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 sho
w that multimodal spectroscopy using a range of bulk- and surface-sensitive techniques provides a po
werful
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 bet
ween long, straight fibrils versus short,
wormlike fibrils. We sho
w using mass spectrometry that the peptide composition of the t
wo fibril types is similar. The overall molecular structure of the fibrils probed
with various bulk-sensitive spectroscopic techniques sho
ws a dominant contribution of the β-sheet core but no difference in structure bet
ween straight and
wormlike fibrils. Ho
wever,
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.,
pan class="NLM_x">www.w3.org/1998/Math/MathML" xmlns:ACS="http://namespace.acs.org/2008/acs" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xml:space="preserve"> pan>2011
pan class="NLM_x">www.w3.org/1998/Math/MathML" xmlns:ACS="http://namespace.acs.org/2008/acs" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xml:space="preserve">, pan>
133pan class="NLM_x">www.w3.org/1998/Math/MathML" xmlns:ACS="http://namespace.acs.org/2008/acs" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xml:space="preserve">, pan>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.