A peptide reacts with glyoxalic acid resulting in transamination of the N-terminal residue to form a 2-oxoacyl group. This further reacts with o-phenylenediamine, leading to a quinoxaline derivative of the original N-terminal amino acid, which is cleavable in mild acid [Dixon & Fields (1972) Methods Enzymol. 25, 409-419]. The 2-oxoacyl peptides are weakly fluorescent with emission maxima around 410 nm and excitation maxima at about 320 nm, depending on the nature and length of the peptide. Formation of the quinoxaline derivative results in a marked increase of fluorescence, with emission maximum of 363 nm when excited at 303 nm. The fluorescence properties of these derivatives change with the nature and length of the peptides and are affected by the presence of organic solvents, NaCl and denaturants. It is suggested that such fluorescent derivatives could be used as probes for the study of the conformation of the N-terminal region of peptides and proteins.
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