Duplex Stability of 7‐Deazapurine DNA: Oligonucleotides containing 7‐bromo‐ or 7‐iodo‐7‐deazaguanine

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Abstract
The oligonucleotide building blocks, the phosphonates 1a, b and the phosphoramidites 2a, b derived from 7‐iodo‐ and 7‐bromo‐7‐deaza‐2′‐deoxyguanosines 3a, b were prepared. They were employed in solid‐phase oligonucleotide synthesis of the alternating octamers d(Br7c7G‐C)4 (8) and d(I7c7G‐C)4 (9) as well as the homo‐oligonucleotides d[(Br7c7G)5‐G] (11) and d[(I7c7G)5‐G] (12). The melting profiles and CD spectra of oligonucleotide duplexes were measured. The Tm values as well as the thermodynamic data were determined and correlated to the major‐groove modification of this DNA. The self‐complementary octamers 8 and 9 form more stable duplexes compared to the parent oligomer d(G‐C)4. The heteroduplex of d[(I7c7G)5‐G] (12) with d(C6) is slightly destabilized (ΔTm = −12°) over that of d[(c7G)5‐G] with d(C6). However, the complex of 12 with poly(C) is more stable than that of d[(c7G5‐G)] with poly(C).

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