Acid–base properties of the reaction product of sialic acid with fluorogenic reagent, 1,2-diamino-4,5-methylenedioxybenzene (DMB)

N-Acetylneuraminic acid (Neu5Ac) forms the highly fluorophoric quinoxalinone derivative (Q) when treated with 1,2-diamino-4,5-methylenedioxybenzene (DMB). Effects of protonation and deprotonation on the fluorescence of Q were examined at room temperature. The strong fluorescence was found to be caused by the neutral form Q but not the protonated form of its excited state [Q]* and at pH below 1 the emission was completely quenched. The deprotonated singlet form [Q−]* was a less efficient fluorescer than [Q]*.