Characterization of Pathogenic Amyloids by Tip Enhanced Raman Spectroscopy

The aberrant assembly of proteins and peptides in ordered fibrillar aggregates, namely amyloid fibrils, was recently related to many human neurodegenerative diseases, such as Alzheimer's and Parkinson's. This assembly process proceeds through oligomeric intermediates that are thought to be the primary pathogenic species in many of these protein deposition diseases. Amyloids oligomers formation can lead, despite similar structural characteristics, to different toxic activity, and can appears 10 to 20 years before the disease symptoms become apparent. As a consequence, tests focused on monitoring aberrant protein oligomers are expected to represent a promising tool for diagnosing protein misfolding diseases in their earliest, pre-symptomatic, and most treatable stages. The N-terminal domain of the Escherichia coli protein HypF (HypF-N) is a small stably folded ?/? protein with 91-residues (10 kDa) and represents a valuable model system for investigating the structural basis of the cellular dysfunction as it was shown to form amyloid-like fibrils similar to those associated with diseases as well as toxic oligomers.1-2 According to the pH conditions, the same polypeptide sequence can assemble into two distinct types of stable oligomers showing similar morphological properties, but different abilities to cause cellular dysfunction. Here we report on a Tip Enhanced Raman Spectroscopy study of HypF-N, demonstrating how it is possible to distinguish their toxic and non-toxic forms by Raman analysis with nanometric resolution. By analysing the TER spectra it was observed that non-toxic oligomers are characterized by a higher hydrophilicity with respect to the toxic counterparts, in which the hydrophobic regions are structurally more highly disorganized and solvent-exposed.