Structural Characterization of Insulin Fibril Surfaces using Tip Enhanced Raman Spectroscopy (TERS)

Abstract

Amyloid fibrils are protein aggregates that are strongly associated with a variety of highly pathogenic diseases, such as Alzheimer's disease, Creutzfeldt-Jakob disease, Parkinson disease, etc. Direct studies of fibril surfaces become crucial for the understanding of the toxicity and pathogenity of amyloid fibrils, first of all, due to their direct contact with the aqueous media. Tip-enhanced Raman spectroscopy (TERS) is one of very few techniques, which allows to probe the fibril surface directly and with high spatial resolution.1 Our current findings indicate that surface of insulin fibrils is highly heterogeneous.2 More than 30% of the fibril surface is dominated by β-sheet, while the rest of it is composed of α-helix and unordered protein secondary structures. The propensity of various amino acids 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.(1) Deckert-Gaudig, T.; Kammer, E.; Deckert, V. J. Biophotonics 2012, 5, 215-219.(2) Kurouski, D.; Deckert-Gaudig, T.; Deckert, V.; Lednev, I. K. J. Am. Chem. Soc. 2012, 134, 13323-13329.