WHY PENTOSE-NUCLEIC AND NOT HEXOSE-NUCLEIC ACIDS .7. PYRANOSYL-RNA (P-RNA)

Qualitative conformational analysis of the entirety of conceivable hexo- and pentopyranosyl oligonucleotide systems derived from the diastereoisomeric aldohexoses (CH2O)6 and aldopentoses (CH2O)5 predicts the existence of a variety of pairing systems which have not been experimentally investigated so far. In particular, the analysis foresees the existence of a ribopyranosyl isomer of RNA ('p-RNA'), containing the phosphodiester linkage between the positions C(4') and C(2') of neighboring ribopyranosyl units. Double strands of p-RNA oligonucleotides are expected to have a linear structure and to show purine-pyrimidine and purine-purine (Watson-Crick) pairing comparable in strength to that observed in homo-DNA. Experimentally, synthetic beta-D-ribopyranosyl (4'-2')-oligonucleotides derived from adenine and uracil confirm this prognosis: adenine-uracil pairing in p-RNA duplexes is stronger than in the corresponding RNA duplexes. Importantly, adenine in p-Ribo(A8) does not show (reverse-Hoogsteen) self-pairing, in sharp contrast to its behavior in the homo-DNA series. The sheer existence of strong and selective pairing in a system that is constitutionally isomeric to RNA and can be predicted to have a linear structure has implications for the problem of RNA's origin. In this context, a comprehensive experimental study of the pairing properties of p-RNA, of its potential for constitutional assembly, self-replication, and intra-duplex isomerization to RNA seems mandatory.