Why does TNA cross-pair more strongly with RNA than with DNA? An answer from X-ray analysis

The TNA twist: L-α-threofuranosyl (3′↔2′) nucleic acid (TNA) residues adopt a C4′-exo pucker when incorporated into an A-(left) or a B-form DNA duplex (right). The resulting intranucleotide P⋯P distance in TNA is very similar to that in RNA (represented by a C3′-endo puckered adenosine residue; green). The structural data explain earlier observations that TNA hydridizes more stably with RNA than with DNA and that RNA constitutes the better template for ligating TNA fragments.