Ligand precursors for incorporation into peptides

Compositions and methods of incorporating ligand precursors and ligands at any location within a peptide during peptide synthesis are disclosed. Derivatives of 2,4,5-triaminopentanoic acid and gamma -aminoglutamic acid are selectively incorporated into the peptide during solid phase or liquid phase synthesis, depending upon the choice of protecting groups. Ligand synthesis may then be completed at a later time to produce N3S, N2S2, and EDTA type chelating agents.