Detection of streptavidin-biotin intermediate metastable states at the single-molecule level using high temporal-resolution atomic force microscopy.

Although the streptavidin-biotin intermolecular bond has been extensively used in many applications due to its high binding affinity, its exact nature and interaction mechanism have not been well understood. Several reports from previous studies gave a wide range of results in terms of the system's energy potential landscape because of bypassing some short-lived states in the detection process. We employed a quasi-static process of slowly loading force onto the bond (loading rate = 20 pN s) to minimize the force-induced disruption and to provide a chance to explore the system in near-equilibrium. Therein, by utilizing a fast sampling rate for the detection of force by atomic force microscopy (20 μs per data point), several transient states of the system were clearly resolved in our force spectroscopy measurements. These key strategies allow the determination of the states' relative positions and free energy levels along the pulling reaction coordinate for the illustration of an energy landscape of the system.