Structural motif of polyglutamine amyloid fibrils discerned with mixed-isotope infrared spectroscopy

Polyglutamine (polyQ) sequences are found in a variety of proteins, and mutational expansion of the polyQ tract is associated with many neurodegenerative diseases. We study the amyloid fibril structure and aggregation kinetics of K(2)Q(24)K(2)W, a model polyQ sequence. Two structures have been proposed for amyloid fibrils formed by polyQ peptides. By forming fibrils composed of both C-12 and C-13 monomers, made possible by protein expression in Escherichia coli, we can restrict vibrational delocalization to measure 2D IR spectra of individual monomers within the fibrils. The spectra are consistent with a beta-turn structure in which each monomer forms an antiparallel hairpin and donates two strands to a single beta-sheet. Calculated spectra from atomistic molecular-dynamics simulations of the two proposed structures confirm the assignment. No spectroscopically distinct intermediates are observed in rapid-scan 2D IR kinetics measurements, suggesting that aggregation is highly cooperative. Although 2D IR spectroscopy has advantages over linear techniques, the isotope-mixing strategy will also be useful with standard Fourier transform IR spectroscopy.