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Author Topic: Why are amino acids LEAST soluble at it's PI (isoelectric point)?  (Read 5375 times)

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AhBeng

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I came across this statement in a Chemistry book, "An amino acid is LEAST soluble at its isoelectric point."

This statement (which was not further explained by the book) does not make sense. At its isoelectric point, the amino acid exists in its dipolar form, aka zwitterion. Surely this makes it MORE (not least!) soluble in (dipolar) water?

Thanks for any replies!
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AWK

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Re: Why are amino acids LEAST soluble at it's PI (isoelectric point)?
« Reply #1 on: April 02, 2007, 01:00:55 AM »

But not all molecules of aminoacids exist in dipolar form at this point
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AhBeng

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Re: Why are amino acids LEAST soluble at it's PI (isoelectric point)?
« Reply #2 on: April 02, 2007, 03:37:39 AM »

At its isoelectic point, which is to say it the amino acids are neutrally charged (ie. greatest % of the amino acids in any given sample are in this state), it must exist in dipolar aka zwitterion form. If the greatest percentage of any given sample of an amino acid was in either its anionic, dianionic (for amino acids with acidic R groups), cationic, or dicationic (for amino acids with basic R groups) forms, we cannot say such a pH is its isolectric point.

Regardless, the statement "But not all molecules of aminoacids exist in dipolar form at this point" still does not justify the sweeping statement (found in a Chemistry book, which is the point of this thread), "An amino acid is LEAST soluble in water at it's isoelectric point.", now does it?

Thanks for any further replies.
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lavoisier

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Re: Why are amino acids LEAST soluble at it's PI (isoelectric point)?
« Reply #3 on: April 02, 2007, 10:11:12 AM »

Actually, the answer is very simple if you think of each molecule as a whole, rather than as a dipole.

- at pH higher than pI you have individual negatively charged molecules H2N-R-COO-
- at pH lower than pI you have individual positively charged molecules HOOC-R-NH3+

In either case, the molecules push one another away (by Coulombian repulsion), and therefore they dissolve.

On the other hand, at the pH where their electric charge is zero (pI), they can stick together more easily, and therefore the overall substance is less soluble.

By the way, if you don't believe the theory, you can measure the solubility at various pH's and see that it's true.
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AhBeng

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Re: Why are amino acids LEAST soluble at it's PI (isoelectric point)?
« Reply #4 on: April 02, 2007, 07:15:41 PM »

Thank ("snack for") you very much, lavoisier! Yes that certainly makes sense.

I wish more books would explain comprehensively the facts, rather than just state it and expect the student to memorize it dogmatically. Chemistry (heck, everything subject!) is enjoyable when it makes perfect sense!  :D

PS.
For those interested, I just made another post, another similarly puzzling conceptual 'riddle', this time involving the concept of chemical equilibrium, on the Physical Chemistry forum *here*.
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