7-Deaza-2'-deoxyadenosine and 3-deaza-2'-deoxyadenosine replacing dA within d(A6)-tracts: differential bending at 3'- and 5'-junctions of d(A6).d(T6) and B-DNA.

7-Deaza-2'-deoxyadenosine (1, c7Ad) and 3-deaza-2'-deoxyadenosine (2, c3Ad) have been incorporated into d(AAAAAA) tracts replacing dA at various positions within oligonucleotides. For this purpose suitably protected phosphonates have been prepared and oligonucleotides were synthesized on solid-phase. The oligomers were hybridized with their cognate strands. The duplexes were phosphorylated at OH-5' by polynucleotide kinase and self-ligated to multimers employing T4 DNA ligase. Oligomerized DNA-fragments were analyzed by polyacrylamide gel electrophoresis and the bending was determined from anomalies of electrophoretic mobility. Replacement of dA by c3Ad decreased the bending more than replacement by c7Ad. Reduction of bending was much stronger when the modified nucleosides replaced one or several dA residues at the 3'-site of an d(AAAAAA)-tract whereas replacement at the 5'-site showed no significant influence [1, 2].