Multimodal Spectroscopic Study of Amyloid Fibril Polymorphism

Amyloid fibrils are a large class of self-assembled protein aggregates that are formed from unstructured peptides and unfolded proteins. The fibrils are characterized by a universal β-sheet core stabilized by hydrogen bonds, but the molecular structure of the peptide subunits exposed on the fibril surface is variable. Here we show that multimodal spectroscopy using a range of bulk- and surface-sensitive techniques provides a powerful way to dissect variations in the molecular structure of polymorphic amyloid fibrils. As a model system, we use fibrils formed by the milk protein β-lactoglobulin, whose morphology can be tuned by varying the protein concentration during formation. We investigate the differences in the molecular structure and composition between long, straight versus short, worm-like fibrils. We show, using mass spectrometry, that the peptide composition of the two fibril types is similar. The overall molecular structure of the fibrils probed with various bulk-sensitive spectroscopic techniques shows a dominant contribution of the β-sheet core, but no difference in structure between straight and worm-like fibrils. However, when probing specifically the surface of the fibrils with nanometer resolution using tip-enhanced Raman spectroscopy (TERS), we find that both fibril types exhibit a heterogeneous surface structure with mainly unordered or α-helical structures and that the surface of long, straight fibrils contains markedly more β-sheet structure than the surface of short, worm-like fibrils. This finding is consistent with previous surface-specific vibrational sum-frequency generation (VSFG) spectroscopic results (VandenAkker, C. C.; Engel, M. F. M.; Velikov, K. P.; Bonn, M.; Koenderink, G. H. J. Am. Chem. Soc. 2011, 133, 18030-18033). In conclusion, only advanced vibrational spectroscopic techniques sensitive to surface structure like TERS and VSFG are able to reveal the difference in structure that underlies the distinct morphology and rigidity of different amyloid fibril polymorphs that have been observed for a large range of food and disease-related proteins.