Abstract
AbstractMechanisms of the formation of the fluorescent species widely used for determination of amino acids, based on reaction with orthophthalaldehyde (OPA) were unknown. Attempts to elucidate this mechanism based solely on product identification were unsuccessful. Analytical methods based on reaction of OPA with amino acids in the presence of RS− or CN− ions were developed empirically. The reaction conditions reported in more than 700 papers differed considerably. In aqueous solutions OPA exists in three different forms in equilibrium – the unhydrated form Ia (20%), monohydrated acyclic form Ib (8%) and cyclic hemiacetal form Ic (72%). These forms are in equilibria, the rate of establishment of which is acid and base catalyzed. Reactions of OPA and NH3 and 2‐aminoethanol (colamine) take place in a series of equilibria depicted in Scheme 1. The isoindole derivatives are formed in cyclization of the carbinolamine. Formation of an imine is a side reaction.
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