ for the transfer of a single adenosine molecule from a fixed site in solution to a fixed site on the polyuridylic acid. This binding energy falls in the range F̄ = −140 to +620 cal/mole, indicating that binding in the 1:2 (purine: pyrimidine) complex is either very weak or repulsive. The absence of any comparable cooperative stacking of adenosines in solution at the same concentration together with the likely repulsive character of the binding implies that the stacking energy must contain a significant contribution from other processes than simple stacking of adenosines. A generalization of the theory to treat multicomponent binding and longer‐range interactions is effected, and the form applicable to simultaneous binding of both adenosine and guanosine by polyuridylic acid is presented.