9.12: Spheres of Hydration
- Page ID
- 178350
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Water Dynamics in the Hydration Shells of Biomolecules
The Role of Water in Biological Systems
Water is the universal medium for life. Virtually every biochemical reaction occurs in aqueous solution, where water is more than a passive solvent—it actively shapes biomolecular structure and function. Its ability to form hydrogen bonds (H-bonds), exhibit polarity, and generate long-range electric fields allows it to stabilize proteins, DNA, and membranes and to influence processes such as folding, catalysis, and energy transfer (Laage et al., 2017, p. 10694).
The Concept of a Hydration Shell
When a biomolecule is placed in water, a distinct layer of water molecules forms around it. This region, known as the hydration shell, differs from bulk water in both structure and dynamics.
- Definition: The hydration shell consists of the first, or sometimes the first few, layers of water molecules whose properties are measurably altered by the nearby biomolecule (p. 10696).
- Thickness: Typically extends about one molecular layer (~3.5 Å) from the surface, though it varies depending on surface chemistry and topology.
- Function: Acts as a buffer zone mediating energy transfer, electric fields, and molecular motion between biomolecule and solvent.
Schematic of bulk water vs. hydration shell:
Typical hydration shells of (a) a protein, (b) a DNA double strand, and (c) a phospholipid bilayer (snapshots from simulations described in refs 57−59).
https://pubs.acs.org/doi/pdf/10.1021...rticle_openPDF published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.
Heterogeneity of the Hydration Shell
Not all regions of the shell behave identically. Surfaces rich in charged or polar groups attract and strongly orient water molecules, while hydrophobic patches allow looser, faster motion (p. 10704). MD mapping reveals broad distributions of residence times and reorientation rates:
- Water near hydrophilic sites often lingers longer and rotates more slowly.
- Water adjacent to flexible or nonpolar regions behaves almost like bulk water.
This heterogeneity explains why proteins can simultaneously exhibit rigid structural “spines of hydration” and more fluid, exchangeable water elsewhere.
Biological Significance
Hydration dynamics influence numerous biochemical functions:
- Protein Flexibility: Water acts as a “lubricant,” enabling conformational changes and catalysis (p. 10696).
- Enzyme Activity: Proper hydration levels (~0.3 g H₂O per g protein) are critical, but some enzymes retain activity even with minimal water, showing adaptability of the hydration environment.
- Energy Transfer and Dissipation: Vibrational energy from biomolecular excitations dissipates through the water shell, preventing local overheating (p. 10709).
In essence, the hydration shell forms a dynamic interface that couples molecular structure, thermodynamics, and biological function.
Key Takeaways
- Hydration shells differ structurally and dynamically from bulk water but remain mobile and adaptable.
- Most perturbations are limited to the first water layer (~3.5 Å).
- Water dynamics span femtoseconds to nanoseconds, providing flexibility critical for life.
- Techniques such as 2D-IR and MD simulations reveal that hydration water is moderately slowed, not immobilized.
- Understanding these microdynamics helps explain how solvent behavior governs biomolecular stability and activity.
Reference
Laage, D., Elsaesser, T., & Hynes, J. T. (2017). Water dynamics in the hydration shells of biomolecules. Chemical Reviews, 117(16), 10694–10725, under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. https://doi.org/10.1021/acs.chemrev.6b00765
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