3-(4-Carboxybenzoyl)quinoline-2-carboxaldehyde

3-(4-Carboxybenzoyl)quinoline-2-carboxaldehyde (CBQCA) is a fluorogenic amine labeling dye that is not fluorescent itself, but covalently reacts with primary amines to form fluorescent products. It was first reported in 1991.

CBQCA is largely used in the context of quantifying peptides or proteins. Either cyanide or thiols are required as a co-substrate in the fluorogenic reaction, although thiols also react with and mask the CBQCA aldehyde thereby preventing the fluorogenic reaction against the targeted primary amines. Once bound to protein the excitation wavelength is 465 nm (blue) and the emission wavelength is ~550 nm (green).

Reaction
CBQCA, which is non-fluorescent, reacts with primary amines in the presence of cyanide to form fluorescent products: