5-aza-7-deazaguanine DNA: Recognition and strand orientation of oligonucleotides incorporating anomeric imidazo[1,2-a]-1,3,5-triazine nucleosides

Abstract

The base-pairing properties of oligonucleotides containing the anomeric 5-aza-7-deazaguanine 2 ` -deoxyribonucleosides 1 and 5 are described. The oligonucleotides were prepared by solid-phase synthesis, employing phosphoramidite or phosphonate chemistry. Stable `purine' . purine duplexes with antiparallel (aps) chain orientation are formed, when the alpha -D-anomer 5 alternates with the beta -D-anomeric 2 ` -deoxyguanosine (2) within the same oligonucleotide chain. Parallel (ps) oligonucleotide duplexes are observed, when the beta -D anomer 1 alternates with 2. A renewed reversal of the chain orientation (ps -. aps) occurs when compound I pairs with 2 ` -deoxyisoguanosine (6). In all cases, it is unnecessary to change the orientation within a single strand when alpha -D units alternate with their beta -D counterparts. Heterochiral base pairs of 5 (alpha -D) with 2 ` -deoxyisoguanosine (beta -D) are well accommodated in duplexes, with random base composition and parallel chain orientation. Base pairs of 5 (alpha -D) with 2 ` -deoxyguanosine (beta -D) destabilize duplexes with antiparallel chains.