Use of phosphorothioate analogs of poly(dA-dT).poly(dAdT) to study steroidal-diamine induced conformational change in poly(dA-dT).poly(dA-dT).

Introduction of phosphorothioate groups into the backbone of poly(dA-dT) allows one to label the d(ApT) and d(TpA) phosphate resonances in the 31P NMR spectrum. Upon binding the steroidal diamine dipyrandenium to poly d(AsT) and poly d(TsA), 31P NMR shows that it is the d(ApT) phosphodiester bond which is most perturbed. Other work has shown that 2 M Cs+ causes the same 31P shift. The DNA conformational change induced by both cations probably involves a narrowing of the minor groove.