Stereoelectronic effects of modified purine bases on the sugar conformation of nucleosides: pyrrolo[2,3-d]pyrimidines

Abstract

Steric and stereoelectronic effects of 7- and 8-substituents of 7-deaza-2′-deoxy-adenosine and -guanosine on the dynamic sugar puckering as well as on the conformation about the C(4′)–C(5′) bond have been studied. (i) The higher the electron-attracting effect of the substituents, the more the N⇌S (North⇌South) equilibrium of the pentofuranose moiety is biased towards the N-conformation. (ii) The higher the electron-withdrawing effect of the 7-substituents, the higher the γ^(+)g (+sc) rotamer population around the C(4′)–C(5′) bond. The interdependencies of the conformational equilibria of 7-deazapurine nucleosides and their influence on oligonucleotide secondary structures are discussed.