Parallel and antiparallel triple helices with G,A-containing third strands

A triple helix, formed by a 13 nucleotide (nt) all-purine oligonucleotide, containing six contiguous guanines, oriented parallel to a homopurine stran d present in the polypurine tract of Friend leukemia virus, was obtained in 0.1 M LiCl. Its dissociation constant at 25 degrees C, given by electropho retic titration, of the order of 50 nM, is at least ten times lower than th at of the corresponding antiparallel tripler formed on the same target. At 4 degrees C, the parallel orientation of the homopurine strands is favored to the point that the guanine block of 6 nt, present in the 'antiparallel' oligonucleotide, attaches in a parallel fashion to the corresponding block in the target strand, to generate a partial, parallel tripler, that coexist s with the antiparallel one.