PHOSPHOROTHIOATE OLIGONUCLEOTIDE-DIRECTED TRIPLE-HELIX FORMATION

Phosphorothioate oligodeoxyribonucleotides were tested for their abili ty to recognize double-helical DNA in two distinct triple helix motifs . Purine-rich oligonucleotides containing a diastereomeric mixture of phosphorothioate or stereoregular (all R(p)) phosphorothioate linkages are shown to form triple-helical complexes with affinities similar to those of the corresponding natural phosphodiester oligonucleotides. I n contrast, pyrimidine-rich phosphorothioate oligonucleotides containi ng a mixture of diastereomeric or stereoregular (all R(p)) linkages do not bind to double-helical DNA with measurable affinity. These observ ations have implications for triple helix structure and for biological applications.