Spectroscopic and calorimetric characterizations of DNA duplexes containing 2-aminopurine

The base analog 2-aminopurine (AP) strongly promotes A.T to G.C and G.C to A.T transitions in bacteria and bacteriophage. During DNA replication, the primary mutagenic event involves formation of a heteroduplex with an AP.C site at a much higher frequency than formation of the corresponding heteroduplex with an A.C site. It is not known if AP-induced mutagenesis correlates with differences in the thermodynamic properties of an AP.C versus an A.C site, or whether interactions involving DNA polymerases are controlling. To address this specific question, and more generally to characterize AP-containing duplexes, we have used a combination of spectroscopic and calorimetric techniques to determine the thermodynamic properties of six 11-mer duplexes. The sequences of these duplexes are identical except for the identity of the variable central base pair which can be either A.T, A.C, AP.T, AP.C, AP.A, or AP.G, and which we use to designate each duplex. Analyses and interpretation of the optically and calorimetrically derived thermal and thermodynamic data on these six duplexes reveal the relative stabilizing influence of the central base pairs to be A.T > AP.T > AP.C > AP.A > AP.G > A.C, with the AP.C-containing duplex being significantly more stable than the A.C-containing duplex. In the aggregate, our results suggest that during incorporation, base pair discrimination by DNA polymerases is influenced, in part, by differences in the thermodynamic stabilities of the newly formed base pairs.