Determination of the isomeric forms proportion of fluorogenic naphthalene-2,3-dicarboxaldehyde in a binary mixture of water:methanol using electrochemical methods

•Naphthalene-2,3-dicarboxyaldehyde exists in three isomeric forms in aqueous solution.•We propose a procedure for determining the concentration for each NDA isomeric form.•The study underlines that the proportion of each of them varies according to the pH.•We evidence why at pH 9 for the labelling of biomolecules is more efficient.

The electrochemical response of the fluorogenic label naphthalene-2,3-dicarboxyaldehyde (NDA) in a binary mixture of water/methanol was characterized with cyclic voltammetry (CV) and differential pulse voltammetry (DPV) electrochemical techniques. Naphthalene-2,3-dicarboxyaldehyde does exist in three isomeric forms in aqueous solution: the unhydrated dialdehyde (DA), the acyclic monohydrated (MA) and the cyclic hemiacetal (HAC). The study underlines that the proportion of each of them varies according to the working pH. At low and high pH, the dialdehyde form is in larger proportion than the acyclic monohydrated form. Conversely at intermediate pH, the concentration of the acyclic form is in greater proportion than the dialdehyde form. These results allowed us to determine the optimal pH of 9 for which the labeling of biomolecules could be more efficient due to the base catalyzed regeneration of the unhydrated form. At this pH, the data processing from the analysis of measured currents and estimation of diffusion coefficients of each form according to the semi-empirical models of Wilke–Chang, Scheibel, Reddy–Doraiswamy and Lusis–Ratcliff allowed us to obtain the concentration of dialdehyde (0.28 mM), acyclic monohydrated (0.57 mM) and cyclic hemiacetal monohydrated (0.15 mM) forms starting from 1 mM naphthalene-2,3-dicarboxyaldehyde.

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