State Resolved THz Spectroscopy and Dynamics of Crystalline Peptide/Water Systems
Zeeshan Ahmed, Shin G. Chou, Karen M. Siegrist, David F. Plusquellic
State resolved THz studies of crystalline peptides and their hydrates have provided precise measures of the infrared active phonon modes that reflect motions on the longest possible length scales in these systems. The impact of co-crystallized water on the lattice vibrations can range from subtle to dramatic depending on the hydrophobic or hydrophilic nature to the peptide water interactions. Two classes of peptide/water systems that encompass two types of secondary structure are investigated at the quantum state resolved level using waveguide methods and quantum chemical theory. Almost all of these systems can undergo some type of exchange with water at room temperature that alters the hydrogen bonding network in ways easily detectable in the THz region at cryogenic temperatures. Stark differences are observed in the spectra of model α-helical and β-sheet structures upon water removal at hydrophilic binding sites. However, within the confined environment of a hydrophobic nanotube, water in the form of helical wires has little impact on the phonon modes of the tube. Across all systems, theoretical predictions on fully relaxed crystal structures are shown to provide convincing evidence of phonon mode assignments and a detailed view of energetics responsible for the diverse dynamics displayed in these systems. These results provide benchmark data to help validate and refine classical mechanical force field models.
biophysics, crystalline peptides, Quantum Chemical Theory, secondary structure, THz spectra