NBD-Cl is a nonfluorescent reagent which becomes highly fluorescent after reaction with thiol or amino groups. In Vitro: NBD-Cl (NBD chloride) forms highly fluorescent derivatives for detection of all the protein amino acids. In addition, NBD-Cl provides a simple and sensitive method for determination of N-terminal amino acids. The differences in intensity and color of fluorescence can be used to advantage to identify prolyl peptides^[1].