Recent developments and applications of photothermal AFM-IR in characterization of amyloids and amyloids aggregation...

AFM-IR combinates atomic force microscopy and infrared spectroscopy to compensate the limitations of both techniques taken separately. It has been reviewed for a large application field like polymers, geology and life sciences. In biology, it is an important tool to study amyloids and protein aggregation processes. Indeed, misfolding can appear under various circumstances in the process of globular proteins folding. In the case of amyloidosis, fibrillar aggregates are deposited in intracellular inclusions or in tissues as extracellular plaques. These aggregates (oligomers or fibrils) are characterized by high β-sheet content which can be analyzed in AFM-IR thanks to specific absorption band. The main progresses and developments of this technique are summarized since its creation in 2005. The evolution of laser sources and new measurement modes has led to the development of new instruments. They are always more efficient, allowing faster analysis, a wider sample range or more sensitive in order to give more (chemical) information about the sample. An overview of the progress made in photothermal AFM-IR in characterization of amyloids and amyloid aggregation processes is also described. The tapping and resonance-enhanced contact AFM-IR are the most commonly used modes. Generally, the label-free analysis of the conformation of the oligomers and/or fibrils at micromolar concentration is described, either in an aggregation kinetic study or in analysis of fibrils in ex vivo study. The coaggregation of two amyloids is also realized using ¹³C-labeled peptide to distinguish both two spectral signatures.