Discriminating early stage Aβ42 monomer structures using chirality-induced 2DIR spectroscopy in a simulation study

Elucidating the structural features of the A beta monomer, the peptide constituent of amyloid fibrils found in Alzheimer's disease, can enable a direct characterization of aggregation pathways. Recent studies support the view that the ensemble of A beta 42 monomers is a mixture of diverse ordered and disordered conformational species, which can be classified according to the formation of a characteristic beta-hairpin conformation in a certain region. Despite the disparity in the structural features of these species, commonly used spectroscopic techniques such as NMR may not directly trace the conformational dynamics in the ensemble due to the limited time resolution and the lack of well-resolved spectral features for different comformers. Here we use molecular dynamics simulations combined with simulations of two-dimensional IR (2DIR) spectra to investigate the structure of these species, their interchange kinetics, and their spectral features. We show that while the discrimination efficiency of the ordinary, nonchiral 2DIR signal is limited due to its intrinsic dependence on common order parameters that are dominated by the generally unstructured part of the sequence, signals with carefully designed chirality-sensitive pulse configurations have the high resolution required for differentiating the various monomer structures. Our combined simulation studies indicate the power of the chirality-induced (CI) 2DIR technique in investigating early events in A beta 42 aggregation and open the possibility for their use as a novel experimental tool.