Studies on the interaction of adenine and nicotinamide ring systems in aqueous solution by high resolution nuclear magnetic resonance.

A Model system for NAD+ has been investigated using a paramagnetic transition metal ion as a probe. The well-known complexation of Mn2+ by adenine nucleotides was utilized to "label" adenosine 5'-diphosphate. A broadening effect on the 100-MHz proton nuclear magnetic resonance spectrum of N1-methylnicotinamide due to the adenine-metal ion complex was observed. It was found that the nicotinamide species showed no evidence for interaction with Mn2+ in the absence of the adenine nucleotide. These observations have led to the proposal that N1-methylnicotinamide associates with the adenine moiety of the adenine nucleotide-metal complex. This suggests a tendency of adenine and nicotinamide rings to interact in aqueous solution implying some tendency of the coenzyme NAD+ to occur in a folded or stacked conformation.