An 15N NMR study of adenine-uracil base pair in a non-aqueous solvent

[1,3,7,9,10-15N]-2',3',5'-Tri-O-acetyl adenosine (A) and its 8-D and 8-Br derivatives (AD and ABr) were prepared from 95% 15N enriched adenosine obtained from microbial fermentation. The chemical shifts and nuclear Overhauser effects of 15N resonances were measured as a function of the concentration of the mixed 1-cyclohexyluracil. The limiting shift of each 15N resonance was calculated and the structure of the A-U dimer was estimated. From the shifts of 15N-1 and 15N-7 signals it is determined that ABr-U dimers prefer the Watson-Crick type hydrogen bond while the Hoogsteen type pairs are predominant in the A-U dimers.