Conversion of the non-infectious, cellular form of the prion protein (PrP^C) to the infectious form (PrP^Sc) is thought to be driven by an α-helical to β-sheet conformational transition. To reveal the sequence determinants which encourage the transition to β-fold, we study the synthetic peptides associated with hydrophobic conserved fragments of the N-terminal region of the prion protein. The structure of peptides in solution was probed under various thermodynamic conditions employing circular dichroism and steady state fluorescence spectroscopy as well as dye binding assays. The fluorescence methods utilized pyrene residues covalently attached to the end of the model peptides. In aqueous solutions, the structure assessments indicate the formation of metastable peptide aggregates; the molecular conformations within the peptide micelles are largely coiled. This stage in molecular assembly exists without significant β-strand formation, i.e., before the appearance of any ordered secondary structure detectable by circular dichroism. At moderate concentrations of trifluoroethanol and/or acetonitrile, the conformational ensemble shifts towards β-strand formation, and the population of the amorphous aggregates decreases significantly. Overall, the present data indicate that hydrophobic interactions between side chains of the peptide variants prevent, in fact, the formation of the rigid β-sheet structures. Encouragement of β-folds requires the destabilization of local interactions in the peptide chain, which in vivo might be possible within cell membranes as well as within partly folded molecular forms.

Key words: Prion diseases, Fluorescence, Pyrene, Conformational polymorphism, Prion protein, Synthetic peptide, β-pleated sheet.