A cyclic peptide analogue of the loop III region of platelet-derived growth factor-BB is a synthetic antigen for the native protein

We report the synthesis and characterization of a cyclic peptide analogue of the loop III region of platelet‐derived growth factor (PDGF) B‐chain sequence, cyclo(73Arg‐Lys‐lle‐Glu‐lle‐Val‐Arg‐Lys‐Lys81‐Cys), incorporating a C‐terminus cysteine residue for the conjugation to a carrier protein. The synthesis involved solid‐phase chemistry, utilizing Fmoc‐tBu chemistry and acid labile side‐chain protecting groups, followed by ‘head‐to‐tail’cyclization using the allyl‐protected glutamic acid anchored on its side chain to the solid support with HATU/HOAt as the coupling agent. Conformational differences between the cyclic and its linear counterpart PDGF peptides were determined by circular dichroism measurements in aqueous media. High titre antisera were raised to both cyclic and linear peptide immunogens. Antisera raised to the cyclic peptide cross‐reacted with PDGF‐BB in both Western blot and ELISA, whereas antisera raised to the linear peptide had no reactivity with PDGF‐BB. The cyclic peptide (conformational design analogue) produces an immunogen which is able to antigenically mimic the secondary structure of loop III of PDGF‐BB and forms a basis from which further small molecular mimetics of PDGF may be designed for use as both immunogens and also potential agonists/antagonists of PDGF. Similarly constructed immunogens may also be useful in the design of vaccines which direct responses to loop regions in other target proteins.