A
myloid fibrils are a large class of self-asse
mbled protein aggregates that are for
med fro
m 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
multi
modal spectroscopy using a range of bulk- and surface-sensitive techniques provides a powerful way to dissect variations in the
molecular structure of poly
morphic a
myloid fibrils. As a
model syste
m, we use fibrils for
med by the
milk protein β-lactoglobulin, whose
morphology can be tuned by varying the protein concentration during for
mation. We investigate the differences in the
molecular structure and co
mposition between long, straight fibrils versus short, wor
mlike fibrils. We show using
mass spectro
metry that the peptide co
mposition of the two fibril types is si
milar. The overall
molecular structure of the fibrils probed with various bulk-sensitive spectroscopic techniques shows a do
minant contribution of the β-sheet core but no difference in structure between straight and wor
mlike fibrils. However, when probing specifically the surface of the fibrils with nano
meter resolution using tip-enhanced Ra
man 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, wor
mlike fibrils. This finding is consistent with previous surface-specific vibrational su
m-frequency generation (VSFG) spectroscopic results (VandenAkker et al.
J. Am. Chem. Soc.,
mlns:mml="http://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"> 2011
mlns:mml="http://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">, m>133m>
mlns:mml="http://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">, 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.