Caffeine dimerization: effects of sugar, salts, and water structure

Abstract

Sugars and salts strongly affect the dimerization of caffeine in water. Such a change of dimerization, considered to be crucial for bitter taste suppression, has long been rationalized by the change of "water structure" induced by the additives; "kosmotropic" (water structure enhancing) salts and sugars promote dimerization, whereas "chaotropic" (water structure breaking) salts suppress dimerization. Based on statistical thermodynamics, here we challenge this consensus; we combine the rigorous Kirkwood-Buff theory of solution with the classical isodesmic model of caffeine association. Instead of the change of water structure, we show that the enhancement of caffeine dimerization is due to the exclusion of additives from caffeine, and that the weakening of dimerization is due to the binding of additives on caffeine.

Computational analysis of the solvation of coffee ingredients in aqueous ionic liquid mixtures

“…38,43,55–59 The self-aggregation of the co-solvent at the surface of the solute leads to positive Kirkwood–Buff integralsof species j around i . Negative values of Δ G ij indicate less favorable binding of the solvent species.…”

“…However, in literature 14,44,55–57,59–62 the volume integration in eqn (1) is usually performed via spherical shellsfor the sake of simplicity. The integration starts at the surface of the solute and ends a value where Δ G ij has converged.…”

Effect of solute aggregation on solubilization

“…Theoretical analysis, based on the first principles of statistical thermodynamics, will lead to establishment of a simple criterion upon which the negligibility of solute self-association on solubilizaiton can be determined, which, despite extensive studies conducted on solute selfassociation in binary and ternary mixtures [30][31][32][33], has not been addressed previously. We will show that the solute's self association indeed makes negligible contributions in the hydrotrope solubilization of hydrophobic solutes studied in our previous papers [9][10][11][12][13], while it may not be negligible in less hydrophobic solutes, such as caffeine [23,34,35].…”

Characterizing the Effects of a “Switchable Water” Additive on the Aqueous Solubility of Small Molecules

“…However, over the range of TMBD concentrations studied (0–1 m ) the concentration of caffeine can be greater than 150 m m and so this assumption may not be valid. Caffeine has been shown to form dimers in solution, indicating that G uu can play a significant role in the solution behaviour of caffeine . However, accounting for the effects of high solute concentration requires a more involved interrogation method for ternary systems .…”

“…Caffeine has been shown to form dimers in solution, indicating that G uu can play as ignificant role in the solution behaviour of caffeine. [31] However,a ccount-ing for the effects of high solute concentration requires a more involved interrogation methodf or ternary systems. [32] Several systems with solute concentrations greater than 150 mm have also been studied using the same approach employed in this study.…”