The Chaotropic Effect as an Assembly Motif in Chemistry

Abstract

Following up on scattered reports on interactions of conventional chaotropic ions (for example, I^- , SCN^- , ClO₄^- ) with macrocyclic host molecules, biomolecules, and hydrophobic neutral surfaces in aqueous solution, the chaotropic effect has recently emerged as a generic driving force for supramolecular assembly, orthogonal to the hydrophobic effect. The chaotropic effect becomes most effective for very large ions that extend beyond the classical Hofmeister scale and that can be referred to as superchaotropic ions (for example, borate clusters and polyoxometalates). In this Minireview, we present a continuous scale of water-solute interactions that includes the solvation of kosmotropic, chaotropic, and hydrophobic solutes, as well as the creation of void space (cavitation). Recent examples for the association of chaotropic anions to hydrophobic synthetic and biological binding sites, lipid bilayers, and surfaces are discussed.

Keywords: aqueous solvation; cavitation; large anions; superchaotropes; superchaotropic ions.

Figure 1

Principal aqueous solvation patterns for anions. a) Presumed favorable orientations of water molecules around a cavity and at the surface of different solutes: kosmotropic and chaotropic ions, hydrophobic molecules, and void space. Electron lone pairs are visualized in yellow. b) Extended Hofmeister scale with specification of the superchaotropic, hydrophobic ionic, and superhydrophobic regions; see Figure S1 in the Supporting Information for a two‐dimensional illustration of the solvation pattern dependence on charge density and polarizability.

Figure 2

Enthalpy–entropy compensation plot for γ‐CD complexes with dodecaborate anions and previously reported γ‐CD complexes with diverse organic guests.10

Figure 3

Macrocycles with intrinsic affinity for chaotropic and superchaotropic anions.

Figure 4

XRD structures for superchaotrope/CD complexes a) γ‐CD/B₁₂Br₁₂ ²⁻, b) γ‐CD/PMo₁₂O₄₀ ³⁻, c) β‐CD/PMo₁₂O₄₀ ³⁻, d) γ‐CD/[P₂W₁₈O₆₂]⁶⁻/[Ta₆Br₁₂(H₂O)₆]²⁺, and e) [Mo₁₅₄]/γ‐CD/[P₂W₁₈O₆₂]⁶⁻.

Figure 5

Examples of hierarchical and orthogonal solution‐phase architectures exploiting the chaotropic effect as an assembly motif: a) assembly of an amphiphilic calixarene on dodecaborate‐stabilized gold nanoparticles and b) assembly of CB7 in the presence of an amphiphilic dodecaborate‐functionalized azobenzene. The red residue illustrates a superchaotropic unit as recognition site to the exterior surface of CB7 and the blue unit represents an auxiliary hydrophobic unit that shows a preferential affinity for inclusion complexation with CB7.

Figure 6

XRD structures of CBn/superchaotropic anion exclusion complexes a) CB7/B₁₂Cl₁₂ ²⁻ and b) CB8/[H₂O⊂V^IV ₁₈O₄₂]¹²⁻.

Figure 7

Thermodynamics of anion binding to human carbonic anhydrase II: a) Plot showing the thermodynamic parameters for the association of different anions (298.15 K, pH 7.6, 10 mm sodium phosphate buffer). b) Plot of the free energies of binding (ΔG°_bind,anion) versus the free energies of hydration (ΔG°_hydration). Reprinted with permission from Ref. 161.

Figure 8

Leakage of carboxyfluorescein (CF) induced by B₁₂I₁₂ ²⁻ in DSPC liposomes at 37 °C. Concentrations of clusters for the individual traces in the graph are given on the left (mm). Suggested mechanism of the interaction between dodecaborate clusters and liposomes (right). Reprinted with permission from Ref. 36.

Figure 9

Left: Cloud points of a 60 mm triethylene glycol monomethyl ether solution in the presence of POMs: SiW⁴⁻ (SiW₁₂O₄₀ ⁴⁻) and PW³⁻ (PW₁₂O₄₀ ³⁻) and sodium salts with various representative anions of the Hofmeister series: SCN⁻ (chaotropic), WO₄ ⁻ (kosmotropic), and Cl⁻ (intermediate between salting‐in and salting‐out). Inset: Ion‐containing surfactant solution at temperatures below (left) and above (right) the cloud point. Right: Schematic of the adsorption of a POM anion on a hydrophilic interface covered by a polyethoxylated surfactant (right). Reprinted with permission from Ref. 16.

Figure 10

Micellization can be driven by the classical hydrophobic effect (top) or the chaotropic effect (or a combination of the chaotropic and non‐classical hydrophobic effect, bottom) with emphasized changes of the hydration shells for the classical surfactant and the large anion cobalt bis(1,2‐dicarbollide), COSAN. For a specification of the non‐classical hydrophobic effect, see the Supporting Information. Reprinted with permission from Ref. 40.