Weakly hydrated anions bind to polymers but not monomers in aqueous solutions

Abstract

eakly hydrated anions, such as I − , SCN − and ClO 4 − , weaken the hydrophobic effect in aqueous solutions. These large, polarizable anions denature proteins, inhibit supramolecular complexation and dissolve surfactant micelles [1][2][3] . At the molecular level, weakly hydrated anions partially shed their hydration shells and adsorb to nonpolar interfaces, thereby inhibiting hydrophobic assembly [4][5][6][7][8][9][10] . The adsorption of these anions to amide-rich polymers has been characterized by submolar to molar equilibrium dissociation constants, K D = 0.05-1.60 M (refs. 11-14 ). Even tighter adsorption has been observed at macroscopic surfaces, such as the air/water interface (K D = 0.03-0.26 M) and in the concave pockets of cavitands and proteins (K D = 0.003-0.09 M) 2,15-22 . By stark contrast, anions are repelled from small molecules, like N-methyl acetamide and tert-butyl alcohol (K D = 4-8 M) 23,24 . As a consequence, weakly hydrated anions precipitate small non-ionic solutes out of aqueous solutions, including acetone and diacetone alcohol 25 . The dramatic range of anion affinity for chemically similar aliphatic binding sites exposes a critical gap in our knowledge of the mechanisms for anion-specific effects.The surface curvature of nonpolar solutes is known to influence solubility because of the distinct local hydration of curved and flat interfaces 26,27 . The water hydrogen-bonding network can wrap around small and convex solutes to maintain its bulk-like structure. Large solutes, however, have a flatter topography that disrupts the hydrogen bonds between water molecules 27 . As such, small solutes can be incorporated into the water network, while larger ones with broken hydrogen bonds associate with each other and release water molecules into the bulk solution. The cartoon in Fig. 1 depicts a simple model for a polymer chain. The termini are highly curved due to their half-spherical geometry, while the centre of the chain is flatter because it has a cylindrical-like structure. Experimental and computational studies of nonpolar solutes with varying chain lengths have shown that this topography disorders water at the centre of the chain more than at the termini 28,29 .Here, the role of surface curvature on anion-specific effects is explored by systematically measuring the interactions of NaSCN with polyethylene oxides (PEO) of varying molecular weights, ranging from monomers to polymers. The results indicate that SCN − is repelled from monomers but attracted to oligomers of increasing chain length. These interactions are distinct because SCN − binds selectively to the centre of oligomer chains, as opposed to their termini (Fig. 1). Investigations of polyether hydration shells reveal that the water structure at the centre of the chain is more disordered than at the termini. Together, these findings imply that SCN − interacts with low-curvature interfaces to displace water at sites of hydrogen-bonding defects. The correlation of binding affinity and water structure measurements at specific locati...

Counterintuitive Electrostatics upon Metal Ion Coordination to a Receptor with Two Homotopic Binding Sites

“…A possible explanation for this is the relatively higher affinity of the weakly hydrated thiocyanate ion to apolar surfaces compared to more hydrated anions. 31 , 32 This was further supported by the analysis of the distribution of counterions around BCETB with one or two potassium ions bound, where we found a significant enrichment of thiocyanate close to BCETB compared to the other counterions.…”

“…This is in agreement with previously observed higher affinities of the thiocyanate ion to apolar surfaces compared to more strongly hydrated anions. 31 , 32 As previously mentioned, the enrichment of thiocyanate ions around BCETB compared to the other counterions could contribute to the stabilization of the complex with one or two potassium ions bound.…”

Polymeric Surfactant P84/Polyoxometalate α-PW₁₂O₄₀^3-—A Model System to Investigate the Interplay Between Chaotropic and Hydrophobic Effects

“…As a conclusion, HPW binds to P84 unimers which is due to its polymeric nature, while monomeric species (e.g. only one PO repetition unit) are not expected to associate with a chaotropic anion [40] such as PW. Here, the HPW binds selectively to the PPO moiety over to the PEO moieties of P84 unimers.…”

Polymeric Surfactant P84/Polyoxometalate α-PW12O403−—A Model System to Investigate the Interplay between Chaotropic and Hydrophobic Effects

“…In conclusion, HPW binds to P84 unimers due to its polymeric nature (81 repetition units) via the chaotropic effect. In contrast, monomeric species (e.g., only 1 "repetition" unit) are not expected to associate with a chaotropic anion [40] such as PW. Here, the HPW bound selectively to the PPO moiety over to the PEO moieties of P84 unimers.…”