Automated solid-phase synthesis of cyclic peptides bearing a side-chain tail designed for subsequent chemical grafting

Recent developments in allyl chemistry and palladium solubilization allow automated continuous-flow solid-phase synthesis of cyclic or branched peptides, with specific side-chain cleavage and on-line cyclization. In this paper, we adapted the method to the synthesis of cyclic peptides bearing an anchoring tail on a side chain of the cycle. Side products were obtained with the standard procedure and an additional washing step had to be introduced in the synthesis protocol to remove side products resulting from the palladium allyl cleavage step. The method is illustrated by the automated synthesis of cyclo[-DVal-Arg-Gly-Asp-Glu (-εAhx-Cys-NH₂)-] which contains the Arg-Gly- Asp adhesion motif (RGD) recognized by cellular integrins. The tail of the peptide was designed with a thiol at the carboxylic end to allow subsequent grafting by covalent attachment. Such tailed cyclic peptides can be grafted on different supports for new applications in biomaterial design, cell adhesion assays, affinity chromatography, immunization, vaccine development, ELISA kits, and the building of libraries of conformationally constrained peptides.