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

Abstract

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 (A^spU°) (III) and 6, 2'-anhydro-6-oxy-1-β-D-arabinofuranosyluridylyl-(3'-5')-8, 2'-anhydro-8-thio-9-β-D-arabinofuranosyladenine (U°pA^s) (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.