Two-dimensional terahertz-infrared-visible spectroscopy of molecular C-H groups

Low frequency modes (LFMs) of proteins in terahertz (THz) frequency range are suggested to be significant for protein’s activity. However, there is lack of understanding of these motions because of the experimental challenges. One-dimensional Raman and linear absorption spectroscopy in THz range measures them directly, but the spectra are often congested by overlapping resonances of different chemical species. Recently developed two-dimensional terahertz-infrared-visible (2D TIRV) spectroscopy can provide site-specific insight into the THz spectrum by measuring correlations between LFMs and chemically-specific high-frequency vibrational modes (HFMs). Measured 2D TIRV spectra however are affected by instrument response function (IRF) which critically complicates their analysis and interpretation.
In this thesis, we develop a method to eliminate IRF and derive complex-valued response function (S⁽³⁾). S⁽³⁾ solely represents the molecular vibrational dynamics and permits for accurate analysis of lineshapes. To verify the approach, we use liquid dimethyl sulfoxide (DMSO) as a model system and measure coupling between CH₃ stretch modes (HFMs) and low-frequency intramolecular and intermolecular modes (LFMs). Data analysis using newly developed mixed quantum-classical formalism of response function reveals dominating mechanical coupling and correlated fluctuations of HFM and LFM frequencies. Furthermore, we explore selection rules governing the signal generation in 2D TIRV spectroscopy using DMSO as model sample.
Finally, we demonstrate the utilization of 2D TIRV spectroscopy to probe site-specific THz motions of proteins. To this end, we measure THz motions of side chains of lysozyme protein in D2O solution and as a dry film.