Comparison between Free and Immobilized Ion Effects on Hydrophobic Interactions: A Molecular Dynamics Study

TL;DR

This study uses molecular dynamics simulations to compare how free and immobilized ions influence hydrophobic interactions, revealing distinct effects and mechanisms relevant to self-assembly and protein folding.

Contribution

It demonstrates that free and immobilized ions differently modulate hydrophobic interactions, with immobilized ions enabling control through spatial arrangements and size variations.

Findings

Free ions strengthen hydrophobic interactions via water density fluctuations.

Immobilized ions can switch hydrophobic interactions on and off.

Size and type of immobilized ions influence interaction strength and behavior.

Abstract

Fundamental studies of the effect of specific ions on hydrophobic interactions are driven by the need to understand phenomena such as hydrophobically driven self-assembly or protein folding. Using beta-peptide-inspired nano-rods, we investigate the effects of both free ions (dissolved salts) and proximally immobilized ions on hydrophobic interactions. We find that the free ion effect is correlated with the water density fluctuation near a non-polar molecular surface, showing that such fluctuation can be an indicator of hydrophobic interactions in the case of solution additives. In the case of immobilized ion, our results demonstrate that hydrophobic interactions can be switched on and off by choosing different spatial arrangements of proximal ions on a nano-rod. For globally amphiphilic nano-rods, we find that the magnitude of the interaction can be further tuned using proximal ions with varying ionic sizes. In general, univalent proximal anions are found to weaken hydrophobic interactions. This is in contrast to the effect of free ions, which according to our simulations strengthen hydrophobic interactions. In addition, immobilized anions of increasing ionic size do not follow the same ordering (Hofmeister-like ranking) as free ions when it comes to their impact on hydrophobic interactions. The immobilized ion effect is not simply correlated with the water density fluctuation near the non-polar side of the amphiphilic nano-rod. We propose a molecular picture that explains the contrasting effects of immobilized versus free ions.