Pyranosyl-RNA ('p-RNA'): base-pairing selectivity and potential to replicate

A review and discussion, with 42 refs. Base pairing in p-RNA (b-D-ribopyranosyl-(4'->2')-oligonucleotides) is not only stronger than in DNA and RNA, but also more selective in the sense that it is strictly confined to the Watson-Crick mode. Homopurine sequences (tested up to decamers) exist as single strands under conditions where they undergo reverse-Hoogsteen self-pairing in homo-DNA or Hoogsteen self-pairing in DNA. This exceptional pairing selectivity is rationalized as hinging on 2 structural features of p-RNA: the large inclination between backbone axis and base-pair axes in p-RNA duplexes, and the higher rigidity of the p-RNA backbone compared with RNA, DNA, and homo-DNA. The most important consequence of the pairing selectivity refers to the potential of p-RNA to replicate. Replicative copying of sequence information by nonenzymic template-controlled ligation is not hampered by self-pairing of guanine-rich templates, as it is known to be the case in the RNA series. Two replicative cycles are demonstrated in which G-rich p-RNA-octamer templates induce sequence-selective ligation of tetramer-2'-phosphate derivs. to complementary C-rich octamer sequences, and in which the latter, with comparable efficiency, induce corresponding ligation reactions back to the original G-rich octamers. Ligation is most satisfactorily achieved after pre-activation of the 2'-phosphate groups as 2',3'-cyclophosphate derivs.; in this version, the process does not proceed as oligocondensation, but as a genuine oligomerization. This is of considerable promise for the search for potentially natural conditions under which homochiral p-RNA strands might self-assemble and self-replicate. [on SciFinder (R)]