Structural Order of Water Molecules around Hydrophobic Solutes: Length-Scale Dependence and Solute–Solvent Coupling

Abstract

It has been suggested that the structure and thermodynamics of the water molecules in the hydration layer of simple hydrophobic solutes undergo an order-disorder transition around a nanometer length-scale of the solute size. Using extensive atomistic molecular dynamics (MD) and replica exchange molecular dynamics (REMD) simulation studies, we have probed this order-disorder transition around model hydrophobic solutes of varying size and shape (spherical, planar, and linear), as well as flexible hydrophobic homopolymer chains (n-alkanes), where the conformational fluctuations are likely to create both spatial and temporal heterogeneity on the solvent accessible surface. We have explored the structural response of the water molecules in the hydration shell due to the local variations of the length-scale (or curvature) upon hydrophobic collapse and/or local conformational changes of these polymers. We have shown that the tetrahedral order of the water molecules in the hydration shell is practically independent of the polymer size in the extended state of the polymer due to the availability of a subnanometer cross-sectional length-scale, allowing the water molecules to form hydrogen bonds around the polymer chain. Beyond a certain length of the polymer chains, the collapsed states (associated with larger solute length-scale) start to induce disorder in the surface water molecules. We demonstrate that the local structure (both local number density and tetrahedral order) of the hydration layer is dynamically coupled to the local topology of the polymer. Thus, we envisage that in a flexible (bio)polymer, the hydration shell properties will be sensitive to the local conformational state of the molecule (both spatially and temporally), and the overall observed water structure and dynamics will be dependent on the topological/chemical heterogeneity, and the time-scale of fluctuations in the local curvature (length-scale) of the solvent accessible surface. Moreover, we have demonstrated the direct coupling between the local density fluctuations of water and the local hydrophobic collapse of the polymer. For the extended state of the polymer, the local solvent density fluctuation is practically independent of the solute coordinate (length-scale), and the hydrophobic collapse of the polymer is prompted by a "local dewetting" process induced by these fluctuations.

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

“…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.…”

“…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 .…”

Comparative Study of Anomalous Size Dependence of Charged and Neutral Solute Diffusion in Water

“…Although we find that compared to bulk water, the κ values are higher for some of the solutes (like for σ sO = 5.5 Å) suggesting that the water structure around them is more flexible, we are hesitant to call them the water-structure breaker, that is, chaotrope. This is because other studies have shown that water around a hydrophobic solute, smaller than a nanometer, maintains bulk-like structure and there is a size-dependent order–disorder transition around 1 nm. , Moreover, Yethiraj and co-workers have shown that simulation studies using similar ion–water models fail to show the structure-breaking properties of certain salts which are experimentally known to have chaotropic properties …”

Explicit solvation thermodynamics in ionic solution: extending grid inhomogeneous solvation theory to solvation free energy of salt–water mixtures

“…However, the high peak in g inh shows that this is not the case, and that a water molecule at this position is indeed coordinated rather strongly, resulting in a second-order entropy that is lower than in bulk. This is expected, since it is well known [90] that water molecules near small hydrophobic solutes tend to keep up their hydrogen bond network at the cost of an entropic penalty.…”