o-Phthalaldehyde is the chemical compound with the formula C6H4(CHO)2. Often abbreviated OPA, the molecule is a dialdehyde, consisting of two formyl (CHO) groups attached to adjacent carbon centres on a benzene ring. This pale yellow solid is a building block in the synthesis of heterocyclic compounds and a reagent in the analysis of amino acids. OPA is well soluble, and stable in water solution at pH < 11.5. It is however sensitive to UV illumination and air oxidation. OPA is used as a very sensitive fluorescent reagent for assaying amines or sulfhydryls in solution, notably contained in proteins, peptides, and amino acids, by capillary electrophoresis and chromatography. OPA reacts specifically with primary amines above their isoelectric point Pi in presence of thiols. OPA reacts also with thiols in presence of an amine such as n-propylamine or 2-aminoethanol.
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