7-deaza-deoxyxanthosine (7-deaza-dX) is a deoxyribonucleoside in which the 7-nitrogen (N7) of the base is replaced by C-H. The resulting modified dX is unable to form a hydrogen bond at position 7. 7-deaza-dX is a useful research tool for DNA structural studies. For example, in one study on triple helix formation, the authors showed that 7-deaza-X:A-T triplets are stable and can be used to facilitate formation of triple helices in the anti-parallel motif (1). In a different study related to development of an expanded genetic alphabet, the ability of 7-deaza-dX to form non-standard base pair with an 2,4-diaminopyrimidine analog (2). Note that in both studies, their authors used H-phosphonate chemistry to incorporate 7-deaza-dX into the oligonucleotide.
1. Milligan, J.F., Krawczyk, S.H., Wadwani, S., Matteucci, M.D. An anti-parallel triple helix motif with oligodeoxynucleotides containing 2-deoxyguanosine and 7-deaza-2’-deoxyxanthosine.Nucleic Acids Res. (1993), 21: 327-333.
2. Lutz, M.J., Held, H.A., Hottiger, M., Hubscher, U., Benner, S.A. Differential discrimination of DNA polymerase for variants of the non-standard nucleobase pair between xanthosine and 2,4-diaminopyrimidine, two components of an expanded genetic alphabet.Nucleic Acids Res. (1996), 24: 1308-1313.