Limited proteolysis of the scrapie prion protein (PrPSc) generates PrP 27-30, which polymerizes into amyloid. By attenuated total reflection-Fourier transform infrared spectroscopy, PrP 27-30 polymers contained 54% beta-sheet, 25% alpha-helix, 10% turns, and 11% random coil; dispersion into detergent-lipid-protein-complexes preserved infectivity and secondary structure. Almost 60% of the beta-sheet was low-frequency infrared-absorbing, reflecting intermolecular aggregation. Decreased low-frequency beta-sheet and increased turn content were found after SDS/PAGE, which disassembled the amyloid polymers, denatured PrP 27-30, and diminished scrapie infectivity. Acid-induced transitions were reversible, whereas alkali produced an irreversible transition centered at pH 10 under conditions that diminished infectivity. Whether PrPSc synthesis involves a transition in the secondary structure of one or more domains of the cellular prion protein from alpha-helical, random coil, or turn into beta-sheet remains to be established.