Measuring ion-pairing and hydration in variable charge supramolecular cages with microwave microfluidics

Angela C Stelson,Cynthia M Hong,Robert G Bergman,Mitchell C Groenenboom,James C Booth,Nathan D Orloff,Christian J Long,Charles A E Little,Kathleen A Schwarz,F Dean Toste,Kenneth N Raymond

doi:10.1038/s42004-019-0157-9

Abstract

Metal-organic supramolecular cages can act as charged molecular containers that mediate reactions, mimic enzymatic catalysis, and selectively sequester chemicals. The hydration of these cages plays a crucial role in their interactions with other species. Here we use microwave microfluidics to measure the hydration and ion pairing of two metal-organic cage assemblies that are isostructural but have different overall anionic charge. We supplement our measurements with density functional theory calculations to compare binding site energies on model metal-organic cage vertices. We find that the cage with dianionic vertices is more strongly hydrated and forms a distinct ion pair species from the cage with trianionic vertices. We evaluate multi-ion species and distinct ion pair solvations as possible sources for differences in ion dynamics and hydration. Broadly, this work highlights the utility of microwave microfluidics to elucidate the consequences of charge states on metal-organic complexes in solution.

Highlights

Metal-organic supramolecular cages can act as charged molecular containers that mediate reactions, mimic enzymatic catalysis, and selectively sequester chemicals
To characterize the hydration and ion pairing in solution, we used broadband (40 kHz–110 GHz) microwave microfluidic measurements
The broad frequency range allows microwave microfluidics to accurately correct for both the lowfrequency effects associated with the electrical double layer (EDL) at the electrode surface and the high-frequency effects from water relaxation

Summary

Introduction

Metal-organic supramolecular cages can act as charged molecular containers that mediate reactions, mimic enzymatic catalysis, and selectively sequester chemicals The hydration of these cages plays a crucial role in their interactions with other species. A common design motif for such supramolecular catalysts are selfassembled metal–organic cages that consist of organic ligands coordinating cationic metal ions to generate a variety of charged, polyhedral architectures. Understood issues include the distribution of charge throughout these supramolecular systems, as well as the localization and rates of ion-pairing interactions[5,6,7,8,9] These factors are often ignored in the design of supramolecular cages because they are not well understood, despite the fact that they have consequences for chemical applications[4,6,10]. Bulkfluid dielectric spectroscopy can measure hydration and ionic interactions in complex biomolecular systems including proteins, a

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Conclusion