Insights into the aggregation mechanism of a hydrophobic peptide

The hydrophobic fragment of the Alzheimer’s related β-amyloid (Aβ) peptide, Aβ(25-40), aggregates and forms insoluble amyloid fibrils at a similar rate to the full-length peptide. In order to gain insight into the fibrillization of this peptide and the ability of the flavonoid myricetin to inhibit this aggregation, the isoleucine at position 32 (I32A) and the glycine at position 37 (G37A) in the full-length peptide were replaced with alanine.  Substitution of isoleucine should slightly reduce the potential for hydrophobic interactions, while substitution of glycine should reduce the potential of the flavonoid from hydrogen bonding with the peptide backbone. Thioflavin T assays indicated that the I32A peptide fibrillized at a significantly reduced rate compared to the Aβ(25-40) and G37A peptides. Although all three peptides are fully disordered initially, circular dichroism studies suggest the structure of the I32A and G37A peptides are different from the parent peptide Aβ(25-40). Introduction of myricetin to the peptide samples resulted is modest structural changes for the Aβ(25-40) and G37A peptides but not the I32A peptide. Size exclusion chromatography was employed to determine the initial oligomer distribution of the peptides. While Aβ(25-40) oligomers were predominantly tetramers, I32A and G37A oligomers were a mixture of trimers and dimers. After 48 hours of incubation at 37°C, the amount of tetramers and trimers in solution dropped for the Aβ(25-40) and G37A peptides but remained similar for the I32A peptide. Incubation of Aβ(25-40) with myricetin increased the relative proportion of trimers to tetramers. Ultraviolet resonance Raman studies suggests that the I32A peptide may be more hydrated than the Aβ(25-40) and G37A peptides. Taken together, these data indicate the structural changes observed for the Aβ(25-40) and G37A peptides upon introduction of myricetin are localized around residue 32 and could arise from hydrophobic interactions between the peptide and the flavonoid or interference with the self-association of the peptide in this region. Substitution of isoleucine at position 32 with alanine had little effect on the peptide’s secondary structure but dramatically decreased the propensity of the peptide fibrillize.