Single strand targeted triplex formation: targeting purine-pyrimidine mixed sequences using abasic linkers.

Foldback triplex-forming oligonucleotides (FTFOs) that contain an abasic linker, [2-(4-aminobutyr-1-yl)-1,3-propanediol] (APD linker), in the Hoogsteen domain against pyrimidine bases of a C:G and a T:A base pair were studied for their relative stability and sequence specificity of triplex formation. In general, the APD linker has less destabilizing effect against a C:G base pair than a T:A base pair. Incorporation of three APD linker moieties resulted in decreased binding to the target, which was comparable to results observed with three imperfectly matched natural base triplets. The APD linker incorporation did not result in the loss of sequence specificity of FTFOs, unlike in the case of normal triplex-forming oligonucleotides (TFOs). The introduction of a positively charged abasic linker, however, resulted in decreased stability of the triplex, because of loss of hydrogen bonding and stacking interactions in the major groove. The results of a molecular modeling study show that APD linker can be readily incorporated without any change in the conformation of the natural sugar-phosphate backbone conserving overall triple helix geometry. Further, the modeling study suggests a hydrogen bond formation between the amino group of linker and N4 of cytosine mediated by a solvent molecule (water) in the floor of the base triplet in addition to a contribution from the positive charge on the APD linker amino group. Either a direct or water-mediated hydrogen bond between the amino group of the APD linker and the O4 of thymine is unlikely when the linker is placed against a T:A base pair.