Difference in conformational diversity between nucleic acids with a six‐membered ‘sugar’ unit and natural ‘furanose’ nucleic acids

Natural nucleic acids duplexes formed by Watson–Crick base pairing fold into right‐handed helices that are classified in two families of secondary structures, i.e. the A‐ and B‐form. For a long time, these A and B allomorphic nucleic acids have been considered as the ‘non plus ultra’ of double‐stranded nucleic acids geometries with the only exception of Z‐DNA, a left‐handed helix that can be adopted by some DNA sequences. The five‐membered furanose ring in the sugar–phosphate backbone of DNA and RNA is the underlying cause of this restriction in conformational diversity. A collection of new Watson–Crick duplexes have joined the ‘original’ nucleic acid double helixes at the moment the furanose sugar was replaced by different types of six‐membered ring systems. The increase in this structural and conformational diversity originates from the rigid chair conformation of a saturated six‐membered ring that determines the orientation of the ring substituents with respect to each other. The original A‐ and B‐form oligonucleotide duplexes have expanded into a whole family of new structures with the potential for selective cross‐communication in a parallel or antiparallel orientation, opening up a new world for information storage and for molecular recognition‐directed self‐organization.