Investigation of the open ring form of nicotinamide adenine dinucleotide.

In strong alkali, nicotinamide adenine dinucleotide (NAD+) undergoes a ring opening of the nicotinamide ring. The open form of NAD+, ONAD, has two pKa values at--1.9 and 11.2 and absorbs maximally at 350 nm in its acidic form, at 372 nm in its neutral form, and at 340 nm in its aniomic form. ONAD has the chemical properties expected for a Schiff base of 2-carboxamideglutacondialdehyde (CGDA) and adenosine diphosphate ribosylamine. The decomposition of ONAD has been studied over a wide range of pH. A final product of ONAD hydrolysis is the base fluorescent compound 2-hydroxynicotinaldehyde. In the pH range 10--13, CGDA can be trapped as an intermediate, which absorbs maximally at 345 nm in its anionic form and at 320 nm in its neutral form, pKa = 2.9. The yield of 2-hydroxynicotinaldehyde from ONAD has been estimated as 95% at NaOH concentrations of 5 N and above, and is postulated to result from ring closure of CGDA. The pseudobase hydroxide ring addition adduct of NAD+, psiNAD-OH, is reversibly formed from NAD+ and is the 370-nm precursor of ONAD.