8-Aza-2'-deoxyguanosine: base pairing, mismatch discrimination and nucleobase anion fluorescence sensing in single-stranded and duplex DNA.

Oligodeoxyribonucleotides containing 8-aza-2-deoxyguanosine 9 were synthesized and a new phosphoramidite 11, showing a high coupling yield in solid-phase synthesis, was prepared. Nucleoside 9 was found to be a perfect shape mimic of dG; it forms a strong base pair with dC and shows an excellent mismatch discrimination. Nucleoside 9 appears strongly fluorescent as the anion, and thus, it has unique reporter group properties as a replacement for dG. The pKa-value of 9 (8.4) is lower than that of dG (9.3) and increases from the single-stranded DNA (8.8) to duplex DNA (9.1). The fluorescence of the nucleoside 9 anion is decreased in ss-oligonucleotides and further reduced in duplex DNA. When mismatches of 9 are formed with the four DNA canonical bases, the fluorescence of mismatches is significantly higher than that of the perfectly paired duplex. The fluorescence of the nucleoside 9 anion correlates with the base pair stability.