Selective 'in synthesis' labelling of peptides by fluorochromes

A new method is described for producing fluorescently-tagged peptides containing specific internal derivatives of lysyl residues. The technique employs the base-labile Boc-Lys(Fmoc)-COOH derivative with base-catalyzed removal of the Fmoc protecting group during peptide synthesis and subsequent fluorescent derivatization of the deprotected epsilon-amino group of lysine. By this technique, other lysine residues and the alpha-amino group of the fragment remain unmodified, which could have some value in studies where it might be required to tag a single individual lysine residue within the peptide, but not the amino terminus. In spite of the fact that poly-substituted peptides are badly soluble and might seldom find a practical application, this technique also allows the introduction of different fluorochromes at different lysyl residues within the peptide, thus obtaining double fluorescence. The method, fast and easy, requires a limited number of manual operations during the automatic synthesis of peptides. Although peptide synthesizers provided with an oscillating glass reactor are more suitable for the manual interventions described, this technique might be also adapted to the newer instruments utilizing continuous-flow columns.