Insights into the aggregation mechanism of Aβ(25-40)

- Substitution of alanine at position 32 or 37 shifts the oligomeric distribution from tetramers/trimers to trimers/dimers.
- Increased backbone hydration reduces Aβ(25-40)'s propensity to fibrillize.
- Structural changes in Aβ(25-40) induced by myricetin are eliminated when the hydrophobicity at residue 32 is reduced.
- Fibrillization depletes the store of the largest oligomeric species, with little impact on the amountof smaller oligomers.

The hydrophobic fragment of the Alzheimer's related β-amyloid (Aβ) peptide, Aβ(25-40), aggregates and forms insoluble amyloid fibrils at a rate similar to the full-length peptide. In order to gain insight into the fibrillization of Aβ(25-40) and the ability of the flavonoid myricetin to inhibit its aggregation, the isoleucine at position 32 (I32A) and the glycine at position 37 (G37A) in the full-length peptide were replaced with alanine. Thioflavin T assays indicate that substitution of isoleucine for alanine significantly reduces the rate and extent of fibrillization 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 results in modest structural changes for the Aβ(25-40) and G37A peptides but not the I32A peptide. Aβ(25-40) oligomers were predominantly tetramers, whereas I32A and G37A oligomers were a mixture of trimers and dimers. After 48 h 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.