13 C- and 15 N-labeling of amyloid-β and inhibitory peptides to study their interaction via nanoscale infrared spectroscopy

The study presents a method for isotope-editing polypeptides to enable their spatial distinction via nanoscale infrared spectroscopy, specifically scattering-type scanning near-field optical microscopy (s-SNOM). The key result is that the approach successfully distinguished the locations of labeled versus unlabeled peptide aggregates based on isotope-induced shifts in the amide I and amide II infrared absorption bands, even for fibrils as thin as 2 nm. The authors obtained evidence suggesting mixing of the two polypeptides, which could indicate specific molecular interactions, but note that for large aggregates, this cannot be distinguished from random co-localization without detailed spectral analysis. A limitation is the small isotope shift (~40 cm⁻¹) and the current signal-to-noise ratio, which limits unequivocal detection of mixing within very small aggregates. The implications are primarily for future in vitro studies of peptide interactions relevant to amyloidogenic diseases, as the method provides a high-resolution tool to map morphologies without bulky fluorescent labels.