Selective 'in synthesis' labeling of peptides with biotin and rhodamine

A new method is described for the selective 'in synthesis' labeling of peptides by rhodamine or biotin at a single, predetermined epsilon-amino group of a lysine residue. The alpha-amino group and other lysyl residues of the peptide remain unmodified. Peptides are assembled by the Fmoc approach, which requires mild operative conditions for the final deprotection and cleavage, and ensures little damage of the reporter group. The labeling technique involves the previous preparation of a suitable Lysine derivative, easily obtained from commercially-available protected amino acids. This new derivative, where the reporter group (biotin, or rhodamine) acts now as permanent protection of lysyl side chain functions, is then inserted into the synthesis program as a conventional protected amino acid, and linked to the preceding residue by aid of carbodiimide. A simpler, alternative method is also described for the selective 'in synthesis' labeling of peptides with N-terminal lysyl residues. Several applications of labeled peptides are reported.