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

摘要

Fundamental studies of the effect of specific ions on hydrophobicinteractions are driven by the need to understand phenomena such ashydrophobically driven self-assembly or protein folding. Usingbeta-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 densityfluctuation near a non-polar molecular surface, showing that such fluctuationcan be an indicator of hydrophobic interactions in the case of solutionadditives. In the case of immobilized ion, our results demonstrate thathydrophobic interactions can be switched on and off by choosing differentspatial arrangements of proximal ions on a nano-rod. For globally amphiphilicnano-rods, we find that the magnitude of the interaction can be further tunedusing proximal ions with varying ionic sizes. In general, univalent proximalanions are found to weaken hydrophobic interactions. This is in contrast to theeffect of free ions, which according to our simulations strengthen hydrophobicinteractions. In addition, immobilized anions of increasing ionic size do notfollow the same ordering (Hofmeister-like ranking) as free ions when it comesto their impact on hydrophobic interactions. The immobilized ion effect is notsimply correlated with the water density fluctuation near the non-polar side ofthe amphiphilic nano-rod. We propose a molecular picture that explains thecontrasting effects of immobilized versus free ions.