Polynucleotides. LV. Synthesis and properties of dinucleoside monophosphates derived from adenine 8,2'-S- and uracil 6,2'-O-cyclonucleosides. Further support for the left-handed stacking of oligonucleotides having high-anti base torsion angles

Two sequence isomers of dinucleoside monophosphates, 8, 2'-anhydro-8-thio-9-β-D-arabinofuranosyladenylyl-(3'-5')-6, 2'-anhydro-6-oxy-1-β-D-arabinofuranosyluracil (AspU°) (III) and 6, 2'-anhydro-6-oxy-1-β-D-arabinofuranosyluridylyl-(3'-5')-8, 2'-anhydro-8-thio-9-β-D-arabinofuranosyladenine (U°pAs) (IV), were synthesized by condensation of suitably protected nucleoside and nucleotide units using dicyclohexyl carbodiimide as a condensing reagent. Examination of the UV, CD and NMR spectra of these dimers led us to the conclusion that, whereas compound III did not take a stacked conformation, compound IV took a well-stacked conformation, in which the bases were stacked along a left-handed screw axis. The adoption of this conformation could be interpreted in terms of the high base torsion angles in both nucleoside units.