Hydrophobic effect

The hydrophobic effect arises when non-polar and electrically uncharged atoms, molecules or groups of molecules are ( as parts of larger molecules ) in aqueous solution. He referred, among others, the then possible storage assembly ( " hydrophobic association " ) of non-polar molecules in water and aqueous solutions, such as those occurs in lipid bilayer and micelles. The hydrophobic effect also leads to changes in water structure and water dynamics (diffusion and rotational molecular motion ) near the apolar particles dissolved in the water.

Cause

The cause of the effect lies in the fact that the water molecules in the immediate vicinity can not form hydrogen bonds to a non-polar molecule and therefore among themselves bind somewhat stronger than in the "free " water. These adjacent water molecules are so arranged slightly higher and therefore limited in their translational and rotational motion. But it is, as described below, may total a dramatic decrease of local water mobility, as has been previously suspected. One speaks in connection with this particular Solvatsphäre also of " hydrophobic hydration " of the inert species.

Because according to the second law of thermodynamics, the entropy can never decrease in a closed system, several hydrophobic hydrated, non-polar molecules assemble together. This reduces the surface area to the water and thus the number of the more-ordered water molecules in the medium. Thus, the entropy increases. This hydrophobic association is certain (eg, elongated ) molecules, the basis of Biomembranbildung.

So a general rule:

  • The entropy increases due to the release of solvent molecules, but it also results in an enthalpy by stronger interactions between the released solvent molecules ( water, especially in dipole -dipole interactions ). There is thus an entropic and enthalpic a share of the hydrophobic effect.
  • There is therefore no intra - energetic hydrophobic force, the effect is based on the specificity of Hydrogen- gebundenden solvent water. However, the hydrophobic effect due to the effect of entropic force can be understood.

The amount of the hydrophobic effect of a molecule in water is described by the hydrophobicity.

Observations

Significant findings on the water structure around hydrophobic particles and thus on the causes of the hydrophobic effect is obtained from molecular dynamics computer simulations.

The hydrophobic effect may be quantified by determining the partition coefficient of non-polar particles between water and non-polar solvents. The distribution coefficients ( in the NPT ensemble) into the free transfer of energy? G = AH - TΔS be transferred, consisting of the enthalpic component AH and the entropic component TΔS. These two components can be measured calorimetrically.

In the solvation shell of small apolar particles slowing down the water molecules due to the stronger interaction with other water molecules at room temperature is some 10 %, for example at the noble gas xenon 30%. With larger non-polar molecules, and a slowing of the rotational diffusion of the water in the Solvatsphäre can also occur by a factor of 2 to 4. This means for example that the Umorientierungskorrelationszeit of water at 25 ° C of 2 ps rises to 4-8 picoseconds.

In protein folding, the hydrophobic effect is also very important. For although the entropy of the protein by the convolution decreases sharply (), outweighs the entropy gain in the surrounding aqueous medium ( ). Therefore, the Gesamtentropieänderung

Which applies to the entropic contribution. Thus, the convolution is " self " and requires no further energy input.

Another observation related to the hydrophobic effect is to make if salts are added to the aqueous solution of the hydrophobic particles or molecular groups. About nuclear magnetic relaxation measurements showed namely that anions experienced an attraction to the hydrophobic interface, whereas this is not true for the cations. This effect can possibly details in the so-called Hofmeister series explain.

Influences

Weakening of the hydrophobic effect by temperature and / or pressure change can cause denaturation of biomolecules. One brings additives, for example as salts in the aqueous solution, the hydrophobic effect is also changed, it can be both amplified and reduced. Addition of large amounts of urea, for example, leads to a denaturation of proteins. The strength of the influence of salts and thus of dissolved ions in the water on the hydrophobic effect is characterized by the so-called " Hofmeister series " one since the 19th century empirically determined and thus known ion - sequence, but which until today still is not fully understood theoretically.

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