| Abstract: | Two phosphorothioate analogues of polyd(A$-T)] have been synthesized enzymatically. In one, polyd(A$-T)], dTMP is replaced by thymidine 5'-O-phosphorothioate; in the other, polyd(T$-A)], dAMP is replaced by 2'-deoxyadenosine 5'-O-phosphorothioate. The 31P NMR spectrum of polyd-(A-T)] in solutions at low salt concentration shows two resonances at 51.80 and -4.25 ppm relative to trimethyl phosphate. The corresponding values for polyd(T$-A)] are 51.51 and -4.43 ppm. These data allow the assignment of the downfield resonance at -4.23 ppm in polyd(A-T)] to the phosphate group of d(TpA) and the resonance at -4.41 ppm to that of d(ApT). Thus, strong evidence is provided for a repeating dinucleotide structure. A comparison of the 31P NMR spectra of the various polymers in solutions of 2 M CsF reveals that both resonances are shifted upfield by approximately 0.9 ppm in the case of the phosphorothioates and by 0.2 or 0.4 ppm in the case of the phosphates. An upfield shift of about 0.18 ppm can also be observed for the two corresponding dinucleoside monophosphates. Thus, the upfield shift induced by high concentrations of CsF is not specific for the polymer backbone. |
