Direct and Simultaneous Measurement of the Stiffness and Internal Friction of a Single Folded Protein

The nanomechanical response of a folded single protein, the natural nanomachine responsible for myriad biological processes, provides insight into its function. The conformational flexibility of a folded state, characterized by its viscoelasticity, allows proteins to adopt different shapes to perform their function. Despite efforts, its direct measurement has not been possible so far. We present a direct and simultaneous measurement of the stiffness and internal friction of the folded domains of the protein titin using a special interferometer based atomic force microscope. We analyzed the data by carefully separating different contributions affecting the response of the experimental probe to obtain the folded state's viscoelasticity. Above similar to 95 pN of force, the individual immunoglobulins of titin transition from an elastic solid-like native state to a soft viscoelastic intermediate.