Pyrazolo[3,4-d]pyrimidine nucleic acids: adjustment of dA-dT to dG-dC base pair stability

Oligonucleotides incorporating 8-aza-7-deazapurin-2,6-diamine (pyrazolo[3,4-d]pyrimidin-4,6-diamine) nucleoside 2a or its 7-bromo derivative 2b show enhanced duplex stability compared to those containing dA. While incorporation of 2a opposite dT increases the T(m) value only slightly, the 7-bromo compound 2b forms a very stable base pair which is as strong as the dG-dC pair. Compound 2b shows a similar base discrimination in duplex DNA as dA. The base-modified nucleosides 2a,b have a significantly more stable N-glycosylic bond than the rather labile purin-2,6-diamine 2'-deoxyribonucleoside 1. Base protection with acyl groups, with which we had difficulties in the case of purine nucleoside 1, was effective with pyrazolo[3,4-d]-pyrimidine nucleosides 2a,b. Oligonucleotides containing 2a,b were obtained by solid phase synthesis employing phosphoramidite chemistry. Compound 2b harmonizes the stability of DNA duplexes. Their stability is no longer dependent on the base pair composition while they still maintain their sequence specificity. Thus, they have the potential to reduce the number of mispairs when hybridized in solution or immobilized on arrays.