Enhancing peptide antigenicity by helix stabilization.

The engineering of antigenic determinants on super secondary structures using de novo design approaches often involves synthesis of long peptide chains (35-80 residues long). This communication illustrates that the stabilization of secondary structure by rational design can also greatly enhance immunogenicity and antigenicity, but in much shorter peptide sequences (21 residues long). A peptide epitope the sequence of which has been derived from the C-terminus of the chicken riboflavin carrier protein (cRCP), H2N-Tyr-His-Ala-Cys-Gln-Lys-Lys-Leu-Leu- Lys-Phe-Glu-Ala-Leu-Gln-Gln-Glu-Glu-Gly-Glu-Glu-OH, has been chosen for analysis. Helical conformations were induced in the peptide in aqueous trifluoroethanol. Analogs were designed to stabilize this conformation in water by either the introduction of appropriately spaced ion pairs or the strongly helix nucleating residue alpha-aminoisobutyric acid (Aib), substituted for Ala/Gly, thus affording a comparison of the helix stabilization strategies. Circular dichroism (CD) results demonstrate that all the designed analogs are appreciably more helical than the parent peptide in 50% aqueous trifluoroethanol. Peptide antisera were raised for all analogs in rabbits. The affinities of these antisera for the native protein antigen, determined using a chaotrope disrupted binding assay, correlated very well with the helix content determined by CD.