N1...N3 hydrogen bonds of A:U base pairs of RNA are stronger than those of A:T base pairs of DNA.

Trans-hydrogen-bond deuterium isotope effects of Watson-Crick A:U and A:T base pairs of 10 homologous RNA and DNA duplexes are compared. The isotope effect at 13C2 of adenosine residues due to deuterium/protium substitution at the imino H3 site, 2hDelta13C2, is larger in RNA than in DNA. The virtually consistent larger isotope effects in RNA suggest that the N1...N3 hydrogen bonds of A:U base pairs of RNA are stronger than those of the A:T base pairs of DNA