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.
Highlights
Affinity determinations in organic media can successfully treat the solvent as purely a spectator, in aqueous supramolecular chemistry[1] water can seldom, if ever, be ignored
The classical treatment of electrostatic interactions in solution based on the Poisson−Boltzmann equation is routine,[10] and the net hydration thermodynamics of common ions are known,[11] the map of the plasticity of the solvation shells of ions, and the manner in which they can interact noncovalently with other chemical entities, has yet to be drawn, despite the ubiquity of buffers in the biosciences[12,13] and the appreciation that they interact with other ions[14−17] and biomacromolecules.[18−21] By probing and mapping the supramolecular properties of buffers and more generally ions, supramolecular chemists can assist the biological sciences in their studies of biomacromolecules
Using the standard unscreened model for affinity determinations, we have measured the affinity of halide ions and the buffer species
Summary
Affinity determinations in organic media can successfully treat the solvent as purely a spectator, in aqueous supramolecular chemistry[1] water can seldom, if ever, be ignored. Our experiments reveal that the generally larger changes in affinity observed between unbuffered versus buffered solutions are mostly due to simple direct guest competition for the host We anticipate that these findings will help address the uncertainly often associated with binding constant determinations in water and buffered solutions and contribute to the long-term goal of understanding the supramolecular properties of buffers and ions in general. Supramolecular chemists the affinity is independent of ionic strength (I), as the concentration of salt or buffer is increased, the affinity described by the screened model is expected to continuously decrease.[36] The logical common frame of reference here to compare the two models is at the theoretical situation where I is zero, i.e., KaU,[0] and KaS,[0] for the unscreened and screened model, respectively These are the values we report below (Table 2)
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