Hydrophobic effect

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The hydrophobic effect is the property that nonpolar molecules tend to self-associate in the presence of aqueous solution. In the most extreme case, oils will pool together and fail to be miscible with water; detergents forming micelles and bilayers (as in soap bubbles) are another dramatic consequence of the hydrophobic effect.

The hydrophobic effect is usually described in the context of protein folding, protein-protein interactions, nucleic acid structure, and protein-small molecule interactions.

In the case of protein folding, it is used to explain why many proteins have a hydrophobic core which consists of hydrophobic amino acids, such as alanine, valine, leucine, isoleucine, phenylalanine, and methionine grouped together; often coiled-coil structures form around a central hydrophobic axis.

The energetics of DNA tertiary structure assembly were determined by Eric Kool to be mostly caused by the hydrophobic effect, as opposed to Watson-crick base pairing. However there is also a significant contribution from stacking interactions between the aromatic bases; this is distinct from the hydrophobic effect proper.

The hydrophobic effect can be nullified to a certain extent by lowering the temperature of the solution to near zero degrees Celsius; at such temperatures, water tends toward an ordered structure and the order generated by hydrophobic patches is no longer as energetically unfavorable. This is neatly demonstrated by the increased solubility of benzene in water at temperatures lower than room temperature.

The transfer free energy of nonpolar molecule from nonpolar solvent to aqueous solvent is often used to quantify the hydrophobic effect. The transfer free energy of hydrophobic molecule, $Δ G t$ , is positive. The $Δ G t$ can be decomposed to the enthalpy component $Δ H t$ and entropy component $- T Δ S t$ by the thermodynamic relation $G = H - T S$ . In room temperature, $Δ H t$ is approximately zero, and $Δ S t$ is negative. In other words, the hydrophobic effect is entropy driven at room temperature. The other characteristic thermodynamic quantity of the hydophobic effect is heat capacity change in transfer, $Δ C p,t$ , which has a positive value as contrasted to a negative value in the transfer of a hydrophilic molecule.

Another way of understanding the hydrophobic effect is the example of a hydrophobic substance in water. Pure water molecules adopt a structure which maximizes entropy (S). A hydrophobic molecule will disrupt this structure and decrease entropy, and creates a 'cavity' as it is unable to interact electrostatically with the water molecules. When more than one 'cavity' is present, the surface area of disruptions is high, meaning there are less free water molecules. To counter this, the water molecules push the hydrophobic molecules together and form a 'cage' structure around them which will have a smaller surface area than the total surface area of the cavities. This maximises the amount of free water and thus the entropy.