Preferential Solvation: Dividing Surface vs Excess Numbers

Abstract

How do osmolytes affect the conformation and configuration of supramolecular assembly, such as ion channel opening and actin polymerization? The key to the answer lies in the excess solvation numbers of water and osmolyte molecules; these numbers are determinable solely from experimental data, as guaranteed by the phase rule, as we show through the exact solution theory of Kirkwood and Buff (KB). The osmotic stress technique (OST), in contrast, purposes to yield alternative hydration numbers through the use of the dividing surface borrowed from the adsorption theory. However, we show (i) OST is equivalent, when it becomes exact, to the crowding effect in which the osmolyte exclusion dominates over hydration; (ii) crowding is not the universal driving force of the osmolyte effect (e.g., actin polymerization); (iii) the dividing surface for solvation is useful only for crowding, unlike in the adsorption theory which necessitates its use due to the phase rule. KB thus clarifies the true meaning and limitations of the older perspectives on preferential solvation (such as solvent binding models, crowding, and OST), and enables excess number determination without any further assumptions.

Hydrotropy: Monomer–Micelle Equilibrium and Minimum Hydrotrope Concentration

“…Now, let us divide the solution into two parts; the first part (called the "solute's vicinity") contains a solute molecule, the other part (called the "bulk") is far away from the solute. 6,7,23,27,40,41 To explore the thermodynamic consequence of this soluteinduced concentration change of the solvent species, let us first write down the Gibbs−Duhem equation for each part, vicinity (represented by *) and bulk: 6,7,23,27,40,41 …”

“…Thus, the Gibbs−Duhem eqs 1 and 2 have now been rewritten explicitly in terms of the concentration change (n i * − n i ) in the solute's vicinity. 6,7,23,27,40,41 Now we introduce the excess solvation number of the species i around the solute defined as follows:…”

“…6,7,23,27,40,41 By equating the species u in eq 7 with the species 2 and 3 at dT = 0, respectively, we obtain the following:…”

Practical molecular thermodynamics for greener solution chemistry

“…1) one obtains a number which would be 0 if the distribution were entirely average (no special interactions), positive for typical small molecules if the molecules have fairly strong interactions and negative for systems with unfavourable interactions. [14][15][16][17][28][29][30][31][32][33][34][35][36] The values for systems of interest are readily found from the hydrotrope app discussed below. These numbers are called the Kirkwood-Buff Integrals (KBI) and the free energy of the system can be calculated on a rigorous, assumption-free basis from the KBI.…”

Macromolecular Stabilization by Excluded Cosolutes: Mean Field Theory of Crowded Solutions

“…As discussed in the Introduction, the magnitude of stabilization or destabilization is quantified by the change in preferential hydration, W ∆Γ , 10,25,64,65 which in the following we designate ∆Γ for short. Since the lattice model is incompressible, so that ( ) 0 PV ∆ = (where P and V are the system pressure and volume, respectively), the enthalpic and energetic changes of any process 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 15 are necessarily identical.…”