The forced unfolding of single domains of muscle protein titin using atomic force microscopy (AFM) established the basis of single molecule force spectroscopy and opened the field of single molecule mechanics 1. Computational steered molecular dynamics (SMD) simulations of unfolding processes provided then a description of the underlying events at the atomic level 2,3. However, experimental velocities (10 nm/s – 10 µm/s) were orders of magnitude slower than computational ones (~1 m/s), not allowing direct comparison. Recently, we have developed high-speed force spectroscopy (HS-FS) that allows pulling at velocities in the mm/s range 4, which are now accessible to SMD. We present the application of HS-FS for forced unfolding of titin I91 domain at velocities up to 4 mm/s 4. We found that a small beta-strand pair of an immunoglobulin domain dynamically unfolds and refolds, buffering pulling forces up to ~100 piconewtons. The distance to the unfolding transition barrier is larger than previously estimated but is in better agreement with atomistic predictions. The ability to directly compare experiment and simulation is likely to be important in studies of biomechanical processes.
4 Rico, F., Gonzalez, L., Casuso, I., Puig-Vidal, M. & Scheuring, S. High-Speed Force Spectroscopy Unfolds Titin at the Velocity of Molecular Dynamics Simulations. Science 342, 741-743, doi:10.1126/science.1239764 (2013).