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

Abstract

A combination of H 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-CO) complex reveals that the reverse Hofmeister effect is attenuated by blocking anion binding/charge attenuation at the nonpolar pocket.

Dynamic light scattering studies of the effects of salts on the diffusivity of cationic and anionic cavitands

“…3). In a previous work, our DLS studies of this host involved solutions buffered with 40 mM phosphate (pH 7.3) [14]. Under these conditions, the initially measured sizes in the absence of added salt were consistently 1.9–2.1 nm; values that match the modeling of the host.…”

“…Recently we demonstrated how host molecules can engender the Hofmeister [12–13] and the reverse Hofmeister effects [14]. In regards to the former, we have shown how poorly solvated anions such as SCN − have an affinity for non-polar surfaces.…”

“…To explore these ideas further we report here the responses of two deep-cavity cavitands, octacarboxylate 1 (counter ion Na + ) [15–16] and positand 2 (counter ion Cl − ) [14] (Fig. 1), to different salts using dynamic light scattering (DLS) [17–20].…”

“…More specifically, both octacarboxylate 1 and positand 2 have a non-polar cavity that can function as an anion (but not to our knowledge a cation) binding site. Anion binding to the cavity of positively charged 2 is stronger than to negatively charged 1 [13–14], but nevertheless anion binding to 1 can be as strong as 4.60 kcal mol −1 . Host 2 has a second anion binding site in the form of the crown of trimethylammoniums “under” the primary bowl [14], and correspondingly the four chelating carboxylates of the crown of 1 may be a reasonable cation binding site.…”

“…Anion binding to the cavity of positively charged 2 is stronger than to negatively charged 1 [13–14], but nevertheless anion binding to 1 can be as strong as 4.60 kcal mol −1 . Host 2 has a second anion binding site in the form of the crown of trimethylammoniums “under” the primary bowl [14], and correspondingly the four chelating carboxylates of the crown of 1 may be a reasonable cation binding site. Furthermore, in addition to these specific cavity and crown sites, the individual charge groups of 1 and 2 can function as weak (pseudo-specific) binding sites for ions of opposite charge.…”