Effect of 6-thioguanine on the stability of duplex DNA

The incorporation of 6-thioguanine (S6G) into DNA is a prerequisite for its cytotoxic action, but duplex structure is not significantly perturbed by the presence of the lesion [J. Bohon and C. R. de los Santos ( 2003) Nucleic Acids Res., 31, 1331-1338]. It is therefore possible that the mechanism of cytotoxicity relies on a loss of stability rather than a pathway involving direct structural recognition. The research described here focuses on the changes in thermodynamic properties of duplex DNA owing to the introduction of S6G as well as the kinetic properties of base pairs involving S6G. Replacement of a guanine in a G center dot C pair by S6G results in similar to 1 kcal/ mol less favorable Gibbs free energy of duplex formation at 37 degrees C. S6G center dot T and G center dot T mismatch-containing duplexes have almost identical Gibbs free energy at 37 degrees C, with values similar to 3 kcal/ mol less favorable than that of the control. Base pair stability is affected by S6G. The lifetime of the normal G center dot C base pair is similar to 125 ms, whereas that of the G center dot T mismatch is below the detection limit. The lifetimes of S6G center dot C and S6G center dot T pairs are similar to 7 and 2 ms, respectively, demonstrating that, although S6G significantly decreases the stability of the pairing with cytosine, it slightly increases that of a mismatch.