Synthesis and application of a fluorescent imido ester for specific labelling of amino groups in proteins.

2-(5'-Dimethylaminonaphthalene-1'-sulfonamido)methylimidic acid methyl ester has been synthesized for fluorescence labelling of amino groups in proteins. The incorporation of the dansyl group serving as an extrinsic fluorescent probe is determined spectrophotometrically. Glucose dehydrogenase (beta-D-glucose: NAD(P+) 1-oxidoreductase, EC 1.1.1.47) from Bacillus megaterium having a reactive lysine residue which belongs to the active site has been labelled. To give proof of the selectivity of the modification, the enzyme preparation having 1.3 dansyl groups per subunit has been digested with trypsin and the major labelled peptide has been isolated and sequenced.