justadream

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TBR OChem page 215 #64

TBR says that at the pI, the amino acid exists primarily as a zwitterion (NH3+, COO-). That means it has two charged ends but the overall charge is zero.

Because the overall charge is 0, the zwitterion will mostly dissolve in the organic layer in an extraction.

Why don't the charges matter? I get that the OVERALL charge is neutral but it seems to me that something with charged ends should be somewhat soluble (as in, non-negligent amount) in the aqueous layer.

Using this logic, would a molecule with 9999 positive charges, and 9999 negative charges be LESS soluble in aqueous layer than a molecule with a single positive charge?
 
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justadream

justadream

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Apr 29, 2011
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@Teleologist

The question is:

"Tyrosine best separates from cysteine at a pH equal to"

Answer: either pI of Cysteine or pI of Tyr, when using ether/water extraction

Explanation: "To separate two compounds using extraction in an ether/water biphasic solution, it is best when one is charged and the other is neutral. It is at the isoelectric pH that the species carries no net charge, thus when the pH is equal to the pI of lysine, then lysine is neutral and therefore more soluble in ether than in water. At this same pH, tryosine is partially negative and thus more water soluble. When the pH is equal to the pI of tryosine, then tyrosine is neutral and therefore more soluble in ether than in water. At this same pH, lysine is positive and thus water-soluble. At all other pH values, both lysine and tyrosine are charged"