Xanthosine (X) and 2'-deoxyxanthosine (dX), the products of deamination of G and dG, have been incorporated into oligonucleotides,⁽¹⁾ imparting a variety of properties to these nucleic acids. The pKa of the proton at N3 of dX is 5.5-5.7.^1b,1h therefore dX is present as a mono-anion to the extent of about 95% at neutral pH. dX residues have the same hydrogen-bond donor/acceptor pattern as thymidine, thus allowing base-pairing with dA.^1h This leads to a distortion of the helix, since two purines are paired. Stronger base stacking interactions are apparent. Duplex stability variations as a function of the opposing base pair have been studied.^1d,1h dX is susceptible to depurination under acidic conditions, forming abasic sites.^1b,1h At pH 4, dX depurinates more quickly than dG by a factor of about 1000. At neutral pH, dX has about the same stability as dG. The recognition and excision of dX by base excision repair enzymes has been studied.^1b dX has been considered to be a promiscuous or universal nucleoside.^1a,1h dX may expand the genetic code when paired to a pyrimidine-2,4-diamine C-nucleoside.^1e Studies of polymerase-mediated incorporation of various nucleotides opposite dX have been carried out.^1b,1d The presence of dX residues may lead to triplexes, quartets, and other G-stranded structures.^1h We offer the 2'-Deoxyxanthosine-CE Phosphoramidite (BA0313), featuring 2-(4-nitrophenyl)ethyl (NPE) protecting groups at O2 and O6.^1a,1b

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