The 5-Methylisocytosine β-D-Ribopyranosyl (4′ 2′)-Oligonucleotide

In the context of Eschenmoser's work on pyranosyl-RNA (‘p-RNA’), we investigated the synthesis and base-pairing properties of the 5-methylisocytidine derivative. The previously determined clear-cut restrictions of base-pairing modes of p-RNA had led to the expectation that a 5-methylisocytosine β-D-ribopyranosyl (= D-pr(MeisoC)) based (4′  2′)-oligonucleotide would pair inter alia with D-pr(isoG) and L-pr(G) based oligonucleotides (D-pr and L-pr = pyranose form of D- and L-ribose, resp.). Remarkably, we could not observe pairing with the D-pr(isoG) oligonucleotide but only with the L-pr(G) oligonucleotide. Our interpretation concludes that this – at first hand surprising – observation is caused by a change in the nucleosidic torsion angle specific for isoC.