NAD+

NAD+ is a dinucleotide coenzyme of molecular formula C₂₁H₂₇N₇O₁₄P₂, with an average molecular mass of 663.43 Da and a monoisotopic mass of 663.1091 Da. It is not a peptide and contains no amino acid residues, so the sequence and residue-count fields are empty. Its architecture is two nucleotides joined tail to tail: an adenine base on a ribofuranose ring, and a nicotinamide base on a second ribofuranose ring, the two sugars bridged by a pyrophosphate linkage that accounts for both phosphorus atoms in the formula. The glycosidic bond at the nicotinamide ring is beta-configured, which is the distinction carried by the beta-NAD synonym; the alpha anomer is a separate compositional isomer of identical formula and mass and is resolved chromatographically rather than by mass.

The nicotinamide ring in this compound is the quaternised, oxidised pyridinium form, and that is what separates it analytically from the reduced form. Reduction adds two hydrogen atoms to give C₂₁H₂₉N₇O₁₄P₂ at 665.45 Da average mass, a difference of 2.02 Da that is trivially resolved by mass spectrometry. The two forms are more easily distinguished still by ultraviolet spectroscopy: both absorb near 259 nm through the adenine chromophore, but only the reduced form carries the dihydronicotinamide band near 340 nm. The ratio of absorbance at 259 nm to that at 340 nm is therefore the routine purity criterion for the oxidised form, in which the longer-wavelength band should be essentially absent.

A related coenzyme, the 2'-phosphorylated derivative, carries one additional phosphate group on the adenosine ribose, giving C₂₁H₂₈N₇O₁₇P₃ at 743.41 Da average mass. The 79.98 Da increment separates the two unambiguously. In this compound class purity is generally assessed by ion-pairing or anion-exchange reversed-phase HPLC with detection at 259 nm, identity by mass spectrometry against the calculated monoisotopic mass, and the oxidation state by the ultraviolet band ratio described above.

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