Structural analysis and mapping of individual protein complexes by infrared nanospectroscopy

TLDR

Mid‑infrared spectroscopy is widely used for material identification and secondary‑structure analysis in chemistry, biology, and biochemistry, yet its diffraction limit blocks nanoscale protein studies. The study introduces nano‑FTIR mapping of protein structure at 30 nm resolution and envisions broad applications, from cellular receptor mapping to analysis of proteins in quaternary structures. Nano‑FTIR achieves 30 nm lateral resolution by combining Fourier transform infrared spectroscopy with near‑field scanning, enabling sensitive detection of individual protein complexes. Nano‑FTIR produced local broadband spectra of diverse protein complexes—including a virus, ferritin, purple membranes, and insulin aggregates—revealing α‑helical and β‑sheet content, and showed that 3‑nm‑thin amyloid‑like insulin fibrils possess substantial α‑helical structure at their periphery, highlighting high protein organization.

Abstract

Mid-infrared spectroscopy is a widely used tool for material identification and secondary structure analysis in chemistry, biology and biochemistry. However, the diffraction limit prevents nanoscale protein studies. Here we introduce mapping of protein structure with 30 nm lateral resolution and sensitivity to individual protein complexes by Fourier transform infrared nanospectroscopy (nano-FTIR). We present local broadband spectra of one virus, ferritin complexes, purple membranes and insulin aggregates, which can be interpreted in terms of their α-helical and/or β-sheet structure. Applying nano-FTIR for studying insulin fibrils—a model system widely used in neurodegenerative disease research—we find clear evidence that 3-nm-thin amyloid-like fibrils contain a large amount of α-helical structure. This reveals the surprisingly high level of protein organization in the fibril’s periphery, which might explain why fibrils associate. We envision a wide application potential of nano-FTIR, including cellular receptor in vitro mapping and analysis of proteins within quaternary structures. Mid-infrared spectroscopy offers important chemical and structural information about biological samples but diffraction prevents nanoscale studies. Amenabar et al.demonstrate Fourier transform infrared nanospectroscopy for analysing the secondary structure of protein complexes with 30 nm spatial resolution.