Mapping amyloid-β(16-22) nucleation pathways using fluorescence lifetime imaging microscopy.

The cross-β peptide architecture is associated with numerous functional biomaterials and deleterious disease related aggregates. While these diverse and ubiquitous paracrystalline assemblies have been widely studied, a fundamental understanding of the nucleation and aggregation pathways to these structures remains elusive. Here we highlight a novel application of fluorescence lifetime imaging microscopy in characterising the critical stages of peptide aggregation. Using the central nucleating core of the amyloid-β (Aβ), Aβ(16-22), as a model cross-β system, and utilising a small fraction of rhodamine labelled peptide (Rh110-Aβ(17-22)), we map out a folding pathway from monomer to paracrystalline nanotube. Using this intrinsic fluorescence reporter, we demonstrate the effects of interfaces and evaporation on the nucleation of sub-critical concentration solutions, providing access to previously uncharacterised intermediate morphologies. Using fluorescence lifetime we follow the local peptide environment through the stages of nucleation and hydrophobic collapse, ending in a stable final structure. This work provides a metric for future implementations of measuring fluorescence lifetimes of intrinsic fluorescence reporters during the very dynamic processes relating to peptide nucleation and maturation.