Synthetic route for heterobivalent ligands. The inset on left, top shows addition of PEGO sequence. The inset on bottom, right shows the structure of protected Ac-MSH-7 ligand on resin. The reagents for each step are as follows—(i) Fmoc-amino acid-OH (3 eq), Cl-HOBt (3 eq), diisopropylcarbodiimide (6 eq); (ii) 20% Piperidine/DMF; (iii) PEGO linker assembly; refer text for details. (iv) 50% Ac2O in Pyridine; (v) TFA (91%), H2O (3%), Thioanisole (3%), Triisopropylsilane (3%) m is 1 or 2; n is 3, 6, 9, 12, 15