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

Abstract

Although alkali halide salts play key roles in all living systems, the physical models used to describe the properties of aqueous solutions of salts do not take into account specific ion–ion interactions. To identify specific ion–ion interactions possibly contributing to the aggregation of proteins, we have used dynamic light scattering (DLS) to probe the aggregation of charged cavitands. DLS measurements of negatively charged 1 in the presence of a range of alkali metal halides reveal no significant aggregation of host 1 as a function of the nature of the cation of the added salt. Only at high concentrations could trace amounts of aggregation be detected by 1H NMR spectroscopy. Contrarily, 1 was readily aggregated and precipitated by ZnCl2. In contrast, although fluoride and chloride did not induce aggregation of positively charged host 2, this cavitand exhibited marked aggregation as a function of bromide and iodide concentration. Specifically, bromide induced small but significant amounts of dimerization, whilst iodide induced extreme aggregation. Moreover, in these cases aggregation of host 2 also exhibited a cationic dependence, with an observed trend Na+ > Li+ > K+ ≈ Cs+. In combination, these results reveal new details of specific ion pairings in aqueous solution and how this can influence the properties of dissolved organics.

Electrostatic Control of Macrocyclization Reactions within Nanospaces

“…This free energy difference corresponds to a p K a shift between the capsules of 3.0, which again is in reasonable agreement with those reported (Figure ). Overall, these agreements are excellent given the inherent assumptions made in this simple model, not the least of which is the neglect of ion-specific binding to the outside of the two capsules. , Furthermore, the model confirms–at least for the two host discussed here–the primary role of Coulombic forces in promoting cyclizations within the capsules. That stated, a potential cautionary note here is that the modeling does not reveal any significant difference in the internal EP field that may explain the differences in stabilization of the thiolate and TS.…”

Electrostatic Potential Field Effects on Amine Macrocyclizations within Yoctoliter Spaces: Supramolecular Electron Withdrawing/Donating Groups

“…Regarding ion association, we have previously demonstrated both specific ion binding to the crown of four ammonium groups defined by the pendent groups in 1 , 60 as well as nonspecific association (ion condensation) to the host and its dimer capsule. 41 , 61 In contrast, there are no specific cation binding sites in 2 or its capsule; only weak ion condensation has been observed. 41 , 62 Irrespective of the type of binding, increases in the ionic strength and/or swapping the capsule counterions by the addition of ions with higher cavitand affinity lead to a reduction in its EPF.…”

“…The effective charge of the two host capsules is less than their formal ±16 because of attenuation from associating counterions, and charge transfer to the solvation shell waters. Regarding ion association, we have previously demonstrated both specific ion binding to the crown of four ammonium groups defined by the pendent groups in 1 , as well as nonspecific association (ion condensation) to the host and its dimer capsule. , In contrast, there are no specific cation binding sites in 2 or its capsule; only weak ion condensation has been observed. , Irrespective of the type of binding, increases in the ionic strength and/or swapping the capsule counterions by the addition of ions with higher cavitand affinity lead to a reduction in its EPF. In the case of the capsules, this can induce a change in the nature of an assembly or modulate the chemical reactivity of an internalized guest …”

“…To gain more insight into the mechanism of the cyclization of 3 within the capsules, we investigated the effects on the nature of exogenous salts upon reaction rates within the two hosts (SI, Section 5). To minimize pairing between the ions of the added salt, we selected highly solvated sodium and chloride counterions for, respectively, studying how anions and cations affect cyclization rates within 1 2 and within 2 2 . , In all cases, the rate constants for cyclization were measured in the presence of 10 mM added salt and compared to the rate of the reaction with only base (NaOD) present. In most cases, 1 H NMR spectroscopy revealed that the added salt had no direct effect on the bound guest.…”

Nanopore Sensing Technique for Studying the Hofmeister Effect

“…Dynamic light scattering (DLS), NMR spectroscopy, and dark-field microscope are very useful tools for studying the Hofmeister effect. [12,13,16,[28][29][30] DLS technique mainly depends on the Brownian motion of particles. It can provide quite quantita-tive information such as hydrodynamic diameters for studying the Hofmeister effect.…”