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Abstract

Protein-misfolding, aggregation and accumulation of insoluble deposits are the hallmark features of a variety of human diseases such as Parkinson's disease and diabetes mellitus type II. The latter is a systemic disorder characterized by insulin resistance, impaired insulin secretion, beta-cell apoptosis and islet amyloid formation. Fibrillar aggregates from the selfassembly of human islet amyloid polypeptide (hIAPP) are major component of islet amyloids. Accumulating evidence suggests that not the mature fibrils, but rather smaller, soluble, polymorphic and highly dynamic oligomers preceding the formation of the fibrils are the cytotoxic species. A detailed insight into the structures of the oligomeric intermediates is crucial for identification of potential targets and development of therapeutic strategies. However, the polydisperse nature of these peptides makes their structural characterization challenging. The traditional condensed-phase analytical techniques provide only averaged structural information on the dynamic ensemble. However, information on the structure of isolated species can be obtained by employing gas-phase techniques. Here, infrared action spectroscopy in combination with ion-mobility spectrometry is used to gain insight into the secondary structure of these isolated peptides. The orthogonal combination of these techniques allows to obtain fingerprint vibrational spectra of m/z- and conformer-selected species. The secondary structure of the full-length hIAPP, as well as of metal-associated hIAPP and hIAPP in heterogeneous co-assemblies with a fragment of the prion protein (PrP_106-126) and the hexapeptide VEALYL were investigated. The obtained data suggests that the secondary structure of the monomeric hIAPP contains a significant fraction of alpha-helical motifs, which is seemingly maintained upon metal binding, self-assembly, or heterogeneous co-assembly with other peptides. These results provide solid evidence for the importance of helical intermediates in the formation of amyloids.