Supplementary Figure 9: Determining the effective spring constant keff of the cantilever-PEG-polypeptide linker system.
From: Imaging G protein–coupled receptors while quantifying their ligand-binding free-energy landscape
(a) The effective spring constant keff is the combination of the cantilever stiffness kc and the stiffness kL of the PEG-polypeptide linker. As described (Supplementary Note), kL was estimated using the combination of the elasticity of the PEG linker (kPEG) and the elasticity of the polypeptide linker (kpolypeptide). The linker consists of the 27-unit long PEG spacer and the 28 aa long polypeptide, which mimics the thrombin cleaved N-terminal end of PAR1 (comp. Fig. 1). (b) The extension of the PEG-polypeptide linker applied to a stretching force F in water is described by the combination of both the PEG elasticity model6 and the WLC model7. (c) Calculated stiffness kL of the PEG-polypeptide linker (blue curve) vs extension and effective stiffness keff of cantilever and linker (red curve). (d) The force vs keff relationship was used to determine the effective spring constant of cantilever-PEG-polypeptide linker system exposed to a given force Feq and to subsequently calculate ΔGbu.
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