Probing hydration-driven ion recognition using cryogenic ion trap infrared spectroscopy

Ion recognition, in which a molecule (ionophore) binds to a selected ion, is one of the fundamental chemical and biological phenomena controlled by an intricate balance among ions, ionophores, and solvents. To date, however, the molecular-level understanding of ion recognition remains elusive due to the difficulty in directly probing solvation structures. Recent advances in gas-phase cryogenic ion trap spectroscopy have enabled us to probe the structural evolution of the micro-solvated ion-ionophore complexes, providing a benchmark for solvation effects on ion recognition. In this perspective, we review recent progress in elucidating the hydration-driven ion recognition phenomena using cryogenic double ion trap infrared spectroscopy, primarily focusing on prototypical ionophores, valinomycin, 18-crown-6, and beauvericin. Our findings reveal the critical roles of water in the ion selectivity of structurally flexible ionophores, for which molecular mechanisms cannot be rationalized by conventional understanding based on the size-matching model. We also focus on the future of cryogenic ion spectroscopy to study the chemistry of functional molecules.