Here we study the hydrogen bond structure and dynamics of the amide groups forming the backbone of protein chains when mixed with water. Aqueous NMA (N-methylacetamide) solutions were investigated as a model system by polarization-resolved pump-probe and 2D infrared spectroscopy (2D IR), using the amide I mode as a reporter. The 2D IR results are compared with molecular dynamics simulations and spectral calculations to gain insight into the molecular structures in the mixture.<br/><br/>We revealed that adding water to NMA leads to disruption of the hydrogen bond chains of NMA molecules, which were recently reported to be present in bulk NMA. Instead, solvation results in the formation of water and NMA clusters where the amide I frequency dynamics are “frozen”, at least at the ps timescale. Curiously enough, only the frequency and rotational dynamics are frozen while the anisotropy is not, and does decay at the ~1 ps time scale. This is because the excitation is mobile and therefore samples a few amide I modes with similar frequencies but different orientations. The former leads to slowing down of the frequency dynamics, as the excitations tend to be shared between vibrations with similar frequency, while the latter results in the anisotropy decay. Since the system under investigation can be considered as a simplified model of the hydration of proteins, the present study forms a convenient basis for understanding the structural and vibrational dynamics in proteins.

2D IR study of NMA-water mixtures

2019 Fall National Meeting

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