Solutes, ranging from small hydrophobic molecules to macromolecular structures such as proteins, are heavily influenced by the presence of salts, particularly the anionic component. It is well known that anions possessing a high charge density, such as sulfate, are able to reduce the solubility of these solutes, with this effect likely to be entropically driven and based on the influence the anions have on the structure of water. Other anions with a lower charge density, such as perchlorate, can have the opposite effect, improving solubility. The current consensus for the latter effect is that rather than being as a result of changes to water structure, it is due to specific binding of the anions to solute molecules, acting to reduce unfavourable interactions with the solvent. In this paper we report on how both sulfate and perchlorate influence binding and catalysis for the α-cyclodextrin mediated reaction of iodide with 3-chloroperbenzoic acid (MCPBA) for which the cyclodextrin–MCPBA complex is the catalytic species. Kinetic runs have been carried out over the temperature range 15–35 °C in dilute nitric acid and with added sulfate and perchlorate, allowing the calculation of enthalpies and entropies for the rate and equilibria processes involved in this system. We show that both sulfate and perchlorate demonstrate the expected Hofmeister behaviours, with sulfate acting to exclude the MCPBA from solution into the cyclodextrin cavity, thus increasing the amount of the catalytic species, whereas perchlorate acts to solubilise the MCPBA. Perchlorate has a complex range of effects, since as well as forming an association complex with cyclodextrin, the thermodynamic parameters indicate that it may also associate with MCPBA and the MCPBA–cyclodextrin complex. These associations may explain the catalysis that is observed with increasing perchlorate concentration. The effects observed in this study may be relevant to other binding processes in chemistry and biology.
|Journal||Journal of Inclusion Phenomena and Macrocyclic Chemistry|
|Publication status||Published - Apr 2014|