## 3 Results
### Super-Linear Response Regime
On the other hand, they emphasize the small effect of illumination on the protein 3D arrangement. Figure 6: \(I\)-\(V\) characteristics of b\(RC\) in dark and in light within a range of applied bias of interest for experiments. The contacts are in the up-down orientation. Circles refer to results with the protein in dark at pH = 6.5, squares refer to the protein in light at pH = 6.5, diamonds refer to the protein in dark at pH = 10. In the inset the \(I\)-\(V\)\(I\)-\(V\) characteristics are for the protein in dark at pH = 6.5 (triangle up) and at pH = 10 (triangle down) using contacts in the transverse orientation. Lines are guides for the eye. Calculations make use of the following parameters: \(D\) = 10.5 Å, \(\rho_{max}\) = 2.7 \(\times\) 10\({}^{12}\)\(\Omega\) Å, \(\rho_{min}\) = 2.7 \(\times\) 10\({}^{4}\)\(\Omega\) Å, \(\Phi\) = 0.219 eV. Figure 7 reports the \(I\)-\(V\) characteristics of b\(RC\) in the up-down in dark orientation and in light at pH = 6.5 for a range of applied bias of interest for experiments. The contacts are in the up-down orientation. Calculations are performed by using three different value of the barrier height, \(\Phi\) = 0.219 eV (squares), \(\Phi\) = 0.212 eV (diamonds), and \(\Phi\) = 0.206 eV (triangles). Circles refer to the protein in dark, with data calculated assuming \(\Phi\) = 0.219 eV. Lines are guides for the eye. Figure 8 reports the \(I\)-\(V\) characteristics of b\(RC\) in dark and in light for a range of applied bias of interest for experiments, \(\Phi\) = 0.219 eV. The contacts are in the up-down orientation. Calculations are performed by using three different values of \(\rho_{max}\) = 2.7 \(\times\) 10\({}^{12}\)\(\Omega\)\(\AA\), (red diamonds), \(\rho_{max}\) = 2.7 \(\times\) 10\({}^{11}\)\(\Omega\)\(\AA\) (cyan triangles), and \(\rho_{max}\) = 5.4 \(\times\) 10\({}^{10}\)\(\Omega\)\(\AA\) (green circles). All the calculations assume \(\rho_{min}\) = 2.7 \(\times\) 10\({}^{4}\)\(\Omega\)\(\AA\) and \(\Phi\) = 0.219 eV. Continuous lines refer to experimental data from [12] obtained by using samples in which the protein was sandwiched between differently modified electrodes as reported in the figure. Figure 7 reports the \(I\)-\(V\) characteristics of b\(RC\) in dark and in light within a range of applied bias of interest for experiments. The contacts are in the up-down orientation. Calculations are performed by using three different value of the barrier height, \(\Phi\) = 0.219 eV (squares), \(\Phi\) = 0.212 eV (diamonds), and \(\Phi\) = 0.206 eV (triangles). Circles refer to the protein in dark, with data calculated assuming \(\Phi\) = 0.219 eV. Lines are guides for the eye. lines). In these experiments, performed with the CAFM, the protein was immobilized on an Au-electrode and contacted with the CAFM tip. Both the electrode and the tip have been coated with different chemicals. Here we focused on three different assemblies, say, Au/4 MP/protein/Au, Au/2 MP/protein/Au and Au/2 MP/protein/2 MP/Au, where 2 MP indicates 2-mercaptoethanolpyridine and 4 MP 4-mercaptoethanolpyridine [12]. The satisfactory agreement between theory and experiments is obtained by using different values for \(\rho_{max}\) to account for the different chemical modifiers used to anchor the protein, while keeping the same values for the other parameters. Three different values of \(\rho_{max}\) have been used, namely: \(2.7\times 10^{12}\)\(\Omega\) A, \(1.2\times 10^{11}\)\(\Omega\) A and \(5.4\times 10^{12}\)\(\Omega\) A, to fit experiments performed by using, respectively, the Au/4 MP/protein/Au, Au/2 MP/protein/Au, and Au/2 MP/protein/2 MP/Au, assembly. This result suggests that the role played by the chemical modifiers of the electrode and/or the CAFM tip is to increase the effective conductivity of the protein and it is not due to the change of the potential barrier height. As a matter of fact, the shape of the \(I\)-\(V\) curve is practically the same for the three cases, while the absolute value is changed.