| Layered parallel beta-sheets are central to protein and peptide aggregation in numerous amyloid-related diseases. We have used a tau-protein-derived hexapeptide, AcPHF6, as a model system to study the aggregation and inhibition processes which involve layered parallel beta-sheet structures. This dissertation describes the studies of a series of macrocyclic beta-sheet peptides that inhibit the aggregation of AcPHF6. Macrocycles 1 containing the pentapeptide VQIVY in the "upper" strand delay and suppress the onset of AcPHF6 aggregation. Inhibition is particularly pronounced in macrocycles 1a, 1d, and 1f, in which the two residues in the "lower" strand provide a pattern of hydrophobicity and hydrophilicity that matches that of the pentapeptide "upper" strand. Inhibition varies strongly with the concentration of these macrocycles, suggesting that it is cooperative. On the basis of these studies, a model is proposed in which the AcPHF6 amyloid grows as a layered pair of beta-sheets and in which growth is blocked by a pair of macrocycles that cap the growing paired hydrogen-bonding edges. We then systematically studied the macrocycles and found that positions R1, R3, and R7 are especially sensitive to mutations. Reducing hydrophobicity at these positions substantially diminishes inhibition. Although position R5 is not sensitive to mutations that reduce hydrophobicity, it is sensitive to mutations that increase hydrophobicity. Enhanced hydrophobicity at this position substantially enhances inhibition. These studies establish that the hydrophobic surface comprising residues R1, R3, and R7 is crucial to the inhibition process and that the residue R 5, which shares this surface, is also important. Collectively, these studies demonstrate the importance of hydrogen-bonding edges and hydrophobic surfaces in inhibiting amyloid aggregation. Attempts to synthesize covalently-linked macrocyclic beta-sheet peptides in order to promote cooperativity are also described in the Dissertation. |
