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 α-D-anomer 5 alternates with the β-D-anomeric 2′-deoxyguanosine (2) within the same oligonucleotide chain. Parallel (ps) oligonucleotide duplexes are observed, when the β-D anomer 1 alternates with 2. A renewed reversal of the chain orientation (ps→aps) occurs when compound 1 pairs with 2′-deoxyisoguanosine (6). In all cases, it is unnecessary to change the orientation within a single strand when α-D units alternate with their β-D counterparts. Heterochiral base pairs of 5 (α-D) with 2′-deoxyisoguanosine (β-D) are well accommodated in duplexes with random base composition and parallel chain orientation. Base pairs of 5 (α-D) with 2′-deoxyguanosine (β-D) destabilize duplexes with antiparallel chains.
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