Surprisingly small structural changes in Donor–Acceptor Stenhouse Adducts (DASAs) result in predictable, robust and effective photochromic switches.

The quantification of membrane-associated biomolecular interactions is crucial to our understanding of various cellular processes. State-of-the-art single-molecule approaches rely largely on the addition of fluorescent labels, which complicates the quantification of the involved stoichiometries and dynamics because of low temporal resolution and the inherent limitations associated with labeling efficiency, photoblinking and photobleaching. Here, we demonstrate dynamic mass photometry, a method for label-free imaging, tracking and mass measurement of individual membrane-associated proteins diffusing on supported lipid bilayers. Application of this method to the membrane remodeling GTPase, dynamin-1, reveals heterogeneous mixtures of dimer-based oligomers, oligomer-dependent mobilities, membrane affinities and (dis)association of individual complexes. These capabilities, together with assay-based advances for studying integral membrane proteins, will enable the elucidation of biomolecular mechanisms in and on lipid bilayers.

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 desirable because of their simplicity and minimal invasiveness, but they struggle with quantifying tight interactions. Here, we show that mass photometry can accurately count, distinguish by molecular mass, and thereby reveal the relative abundances of different unlabelled biomolecules and their complexes in mixtures at the single‐molecule level. These measurements determine binding affinities over four orders of magnitude at equilibrium for both simple and complex stoichiometries within minutes, as well as the associated kinetics. These results introduce mass photometry as a rapid, simple and label‐free method for studying sub‐micromolar binding affinities, with potential for extension towards a universal approach for characterizing complex biomolecular interactions.

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-µM binding affinities, with potential to be extended towards a universal approach for characterising complex biomolecular interactions.

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 desirable because of their simplicity and minimal invasiveness, but they struggle with quantifying tight interactions. Here, we show that mass photometry can accurately count, distinguish by molecular mass, and thereby reveal the relative abundances of different unlabelled biomolecules and their complexes in mixtures at the single‐molecule level. These measurements determine binding affinities over four orders of magnitude at equilibrium for both simple and complex stoichiometries within minutes, as well as the associated kinetics. These results introduce mass photometry as a rapid, simple and label‐free method for studying sub‐micromolar binding affinities, with potential for extension towards a universal approach for characterizing complex biomolecular interactions.