Structure and Composition
of Insulin Fibril Surfaces
Probed by TERS
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Abstract
Amyloid fibrils associated with many neurodegenerative
diseases
are the most intriguing targets of modern structural biology. Significant
knowledge has been accumulated about the morphology and fibril-core
structure recently. However, no conventional methods could probe the
fibril surface despite its significant role in the biological activity.
Tip-enhanced Raman spectroscopy (TERS) offers a unique opportunity
to characterize the surface structure of an individual fibril due
to a high depth and lateral spatial resolution of the method in the
nanometer range. Herein, TERS is utilized for characterizing the secondary
structure and amino acid residue composition of the surface of insulin
fibrils. It was found that the surface is strongly heterogeneous and
consists of clusters with various protein conformations. More than
30% of the fibril surface is dominated by β-sheet secondary
structure, further developing Dobson’s model of amyloid fibrils
(Jimenez et al. Proc. Natl.
Acad. Sci. U.S.A. 2002, 99, 9196–9201). The propensity
of various amino acids to be on the fibril surface and specific surface
secondary structure elements were evaluated. β-sheet areas are
rich in cysteine and aromatic amino acids, such as phenylalanine and
tyrosine, whereas proline was found only in α-helical and unordered
protein clusters. In addition, we showed that carboxyl, amino, and
imino groups are nearly equally distributed over β-sheet and
α-helix/unordered regions. Overall, this study provides valuable
new information about the structure and composition of the insulin
fibril surface and demonstrates the power of TERS for fibril characterization