Structural constraints regulating triple helix formation by A-tracts.

The study concerns the propensity of triple helix formation by different DNA oligonucleotides containing long A-tracts with and without flanking GxC base pairs in order to probe the role of length of the A-tract and the flanking sequences. From nuclear magnetic resonance (NMR) studies of imino proton spectra and circular dichroism (CD) spectroscopy of samples composed of potential triplex forming strand sequences in correct stoichiometries, we have concluded that 8-mer A-tracts flanked by GxC base pairs exert significant steric hindrance to triple helix formation. When as much as 50 mM Mg2+ was added, no triple helix formation was observed in these samples. In contrast, open-ended 8-mer A-tracts formed triplex with the corresponding two T8 strands under relatively mild ionic conditions (100 mM Na+). Moreover, the shorter the length of the A-tract, the less is the hindrance to form a triple helix.