4-Nitro-1,8-naphthalic anhydride (NNA) was used to distinguish cysteine from homocysteine and other potentially interfering thiols through a novel sequential substitution mechanism. The discrimination involves a blue-fluorescent thioether formation via nucleophilic aromatic substitution of the nitro group by thiol, followed by a second intramolecular nucleophilic aromatic substitution of alkylthio with the amino group to give the green-fluorescent 4-amino derivative. NNA is highly selective towards Cys, and the detection limit of Cys by this method is 0.3 μM.
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