15-mer DNA duplexes containing an abasic site are thermodynamically more stable with adjacent purines than with pyrimidines.

Abasic site (AP)-containing duplexes, with flanking adenine (A) or cytosine (C) bases, were shown to be more stable with flanking A than with C bases [Sági, J., Hang, B., and Singer, B. (1999) Chem. Res. Toxicol. 12, 917-923]. We investigated whether the lower-magnitude destabilization by an AP site, with A neighbors, is a general effect of the purine versus the pyrimidine neighbors. Duplex stability, as compared to that of the corresponding control duplexes, was markedly decreased by the incorporation of the AP site (x) opposite any of the four bases. However, for the duplexes containing T, A, or C opposite the AP site, replacement of the symmetric doublet flanking pyrimidine bases with purines resulted in a smaller destabilization effect. The average stabilizing effect of the symmetric doublet purine neighbors of an AP site opposite T, A, or C bases was 3.2 degrees C (DeltaT(m)) and 1.3 kcal/mol (DeltaDeltaG degrees (37)) compared to those of pyrimidine neighbors. In contrast, a G.AP pair reduced or eliminated the differential effect of the neighbors. Using unrestrained molecular dynamics, it was shown that for the duplexes containing T opposite the AP site, with doublet pyrimidine neighbors, there was a larger magnitude of curvature around the lesion site than for the duplexes with the purines flanking the AP site. Purines flanking the AP site tend to shift toward each other, creating overlap, in contrast to the flanking pyrimidines. This indicates the possibility of stacking between purine bases at the AP site and can be the reason for the observed smaller thermodynamic destabilization of the duplexes with the AAxAA and GGxGG central sequences, as compared to those with TTxTT and CCxCC sequences. This work showed that for an AP site the GC content is not the only determinant of duplex stability, but rather is influenced more by whether purines or pyrimidines flank the AP site.