Anti-parallel triplexes: Synthesis of 8-aza-7-deazaadenine nucleosides with a 3-aminopropynyl side-chain and its corresponding LNA analog
The phosphoramidites of DNA monomers of 7-(3-aminopropyn-1-yl)-8-aza-7-deazaadenine (Y) and 7-(3-aminopropyn-1-yl)-8-aza-7-deazaadenine LNA (Z) are synthesized, and the thermal stability at pH 7.2 and 8.2 of anti-parallel triplexes modified with these two monomers is determined. When, the anti-parallel TFO strand was modified with Y with one or two insertions at the end of the TFO strand, the thermal stability was increased 1.2 °C and 3 °C at pH 7.2, respectively, whereas one insertion in the middle of the TFO strand decreased the thermal stability 1.4 °C compared to the wild type oligonucleotide. In order to be sure that the 3-aminopropyn-1-yl chain was contributing to the stability of the triplex, the nucleobase X without the aminopropynyl group was inserted in the same positions. In all cases the thermal stability was lower than the corresponding oligonucleotides carrying the 3-aminopropyn-1-yl chain, especially at the end of the TFO strand. On the other hand, the thermal stability of the anti-parallel triplex was dramatically decreased when the TFO strand was modified with the LNA monomer analog Z in the middle of the TFO strand (ΔTm = −9.1 °C). Also the thermal stability decreased about 6.1 °C when the TFO strand was modified with Z and the Watson–Crick strand with adenine-LNA (AL). The molecular modeling results showed that, in case of nucleobases Y and Z a hydrogen bond (1.69 and 1.72 Ǻ, respectively) was formed between the protonated 3-aminopropyn-1-yl chain and one of the phosphate groups in Watson–Crick strand. Also, it was shown that the nucleobase Y made a good stacking and binding with the other nucleobases in the TFO and Watson–Crick duplex, respectively. In contrast, the nucleobase Z with LNA moiety was forced to twist out of plane of Watson–Crick base pair which is weakening the stacking interactions with the TFO nucleobases and the binding with the duplex part