All infrared spectra have been calculated based on the excitonic amide I approach originally developed for linear IR spectra by Krimm and Torii , which was later extended to 2D IR spectra by Hamm & Hochstrasser et al. . The calculation is implemented using the framework detailed in the reference . In short, the excitonic amide I Hamiltonian is constructed by analyzing the local modes and the couplings between them, calculated from the atomic coordinates in the PDB file of the actin filament (6BNO). The delocalized vibrational eigenmodes are obtained by solving the time-independent Schrödinger equation. The IR response of amide I can be calculated from the eigenmodes. In the case of the spectrum depicted in Figure and S4, we include an inhomogeneous broadening of 10 cm -1 using the gasdev function of Numerical Recipes and an average over 10 renditions. In this study, the unperturbed amide I local mode frequency employed to obtain an optimal match with the experiments, is blueshifted by 18 cm -1 . In prior studies involving calculations for fibrinogen , we found a similar blueshift, the reason for which remains unclear. Potentially, the typical hydrogen length is longer in filamentous proteins, such as actin and fibrinogen, than in globular proteins, for which a redshifted central frequency leads to a better spectral match, as is the case in refs ). Such elongated hydrogen bonds could be caused by internal strain due to the filamentous structure formation.