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)(5)-G] (12). The melting profiles and CD spectra of oligonucleotide otide 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 (Delta T-m=-12 degrees) 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).