Quantifying protein‐protein interactions by molecular counting with mass photometry

Interactions between biomolecules control the processes of life in health, and their malfunction in disease, making their characterization and quantification essential. Immobilization- and label-free analytical techniques are particular desirable because of their simplicity and minimal invasiveness, but struggle to quantify tight interactions. Here, we show that we can accurately count, distinguish by molecular mass, and thereby reveal the relative abundances of different un-labelled biomolecules and their complexes in mixtures at the single-molecule level by mass photometry. These measurements enable us to quantify binding affinities over four orders of magnitude at equilibrium for both simple and complex stoichiometries within minutes, as well as to determine the associated kinetics. Our results introduce mass photometry as a rapid, simple and label-free method for studying sub-microM binding affinities, with potential to be extended towards a universal approach for characterising complex biomolecular interactions.