Buffer and Salt Effects in Aqueous Host–Guest Systems: Screening, Competitive Binding, or Both?

Abstract

There are many open questions regarding the supramolecular properties of ions in water, a fact that has ramifications within any field of study involving buffered solutions. Indeed, as Pielak has noted (Buffers, Especially the Good Kind, Biochemistry, 2021, in press. DOI:10.1021/acs.biochem.1c00200) buffers were conceived of with little regard to their supramolecular properties. But there is a difficulty here; the mathematical models supramolecular chemists use for affinity determinations do not account for screening. As a result, there is uncertainty as to the magnitude of any screening effect and how this compares to competitive salt/buffer binding. Here we use a tetra-cation cavitand to compare halide affinities obtained using a traditional unscreened model and a screened (Debye–Hückel) model. The rule of thumb that emerges is that if ionic strength is changed by >1 order of magnitudeeither during a titration or if a comparison is sought between two different buffered solutionsscreening should be considered. We also build a competitive mathematical model showing that binding attenuation in buffer is largely due to competitive binding to the host by said buffer. For the system at hand, we find that the effect of competition is approximately twice that of the effect of screening (∼RT at 25 °C). Thus, for strong binders it is less important to account for screening than it is to account for competitive complexation, but for weaker binders both effects should be considered. We anticipate these results will help supramolecular chemists unravel the properties of buffers and so help guide studies of biomacromolecules.

A Tunable Porous β-Cyclodextrin Polymer Platform to Understand and Improve Anionic PFAS Removal

“…Additionally, the type of anion may potentially affect inhibition to a varying extent. We attribute anion valency to either direct-site competition or a screening effect . For instance, one unit of divalent anion sulfate has a greater screening effect due to compression of the electrical double layer than two units of monovalent anion, where the compression is directly related to ionic strength which is proportional to the square of ion valency.…”

Buffer-specific effects arise from ionic dispersion forces

“…This case could be modelled using the idea of specific binding (chemisorption). 33 But the crossing point of cacodylate and hydrogen phosphate lies outside the ion radius, indicating the role of the dispersion interaction as nonelectrostatic physisorption, active at a distance from the surface. The crossing point for citrate lies at 6.4 Å, well past the region of direct surface contact.…”

Understanding specific ion effects and the Hofmeister series

“…The screening and competitive behaviour of buffers has recently been modelled, showing the importance of ionic strength in this respect. 351 The fact that mixed salt effects have also been studied in polyelectrolytes 352,353 and neutral polymer solutions (Section 5) presents alternative opportunities for studying SIEs via ionic surfactants, which are already known to influence mesophase behaviour in mixed salt-surfactant systems in an ion-specific way. 354,355 Ionic surfactants also enable systematic investigation and manipulation of SIEs, for instance by using a constant charge group with increasing aliphatic tail length ( i.e.…”