Vibrational excitons in ionophores: experimental probes for quantum coherence-assisted ion transport and selectivity in ion channels

Despite there being a large body of work, the exact molecular details underlying ion selectivity and transport in the potassium channel have not been fully uncovered. One major reason has been the lack of experimental methods that can probe these mechanisms dynamically on their biologically relevant timescales. Recently, it was suggested that quantum coherence and its interplay with thermal vibration might be involved in mediating ion selectivity and transport. In this paper, we present an experimental strategy for using time-resolved infrared spectroscopy to investigate these effects. We show the feasibility by demonstrating the infrared (IR) absorption and Raman spectroscopic signatures of the potassium-binding model molecules that mimic the transient interactions of potassium with binding sites of the selectivity filter during ion conduction. In addition to guiding our experiments on the real system, we have performed molecular dynamic-based simulations of the FTIR and two-dimensional IR (2DIR) spectra of the entire KcsA complex, which is the largest complex for which such modeling has been performed. We found that by combining isotope labeling with 2DIR spectroscopy, the signatures of potassium interaction with individual binding sites would be experimentally observable, and we identified specific labeling combinations that would maximize our expected experimental signatures.