Coherent Vibrational Dynamics in an Isolated Peptide Captured with Two-Dimensional Infrared Spectroscopy.

Quantum mechanical vibrational coherence transfer processes play important roles in energy relaxation, charge transfer, and reaction dynamics in chemical and biological systems but are difficult to directly measure using traditional condensed-phase nonlinear spectroscopies. Recently, we developed a new experimental capability to obtain two-dimensional infrared (2D IR) spectra of molecular systems in the gas phase that enables the direct measurement of coherence pathways. Herein, we report ultrafast 2D IR spectroscopy of the peptide glutathione (GSH) isolated and cryogenically cooled in the gas phase. Six vibrational modes were simultaneously excited within the amide I and II region. The spectral dynamics of both diagonal and off-diagonal cross peak features exhibit long-lived oscillatory behavior consistent with the presence of coherent vibrational dynamics. The oscillatory signatures deviate significantly from the expected quantum beating pathways predicted from standard nonlinear response theories. These deviations indicate the presence of additional nonlinear pathways, including coherence transfer processes. Quantum chemistry calculations indicate large anharmonic couplings between the excited vibrational modes in GSH and, critically, strong coupling between the excited modes and numerous low-frequency modes that act as a bath to mediate coherence transfer. The data provide important new benchmarks for modeling coherence transfer dynamics and system-bath interactions in open quantum systems free from solvent effects.