The influence of H-bonding network disruption on morphology, structure and autofluorescence of amyloids

Hydrogen bonding plays a pivotal role in amyloid structures. The characteristic feature of amyloids is the presence of a secondary β-sheet structure in which hydrogen bonds are regularly aligned. Previous reports suggested that hydrogen bonds can be involved in the autofluorescence phenomenon of amyloids. Here, we studied a fragment of amyloidogenic amyloid-β (Aβ) peptide, Aβ(35–42), and its analogue with one ester bond replacing an amide, depsiAβ(35–42), to isolate the effect of disrupting the hydrogen bonding network on autofluorescence. We have chosen these sequences because they are lacking aromatic amino acids and contain α-amino and α-carboxyl groups at their N- and C-termini that can undergo protonation and deprotonation. Morphological characterization indicated that under acidic conditions (pH ∼2) Aβ(35–42) rapidly assembles into amyloid fibrils, whereas depsiAβ(35–42) forms spherical aggregates. The presence of a secondary β-sheet structure was confirmed in both samples. Interestingly, the autofluorescence signal was detected in depsiAβ(35–42) despite the absence of fibrillar structures, and its intensity was similar to that of Aβ(35–42). Furthermore, our studies revealed that the autofluorescence is pH-dependent. Overall, our results corroborate the hypothesis that proton transfer between the N- and C-termini of an aggregated peptide in β-sheet conformation is critical for its intrinsic fluorescence properties.