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

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(Br 7 c 7 G‐C) 4 ( 8 ) and d(I 7 c 7 G‐C) 4 ( 9 ) as well as the homo‐oligonucleotides d[(Br 7 c 7 G) 5 ‐G] ( 11 ) and d[(I 7 c 7 G) 5 ‐G] ( 12 ). The melting profiles and CD spectra of oligonucleotide duplexes were measured. The T m 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[(I 7 c 7 G) 5 ‐G] ( 12 ) with d(C 6 ) is slightly destabilized (Δ T m = −12°) over that of d[(c 7 G) 5 ‐G] with d(C 6 ). However, the complex of 12 with poly(C) is more stable than that of d[(c 7 G 5 ‐G)] with poly(C).