Left-handed Helical Polynucleotides with D-Sugar Phosphodiester Backbones

Naturally occurring polynucleotides have right-handed helical confrontations in the solid state¹ and in solution². Poly(dI-dC)poly(dI-dC) was found to form a left-handed helix in spite of the D-sugar backbone. Also, L-adenylyl-(3′–5′)-L-adenosine synthesized by Tazawa et al⁴. takes up the left-handed stacked conformation. We had synthesized a dinucleoside monophosphate, 8,2′-anhydro-8-mercapto-9-β-D-arabinofuranosyladenine phosphoryl-(3′–5′)-8,2′-anhydro-8-mercapto-9-β-D-arabinofuranosyladenine (A^spA^s) (molecular structure Ia; see also ref. 5) and this compound has a left-handed stacked conformation. The two bases in Ia, having the D-sugar backbone, stacked along the left-handed helical axis; these bases are fixed at ?_CN = ?108° (syn-anti region) by the anhydro linkages.