A CONFORMATIONALLY HOMOGENEOUS COMBINATORIAL PEPTIDE LIBRARY

In search for a rational way to convert the information encoded in pep tide structures into peptidomimetics, major progress could be made by coupling the power of selection methods, now enormously increased in n umber as a result of the development of combinatorial peptide librarie s, with the rational design of structure-inducing templates for the se lectable sequences. The availability of libraries of peptides with pre determined structure would enable selection-driven peptidomimetic desi gn, whereby a conformational model for the peptide pharmacophore would be directly derived from the screening, allowing the design of a suit able non-peptidic scaffold to replace the peptide backbone. We describ e here the first example of a conformationally homogeneous combinatori al peptide library, which yields ligands with the expected structure u pon selection. The library was built by randomising five positions in the alpha-helical portion of a 26 amino acid Cys(2)His(2) consensus '' zinc-finger'' motif. Since in zinc-fingers metal coordination and fold ing are coupled, in our library metal-dependent binding represents a b uilt-in control against the selection of structurally undefined sequen ces. The alpha-helical library was produced as both fusion with the pV III protein of filamentous phage and soluble peptides by chemical synt hesis, the latter enabling the expansion of the selectable repertoire by the inclusion of non-coded amino acids. The two libraries were inde pendently screened with the same receptor (a monoclonal IgA reactive a gainst the lipopolysaccharide of the human pathogen Shigella flexneri) , yielding a very similar consensus. In particular, the peptides defin ed by both methods showed very strong, zinc-dependent binding to the I gA. The geometrical arrangement of the side-chains of the selected pep tide pharmacophore was shown by circular dichroism, Co(II)-complex abs orption and high-resolution NMR to be structurally invariant with resp ect to the parent zinc-finger.