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.