On the structure of poly d(A-S4T).

The helical twist of poly d(A-s4T) was determined from the periodicity of the cleavage patterns of the double stranded polydeoxynucleotide adsorbed on calcium phosphate and found to be 14 bp per turn. Both cleavage patterns and 31P NMR spectra indicate a mononucleotide structure rather than an alternating B DNA like poly d(A-T). The failure of nucleosome formation excludes a B type structure. The discrepancy of the mononucleotide structure found in 31P NMR spectra and the dinucleotide structure given by X ray fiber diffraction is explained by an alternating tilt of the planes of the base pairs (base roll) as a consequence of a strong propeller twist. The importance of interstrand stacking interactions of adjacent 4-thiothymidines for the helical stability is discussed.