Selective fluorescent labeling of amino groups of bovine pancreatic tryspin inhibitor by reductive alkylation

Bovine pancreatic trypsin inhibitor (BPTI) was reductively alkylated with 2‐methoxy‐1‐naphthyl aldehyde and sodium cyanoborohydride (NaCNBH₃). All five possible derivatives, each labeled at one of the primary amino groups of BPTI, were obtained. The distribution of yields of the various derivatives can be controlled by changing the reaction conditions. Products were identified by high‐performance liquid chromatography (HPLC) tryptic peptide mapping. This procedure was used for the preparation of three pure 2‐methoxy‐1‐naphthyl‐methylenyl‐BPTI (MNA‐BPTI) derivatives. Purification was achieved by means of affinity chromatography and HPLC. The spectral characteristics of the probe, notably monoexponential decay with a lifetime of 6.8 ± 0.1 ns and moderate limiting fluorescence polarization, P = 0.3 ± 0.015, make it a very useful donor in energy‐transfer measurements.