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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 fibrils versus short, wormlike 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 wormlike 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, wormlike fibrils. This finding is consistent with previous
surface-specific vibrational sum-frequency generation (VSFG) spectroscopic
results (VandenAkker et al. J. Am. Chem. Soc., 2011, 133, 18030−18033, DOI: 10.1021/ja206513r). In conclusion, only advanced vibrational spectroscopic techniques
sensitive to surface structure such as 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