Structural and compositional profiling of individual biomolecular assemblies by infrared absorbance-modulated evanescent scattering

The structure and composition of biomolecular assemblies shape their biological functions and disease processes. Yet existing approaches either require homogeneous samples or lack chemical specificity and sensitivity, limiting the elucidation of transient oligomeric species under native conditions. Here we report infrared absorbance-modulated evanescent scattering (IR-AMES) that enables ultrasensitive structural and compositional profiling of individual biomolecular assemblies in aqueous environments. This capability is achieved by photothermally encoding mid-infrared vibrational absorption into scattering contrast using a total-internal-reflection geometry. Applying IR-AMES to recombinant tau assemblies, we resolve random-coil monomers and the emergence of heterogeneous oligomers beyond ensemble-averaged measurements. In Alzheimer's disease brain-derived tau, IR-AMES reveals enrichment of antiparallel {beta}-sheet structures and RNA components in oligomers. Using lipid nanodiscs as membrane mimics, we find that pathological tau oligomers exhibit enhanced interactions with anionic membranes. IR-AMES enables spectroscopic imaging of diverse biomolecular assemblies, opening opportunities to elucidate structure-function relationships in heterogeneous biomolecular systems.