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. Author manuscript; available in PMC: 2011 Dec 3.
Published in final edited form as: Biochem Biophys Res Commun. 2010 Nov 2;403(1):133–137. doi: 10.1016/j.bbrc.2010.10.133

Figure 1.

Figure 1

The free energy landscape of a molecule under force. A) Typical energy landscape representation commonly used to interpret force spectroscopy data, showing a putative thermodynamic state that is tens of nanometers away from its native folded state that is apparent both under general force F0 and also in the absence of force. B) Free energy representation of an extending molecule that can be simply modeled as the sum of an entropic term (WLC) and an enthalpic term accounting for the short range interactions that define the collapsed protein. The free energy of a simple molecule in bulk (F=0) contrasts with the same molecule under a force of F=12 pN. The energy cost of extending is calculated from the WLC model of polymer elasticity (Lc=30 nm, p =0.4 nm), the drive to collapse is represented by a Morse potential (Umin= 60 pNnm, x0=4 nm). Notice the absence of a stable extended state for F=0. Under a constant stretching force of 12 pN a new entropic energy barrier appears separating the entropic minima at 15 nm, from the collapsed minima at 4 nm. This barrier is completely absent below the critical force Fc.