## 2 Results and Discussion
We therefore decided to conduct dynamic force spectroscopy measurements by repeating the experiments over a range of different loading r  
Figure 4: Peak analysis of probability distributions for both GFOGER (**A**,**B**) and GPP (**C**,**D**). Solid lines represent raw data whereas the dotted line is the best fit solution of a sum of several individual Gaussian distributions. Dashed lines in B and D correspond to the individual Gaussian components. Identified peaks are highlighted by arrows. Such experiments may be modelled by the Bell-Evans model [31, 32, 33]:  
\[F^{*}(r)=\frac{k_{\mathrm{B}}T}{x_{\beta}}\ln\frac{rx_{\beta}}{k_{\mathrm{off}}k _{\mathrm{B}}T} \tag{1}\]  
This model assumes a single energy barrier and demonstrates that the most probable rupture force, \(F^{*}(r)\), is logarithmically dependent on the loading rate, \(r\). By fitting this equation to the experimental data we can find the kinetic off rate, \(k_{\mathrm{off}}\), which is the inverse of the ligand-receptor bond lifetime, \(k_{off}(0)=1/\tau(0)\) at zero applied force, and the distance between the ground state and transition state, \(x_{\beta}\). For both the GFOGER/I-domain interaction and the GPP/I-domain interaction, we observe a linear dependence of the rupture force on the logarithm of the loading rate. Such logarithmic dependence is another indication that we are measuring specific ligand-receptor interactions. For GFOGER we find \(k_{\mathrm{off}}=0.44\,\mathrm{s}^{-1}\), \(x_{\beta}=0.7\,\mathrm{nm}\) and for GPP we find \(k_{\mathrm{off}}=11\,\mathrm{s}^{-1}\), \(x_{\beta}=0.37\,\mathrm{nm}\). This suggests that the bond lifetime for GFOGER is much longer than for GPP at zero applied force. However, when GPP is loaded quickly (above the cross-over point \(r=1.6\times 10^{4}\,\mathrm{pN}\,\mathrm{s}^{-1}\) ), it resists much more than GFOGER. It seems likely that the identity between the force curves may represent the unbinding of GPP triplets, which are common to both peptides. The underlying molecular interaction may reflect the ability of the GPP triple helix to fit the same binding groove on the I-domain as the specific peptide, GFOGER. The difference between the two spectra may reflect the unbinding of the specific GFOGER hexapeptide itself. A crystal structure of the GFOGER/I-domain complex [6] revealed a binding groove long enough to accommodate little more than the two triplets, GFO and GER. Interactions were mainly between the negatively-charged glutamate residue, the positively-charged arginines and the hydrophobic phenylalanine, but in addition, several interactions occurred with the polypeptide main chain within these two triplets that would also be possible if GPP polymers were docked onto the I-domain's collagen binding site. Thus it seems plausible that the common peaks in the spectra may reflect unbinding of such interactions, and be accounted for in the case of unbinding of GFOGER by mis-registration between the peptide and the I-domain such that the flanking GPP triplets occupied the binding groove rather than the specific GFOGER sequence. \begin{table}
\begin{tabular}{l c c} \hline  & GFOGER & GPP \\ Peak & Peak force ( pN) & Peak force ( pN) \\ \hline
1 & \(13\pm 2\) & \(16\pm 2\) \\
2 & \(26\pm 4\) & \(27\pm 4\) \\
3 & \(37\pm 6\) & \(34\pm 5\) \\
4 & \(55\pm 8\) & \(52\pm 8\) \\
5 & \(71\pm 11\) & \(65\pm 10\) \\
6 & \(89\pm 13\) & \(72\pm 10\) \\
7 & - & \(80\pm 12\) \\
8 & - & \(90\pm 14\) \\
9 & - & \(109\pm 16\) \\ \hline \end{tabular}
\end{table} 
Table 1: Summary of peak forces for GFOGER and GPP.