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.
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