The influence of modified purine bases on the stability of parallel DNA

The stability of the parallel-stranded (ps) DNA duplexes is increased when the dA-residues are replaced by the 7-substituted 7-deaza-2′-deoxyadenosine derivatives 3a,b or the dG-residues by the 8-aza-7-deazapurine 2′-deoxynucleosides 6 and 7a,b. Also the N-7-glycosylated adenine 5 forms stable base pairs in ps-DNA while it destabilizes oligonucleotide duplexes with antiparallel chain orientation. The presence of a 2-amino group as in compound 4b is critical for the DNA-structure, leading to a much greater destabilization of the ps-hybrids than of aps-DNA.