Parallel-Stranded Oligonaucleotide Duplexes Containing 5-Methylisocytosine-Guanine and Isoguanine-Cytosine Base Pairs

摘要

Parallel-stranded oligonucleotides containing 5-methylisocytosine-guanine and/or isoguanine-cytosine base pairs were prepared. The coupling efficiency was higher than 99% when the phosphoramidites of 2'-deoxy-5-methylisocytidine (2a) and of 2'-deoxyisoguanosine (3) were employed in solid-phase synthesis. The glycosylic bond of 2'-deoxy-5-methylisocytidine (1a) is stabilized against acid in the case of the amidine derivatives 4a,b. Depyrimidination occurs upon extended exposure to ammonia. The orientation of the parallel-stranded (ps) duplexes was unambiguously determined using the pyrene excimer fluorescence. The antiparallel(aps)duplex d(A_(12))centre dot d(T_(12))(25 centre dot 26) truned into the parallel orientation when two central adenine-thymine base pairs were replaced by isoguanine-cytosine pairs. The thermal stability of ps-duplexes containing 5-methylisocytosine-guanine base pairs was found to be significantly lowere than that of duplexes with isoguanine-cytosine pairs. According to the CD spectra, ps-DNA-DNA or DNA-RNA hybrids show differences in their secondary structures compared to their aps counterparts.