| Abstract
| - Oligonucleotides containing 2‘-deoxyisoguanosine (1, iGd), 7-deaza-2‘-deoxyisoguanosine (2,c7iGd), and its 7-halogenated derivatives 3 and 4 were synthesized on solid phase using the phosphoramiditebuilding blocks 5−7. The hybridization properties of oligonucleotides were studied on duplexes with paralleland antiparallel chain orientation. It was found that the 7-halogenated nucleoside analogues 3 and 4 enhancethe duplex stability significantly in both parallel (ps) and antiparallel (aps) DNA. Moreover, the halogenatednucleosides shift the tautomeric keto−enol equilibrium strongly toward the keto form, with KTAUT [keto]/[enol] ≈ 104 coming close to that of 2‘-deoxyguanosine (104−105), while the nonhalogenated 7-deaza-2‘-deoxyisoguanosine 2 shows a KTAUT of around 2000 and the enol concentration of 1 is 10% in aqueoussolution. Consequently, nucleosides 3 and 4 show a much better mismatch discrimination against dT thancompound 1 or 2 in antiparallel as well as in parallel DNA. 3 and 4 are expected to increase the selectivityof base incorporation opposite to isoCd in the form of triphosphates or in the polymerase-catalyzed reactionin comparison to 1 or 2.
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