PYRANOSYL-RNA (P-RNA) - BASE-PAIRING SELECTIVITY AND POTENTIAL TO REPLICATE - PRELIMINARY COMMUNICATION

Base pairing in p-RNA (beta-D-ribopyranosyl-(4'--> 2')-oligonucleotide s) is not only stronger than in DNA and RNA, but also more selective i n the sense that it is strictly confined to the Watson-Crick mode. Hom opurine sequences (tested up to decamers) exist as single strands unde r conditions where they undergo reverse-Hoogsteen self-pairing in homo -DNA or Hoogsteen seif-pairing in DNA. This exceptional pairing select ivity is rationalized as hinging on two structural features of p-RNA: the large inclination between backbone axis and base-pair axes in p-RN A duplexes, and the higher rigidity of the p-RNA backbone compared wit h RNA, DNA, and homo-DNA. The most important consequence of the pairin g selectivity refers to the potential of p-RNA to replicate. Replicati ve copying of sequence information by nonenzymatic template-controlled ligation is not hampered by self-pairing of guanine-rich templates, a s it is known to be the case in the RNA series. We have demonstrated t wo replicative cycles in which G-rich p-RNA-octamer templates induce s equence-selective ligation of tetramer-2'-phosphate derivatives to com plementary C-rich octamer sequences, and in which the latter, with com parable efficiency, induce corresponding ligation reactions back to th e original G-rich octamers. Ligation is most satisfactorily achieved a fter pre-activation of the 2'-phosphate groups as 2',3'-cyclophosphate derivatives; in this version, the process does not proceed as oligoco ndensation, 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.