l-α-Lyxopyranosyl (4‘→3‘) Oligonucleotides: A Base-Pairing System Containing a Shortened Backbone1

Abstract
The l-α-lyxopyranosyl (4‘→3‘) oligonucleotide system shows cooperative base-pairing in spite of containing only five instead of the usual six covalent bonds per repetitive backbone unit. In contrast, corresponding d-β-ribofuranosyl (4‘→3‘) oligonucleotides do not show adenine−thymine pairing under comparable conditions. The difference in pairing behavior relates to the conformation of the two systems' vicinal 3‘,4‘-phosphodiester substituents, which is diaxial in the lyxopyranosyl system and 3‘-axial-4‘-equatorial in the ribopyranosyl system.