Initial Steps of Amyloidogenic Peptide Assembly Revealed by Cold‐Ion Spectroscopy

Abstract

The early stages of fibril formation are difficult to capture in solution. We use cold-ion spectroscopyt oe xamine an 11-residue peptide derived from the protein transthyretin and clusters of this fibre-forming peptide containing up to five units in the gas phase.F or each oligomer,t he UV spectra exhibit distinct changes in the electronic environment of aromatic residues in this peptide compared to that of the monomer and in the bulk solution. The UV spectra of the tetraand pentamer are superimposable but differ significantly from the spectra of the monomer and trimer.S uchaspectral evolution suggests that acommon structural motif is formed as early as the tetramer.The presence of this stable motif is further supported by the low conformational heterogeneity of the tetraand pentamer,r evealed from their IR spectra. From comparison of the IR-spectra in the gas and condensed phases,w e propose putative assignments for the dominant motif in the oligomers.Parkinsons, Alzheimers, Huntingtons, diabetes type 2, familial amyloid polyneuropathy,and about 38 other diseases have been associated with the deposition of insoluble proteinaceous material in tissues.[1] In those insoluble deposits,p roteins are packed into fibrillar structures rich in bsheets,which are known as amyloid fibres.[2] Recent evidence suggests that the cytotoxic species are found amongst soluble, prefibrillar aggregates.[1] Since aggregation into amyloid fibres is am ulti-faceted process,avariety of techniques have been applied to assess both the kinetics of assembly and the macroscopic details of assembled fibrils.Despite the vast amount of information regarding the kinetics of fibrillation and the structure of mature fibrils,r elatively little is known about the structure of the soluble,prefibrillar,and potentially toxic aggregates.[1] Early events in fibrillogenesis feature adynamic,heterogenously populated ensemble of oligomeric states.Because of their inherently transient nature,their exact characterisation proves difficult using bulk solution and solidstate techniques.[3] To address this,gas-phase techniques,such as mass spectrometry coupled with ion mobility spectrometry (IM-MS), have particular relevance to enable size-specific characterisation of these transient ensembles.[4] Although mass spectrometry enables the selection of single oligomeric states,i tr emains ac hallenge to simultaneously examine changes in secondary structure at the intact protein level. Instead, peptide segments are often used as surrogates for proteins,allowing more details to be discerned regarding the conformational changes as aggregation proceeds.I narecent study,f or instance,acombination of IM-MS and IR spectroscopy revealed as ignature of b-sheet formation in gasphase oligomers of an insulin hexapeptide fragment. [5] Although the use of short isolated fragments greatly increases the capacity of IM-MS and spectroscopy to discern structural features,aquestion arises of whether the use of small peptides is appropriate for modelling the process of amyloid ...

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