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Hydrophobic Effect
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The hydrophobic effect is the observed tendency of nonpolar substances to
aggregate in aqueous solution and exclude water molecules. This occurs
because interactions between the hydrophobic molecules allow water molecules
to bond more freely, increasing the entropy of the system. The word
hydrophobic literally means "water-fearing," and it describes the segregation
and apparent repulsion between water and nonpolar substances.
The hydrophobic effect is responsible for the separation of a mixture of oil and
water into its two components. The hydrophobic effect is also responsible for
the stability of cell membranes, drives protein folding as well as the insertion of
membrane proteins into the nonpolar lipid environment and finally stabilizes
protein-small molecule interactions. Hence the hydrophobic effect is essential to
life. Substances for which this effect is observed are known as hydrophobes.
Amphiphiles
Amphiphiles are molecules that have both hydrophobic and hydrophilic
domains. Detergents are composed of amphiphiles that allow hydrophobic
molecules to be solubilized in water by forming micelles and bilayers (as in
soap bubbles). They are also important to cell membranes composed of
amphiphilic phospholipids that prevent the internal aqueous environment of a
cell from mixing with external water.
Folding of macromolecules
In the case of protein folding, the hydrophobic effect is important to
understanding the structure of proteins that have hydrophobic amino acids (such
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as alanine, valine, leucine, isoleucine, phenylalanine, tryptophan and
methionine) clustered together within the protein. Structures of water-soluble
proteins have a hydrophobic core in which side chains are buried from water,
which stabilizes the folded state. Charged and polar side chains are situated on
the solvent-exposed surface where they interact with surrounding water
molecules. Minimizing the number of hydrophobic side chains exposed to water
is the principal driving force behind the folding process, although formation of
hydrogen bonds within the protein also stabilizes protein structure.
The energetics of DNA tertiary structure assembly were determined to be driven
by the hydrophobic effect, in addition to Watson-Crick base pairing, which is
responsible for sequence selectivity, and stacking interactions between the
aromatic bases.
Protein purification
In biochemistry, the hydrophobic effect can be used to separate mixtures of
proteins based on their hydrophobicity. Column chromatography with a
hydrophobic stationary phase such as phenyl-sepharose will cause more
hydrophobic proteins to travel more slowly, while less hydrophobic ones elute
from the column sooner. To achieve better separation, a salt may be added
(higher concentrations of salt increase the hydrophobic effect) and its
concentration decreased as the separation progresses.
The origin of hydrophobic effect
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The origin of the hydrophobic effect is not fully understood. Some argue that
the hydrophobic interaction is mostly an entropic effect originating from the
disruption of highly dynamic hydrogen bonds between molecules of liquid
water by the nonpolar solute. A hydrocarbon chain or a similar nonpolar region
of a large molecule is incapable of forming hydrogen bonds with water.
Introduction of such a non-hydrogen bonding surface into water causes
disruption of the hydrogen bonding network between water molecules. The
hydrogen bonds are reoriented tangentially to such surface to minimize
disruption of the hydrogen bonded 3D network of water molecules, and this
leads to a structured water "cage" around the nonpolar surface. The water
molecules that form the "cage" (or solvation shell) have restricted mobility. In
the solvation shell of small nonpolar particles, the restriction amounts to some
10%. For example, in the case of dissolved xenon at room temperature a
mobility restriction of 30% has been found. In the case of larger nonpolar
molecules, the reorientational and translational motion of the water molecules in
the solvation shell may be restricted by a factor of two to four; thus, at 25 °C the
reorientational correlation time of water increases from 2 to 4-8 picoseconds.
Generally, this leads to significant losses in translational and rotational entropy
of water molecules and makes the process unfavorable in terms of the free
energy in the system. By aggregating together, nonpolar molecules reduce the
surface area exposed to water and minimize their disruptive effect.
The hydrophobic effect can be quantified by measuring the partition coefficients
of non-polar molecules between water and non-polar solvents. The partition
coefficients can be transformed to free energy of transfer which includes
enthalpic and entropic components, ΔG = ΔH - TΔS. These components are
experimentally determined by calorimetry. The hydrophobic effect was found to
be entropy-driven at room temperature because of the reduced mobility of water
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molecules in the solvation shell of the non-polar solute; however, the enthalpic
component of transfer energy was found to be favorable, meaning it
strengthened water-water hydrogen bonds in the solvation shell due to the
reduced mobility of water molecules. At the higher temperature, when water
molecules become more mobile, this energy gain decreases along with the
entropic component. The hydrophobic effect increases with temperature, which
leads to "cold denaturation" of proteins.
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