Investigations on purine and pyrimidine bases stacking associations in aqueous solutions by the fluorescence quenching method. II. Heteroassociation between 2-aminopurine and thymidine.

Heteroassociation between A and B compounds in liquid solution was considered. Provided that concentration of A molecules is low, a general equation describing fluorescence quantum yield and lifetime of compound A as a function of B molecules concentration was derived. The heteroassociation between 2-aminopurine and thymidine in aqueous solutions was examined within the range of temperatures 0 to 90 degrees C. The equilibrium constants of the first step of association, namely heterodimer formation, were determined and its thermodynamic parameters (deltaH equals - 2.76 kcal/mol, deltaS equals - 5.9 e.u.) were calculated. The observed changes of the stacking rate constants with temperature confirm the two-step mechanism of the reaction. The activation energy (approximately 10.7 angstroms) are only slightly larger than in the case of 2-aminopurine autoassociation, most probably because of a stronger solvation of thymidine molecules.