Nuclear magnetic resonance studies on pyridine dinucleotides: The pH dependence of the carbon-13 nuclear magnetic resonance of NAD + analogs

The pH dependence of the 13C chemical shifts for nicotinamide adenine dinucleotide (NAD +), thionicotinamide adenine dinucleotide (TNAD +), pyridine adenine dinucleotide (PyrAD +), N-methyl-nicotinamide adenine dinucleotide (N-Me-NAD +), acetylpyridine adenine dinucleotide (AcPyAD +), nicotinamide hypoxanthine dinucleotide (NHD +), and nicotinamide adenine dinucleotide phosphate (NADP +) are reported. In these analogs the 13C chemical shifts of the pyridinium moiety reflect the p K a of the opposing purine base, while the 13C chemical shift dependence on pD for the pyridinium carbons of nicotinamide mononucleotide (NMN +) and adenosine monophosphate (AMP), 1,4-dihydronicotinamide adenine dinucleotide (NADH), 1,4-dihydronicotinamide adenine dinucleotide phosphate (NADPH), and nicotinic acid adenine dinucleotide (N(a)AD +) are not influenced by the adenine ring in the pD range tested. Through the use of 13C-labeled NAD +, the source of the pH dependence of the 13C chemical shifts was shown to be intramolecular in origin. However, serious doubt is cast on the utility of employing the pD dependence of chemical shift data to determine the nature of solution conformers or their relative populations.