Coherent Vibrational Dynamics in an Isolated Peptide Captured with Cryogenic Ion 2D IR Spectroscopy

The transfer of quantum mechanical coherences in chemical and biological systems play important roles in energy relaxation and reaction dynamics. The direct observation of coherence transfer events in condensed-phase spectroscopic experiments, however, is often masked by rapid incoherent population relaxation and coherence dephasing dynamics. Here, we report ultrafast two‑dimensional infrared (2D IR) spectroscopy of the peptide glutathione (GSH) isolated and cryogenically cooled in the gas phase. The action-based experiments are insensitive to population relaxation, allowing for the observation of long-lived quantum beating pathways between vibrational coherences during the pump-probe delay time. The dynamics of both diagonal and off‑diagonal cross peak features are inconsistent with predictions based on the common "secular" pathways from perturbative descriptions of nonlinear spectroscopies. These deviations indicate the presence of nonsecular pathways, including coherence-to-coherence and coherence‑to-population transfer processes, between the half-dozen vibrational modes in GSH simultaneously excited within the amide I and II region.