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Salting out and organic solvents

  1. Jan 15, 2012 #1
    Hello!
    I am confused about the protein interactions that occur when either salt concentration is increased or an organic solvent is introduced.
    When salting out, wiki says:

    "Addition of a neutral salt, such as ammonium sulfate, compresses the solvation layer and increases protein-protein interactions. As the salt concentration of a solution is increased, more of the bulk water becomes associated with the ions. As a result, less water is available to partake in the solvation layer around the protein, which exposes hydrophobic patches on the protein surface. Proteins may then exhibit hydrophobic interactions, aggregate and precipitate from solution."

    And for organic solvents:

    "The solvation layer around the protein will decrease as the organic solvent progressively displaces water from the protein surface and binds it in hydration layers around the organic solvent molecules. With smaller hydration layers, the proteins can aggregate by attractive electrostatic and dipole forces."

    Both seem to be saying that the hydration laayer is removed, and yet this leads to different interactions...

    I was reading from this source: http://teachline.ls.huji.ac.il/72682/Booklets/AMERSHAM_HIC_RPC.pdf about reverse phase chromatography, and it states that increasing organic solvent concentrations elutes the protein. From my understanding RPC works by incraesing the hydrophobic character of the eluent to compete with the stationary phase. In which case, hydrophobic interactions are increased, not electrostatic ones, as wiki says.
    Is it just me?
    Thanks in advance.
     
  2. jcsd
  3. Jan 16, 2012 #2

    Borek

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    Staff: Mentor

    This is a false dichotomy - all interactions involved are electrostatic (coulombic) in their nature. They can be further classified as hydrophobic or hydrophilic, depending on how strong the interaction with water molecules is.

    Note that even hydrophobic surface still attracts water. However, it usually attracts non-water molecules more, and the system tends to rearrange itself in such a way that those strong interacting surfaces are in contact.
     
  4. Jan 16, 2012 #3
    Thanks Borek!
     
  5. Jan 16, 2012 #4
    A protein's shape is determined by the electrostatic interaction of its constituent monomers (AAs) with each other and the normal solvent, water. When the solvent is changed to an inorganic solvent, the shape will change. For example, functional groups normally at the surface of a protein are polar or ionic, due to that being the lowest energy configuration, but if the solvent changed to benzene, the lowest energy configuration would be the polar/ionic groups facing each other, and the new surface functional groups being nonpolar, such as tryptophan.

    The specific changes are extremely complicated as the exact sequences of amino acids and the specific amino acids involved will drastically change any configuration and molecular dynamics.
     
  6. Jan 27, 2012 #5
    Solvation phenomena, especially those involving hydrophobic hydration, are a result of a delicate balance between enthalpic and entropic effects. Simple explanation are usually wrong or at leas¨t misleading.

    It is also a common error to think that the interaction between polar and non-polar molecule is weaker than between two non-polar ones.

    Interaction strength (magnitude of dissociation energy) grows in this order:
    non-polar + non-polar < non-polar + polar < polar + polar

    Reason for demixing polar and nonpolar phases is that polar+polar interaction is a LOT more stronger than the mixed one, so it squeezes non-polar solute out.

    Situation is even more complicated when water (or other associating fluid) is the polar solvent. Then the separation of nonpolar solute is driven mainly by entropic gain, because in a solvation shell waters were immobilized.

    When ions are added, with their strong effect on water structure and both enthalpy and entropy, then it is real challenge for explanation. Molecular theory still struggles with this problem.
     
    Last edited: Jan 27, 2012
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