ACS Nano http://doi.org/csbc (2018)

The chemical and structural characterization of single protein aggregates is crucial to understand the protein folding/misfolding processes in living organisms. The photothermal-induced resonance (PTIR) technique, also known as AFM-IR, has been demonstrated to be powerful in accessing protein conformation. However, its application in liquid environments remains challenging due to the strong infrared (IR) adsorption of water and the interference of fluid drag on cantilever oscillation. Now, Centrone from NIST and co-workers report PTIR spectra with high signal-to-noise ratio in water, which enables them to identify the chemical and structural state of single fibrillar aggregates.

The researchers first validate the acquisition of PTIR data in water or air on gold plasmonic resonators coated with a 200-nm-thick PMMA layer. The nanoscale confinement near the resonators gaps results in surface-enhanced IR adsorption, leading to IR spectra and maps in water and air with comparable resolution and signal-to-noise ratio. They subsequently test the viability of this technique on the characterization of single protein aggregate conformation. D2O is used instead of H2O to avoid the spectral overlap with amide. Diphenylalanine (FF), the core recognition module of the Alzheimer’s β-amyloid peptide, and its derivative Boc-FF are chosen as model systems. FF and Boc-FF can form morphologically similar fibrillar aggregates in water. Using the present PTIR approach, the differences in fibril secondary and quaternary structural properties are revealed by their different IR fingerprints.