Observation of beta-sheet aggregation in a gas-phase tau-peptide dimer.

In Alzheimer's disease, the tau protein forms intracellular amyloid fibrils in which the (306)VQIVYK(311) sequence adopts parallel beta-sheets, enabling fibril formation via cross-beta "steric zippers". We investigated aggregation of the protected segment (Ac-VQIVYK-NHMe) using IR/UV hole-burning spectroscopy in the NH stretch region in a cold molecular beam combined with DFT calculations in order to characterize its structure and identify the noncovalent interactions generally responsible for aggregation and stabilization in amyloid peptides. The computed and experimental IR spectra suggest that the tau-protein fragments form extended beta-strands that are combined in a beta-sheet through characteristic backbone hydrogen bonds, indicating that this secondary structure is energetically most attractive and readily forms in the gas phase, without any "guiding" interactions from a solvent or protein environment.