Studies of tautomers and protonation of adenine and its derivatives by nitrogen-15 nuclear magnetic resonance spectroscopy

High-resolution nitrogen-15 NMR spectra of adenine have been obtained both at the S-adenosylmethionine level for a uniformly enriched sample prepared from S-adenosylmethionine isolated from yeast grown with ^(15)NH_4CI as the principal nitrogen source. Specific ^(15)N labeling at N1, N3, N6’, N7, and N9 provided unequivocal assignments of the chemical shifts as well as elucidation of the position of the tautomeric equilibrium. For comparison, the nitrogen resonances of several adenine derivatives were determined at the natural-abundance level. The protonation sites of many of these substances were from the effect of pH on the nitrogen chemical shifts. Both adenine and its conjugate acid clearly exist as the N9-H tautomers in aqueous solution. Conversion of adenine to its conjugate base (pK, ~ 10) results in a 56 ppm downfield shift of the N9 resonance.