Synthesis and Biophysical Characterization of Engineered Topographic Immunogenic Determinants with αα Topology

Model peptides with predetermined secondary, tertiary, and quaternary conformation have been successfully designed, synthesized, and characterized in an attempt to mimic the three-dimensional structure of an antigenic determinant. This work is a continuing effort to map the antigenic structure of the protein antigen lactate dehydrogenase C₄ (LDH-C₄) to develop a contraceptive vaccine. A putative topographic determinant with αα topology which associates into four-helix bundles was designed on the basis of the framework model of protein folding. An idealized amphiphilic 18-residue sequence (α₁) and a 40-residue αα fold (a₃) have been shown to form stable 4-helix structures in solution with a free energy of association on the order of-20.8 kcal/mol (tetramerization of α₁) and −7.8 kcal/mol (dimerization of α₃). Both α₁ and α₃ form stable monolayers at the air-water interface. The CD spectra of Langmuir-Blodgett monolayers are characteristically α-helical. Both CD and FTIR spectroscopic studies reveal a high degree of secondary structure. The SAXS data strongly suggest that the helices are arranged in a four-helix bundle since the radius of gyration of 17.2 Å and the vector distribution function are indicative of a prolate ellipsoid of axial dimensions and molecular weight appropriate for the four-helix bundle. The major contribution to the formation and stabilization of α₁ and α₃ is believed to be hydrophobic interaction between the amphiphilic α-helices. The displayed heptad repeat, helix dipole, ion pairs, and the loop sequence may have also contributed to the overall stability and antiparallel packing of the helices. A detailed structural analysis of a relevant topographic immunogenic determinant will elucidate the nature of antigen-antibody interactions as well as provide insight into protein folding intermediates.