Parallel-stranded DNA formed by new base pairs related to the isoguanine-cytosine or isocytosine-guanine motifs

Parallel-stranded (ps) oligonucleotide duplexes containing several new base pairs formed between 7-deazaisoguanine and cytosine, 8-aza-7-deaza-isoguan ine and cytosine, and 5-aza-7-deaza guanine and guanine are described. The stability of the ps-hybrids increased if the duplex contains 8-aza-7-deazai soguanine instead of isoguanine and is decreased by 7-deazaisoguanine incor poration. The purine-purine base pair between 5-aza-7-deazaguanine and guan ine was found to be more stable than that of 5-methylisocytosine with guani ne.