Different behavior of the octadeoxynucleotides d(A-T)4 and d(T-A)4 at high concentrations of cesium fluoride

High CsF concentrations induce a zig-zag double helix, which we call X-DNA, in poly d(A-T) and also in the octadeoxynucleotide d(T-A)4 while d(A-T)4 remains fixed in a B-DNA form. Intermolecular contacts promote the B-X isomerization of the former oligonucleotide but induce aggregation of the latter. This indicates that there is an intramolecular factor, presumably base stacking in the T-A steps, stabilizing the X-DNA conformation.