Simulation of two-dimensional sum-frequency generation response functions: application to amide I in proteins.

We present the implementation of an approach to simulate the two-dimensional sum frequency generation response functions of systems with numerous coupled chromophores using a quantum-classical simulation scheme that was previously applied successfully to simulate two-dimensional infrared spectra. We apply the simulation to the amide I band of a mechanosensitive channel protein. By examining the signal generated from different segments of the protein, we find that the overall signal is impossible to interpret without the aid of simulations due to the interference of the response generated on different segments of the protein. We do not find significant cross-peaks in the spectra, even when the waiting time is increased. The spectra are thus not sensitive to coupling between different structural elements. Despite this, we conclude that two-dimensional sum frequency generation spectroscopy will be a powerful tool to investigate membrane bound proteins.