Published online 19 May 2005
Article |
Effect of 6-thioguanine on the stability of duplex DNA
Department of Physiology and Biophysics and Department of Pharmacological Sciences, State University of New York at Stony Brook Stony Brook, NY 11794-8651, USA
*To whom correspondence should be addressed. Tel: +1 631 444 3649; Fax: +1 631 444 3218; Email: cds{at}pharm.sunysb.edu
Received January 10, 2005. Revised April 7, 2005. Accepted April 22, 2005.
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, 13311338]. 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 GC pair by S6G results in
1 kcal/mol less favorable Gibbs free energy of duplex formation at 37°C. S6GT and GT mismatch-containing duplexes have almost identical Gibbs free energy at 37°C, with values
3 kcal/mol less favorable than that of the control. Base pair stability is affected by S6G. The lifetime of the normal GC base pair is
125 ms, whereas that of the GT mismatch is below the detection limit. The lifetimes of S6GC and S6GT pairs are
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.
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