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Ion–Hydrocarbon and/or Ion–Ion Interactions: Direct and Reverse Hofmeister Effects in a Synthetic Host

Jordan, Jacobs H., Gibb, Corinne L. D., Wishard, Anthony, Pham, Thu, Gibb, Bruce C.
Journal of the American Chemical Society 2018 v.140 no.11 pp. 4092-4099
ammonium, anions, binding sites, nuclear magnetic resonance spectroscopy, turbidity
A combination of ¹H NMR spectroscopy, DLS, and turbidity measurements reveal that polarizable anions engender both the Hofmeister and reverse Hofmeister effects in positand 2. Host 2 possesses two principal and distinctly different binding sites: a “soft” nonpolar pocket and a “hard” crown of ammonium cations. NMR spectroscopy reveals that anion affinity to both sites is comparable, with each site showing characteristic selectivities. NMR spectroscopy also reveals that anions competitively bind to the pocket and induce the Hofmeister effect in host–guest binding at very low concentrations (∼2 mM). Furthermore, the suite of techniques utilized demonstrates that anion binding to both sites leads to charge attenuation, aggregation, and finally precipitation (the reverse Hofmeister effect). Anion-induced precipitation generally correlated with affinity, and comparisons between the free host and its adamantane carboxylate (Ada-CO₂–) complex reveals that the reverse Hofmeister effect is attenuated by blocking anion binding/charge attenuation at the nonpolar pocket.