Stabilization of triple-stranded oligonucleotide complexes: use of probes containing alternating phosphodiester and stereo-uniform cationic phosphoramidate linkages.

Pyrimidine oligonucleotides containing alternating anionic and stereo-uniform cationic N-(dimethylamino-propyl)phosphoramidate linkages [e.g. d(T+T-)7T, d(T+T-)2(T+C-)5T and (U'+U'-)7dT, where U' is 2'-O-methyluridine)] are shown to bind to complementary double-stranded DNA segments in 0.1 M NaCl at pH 7 to form triple-stranded complexes with the pyrimidine.purine.pyrimidine motif. For each of the sequences investigated, one stereoisomer bound with higher affinity, and the other stereoisomer with lower affinity, than the corresponding all-phosphodiester oligonucleotide. The stereoisomer of d(T+T-)7T that interacted weakly with a dT.dA target in 0.1 M NaCl formed a novel dA.dA.dT triple-stranded complex with poly(dA) or d(Al5C4A15) in 1 M NaCl; in contrast, the stereoisomer that bound strongly to the dT.dA target failed to form a dA.dA.dT triple-stranded complex.