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Topic: Overall Charge at specific pH of Amino Acids  (Read 6387 times)

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Offline x.misosoup

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Overall Charge at specific pH of Amino Acids
« on: October 31, 2014, 07:54:02 AM »


Not sure if that uploaded correctly. Please help with this question! My friends and I have all discussed; answers A, B and C were all possible.

If anyone knows, please let me know why also!! Thanks in advance!!
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Offline Babcock_Hall

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Re: Overall Charge at specific pH of Amino Acids
« Reply #1 on: October 31, 2014, 11:36:41 AM »
It is a forum rule that you must show your attempt before we can help you.  In addition, the question is worded in a somewhat strange way.  It sounds as if they are asking for least charge in magnitude, without regard for the sign of the charge.  Is that also your understanding?
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Offline orgopete

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Re: Overall Charge at specific pH of Amino Acids
« Reply #2 on: October 31, 2014, 01:39:15 PM »
Can you eliminate B as it has a net positive charge? You may have to look up the pKa of an amine at pH three. Can you eliminate any other of the others that are listed as no net charge, but may become charged at pH 3?
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Offline Babcock_Hall

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Re: Overall Charge at specific pH of Amino Acids
« Reply #3 on: October 31, 2014, 02:55:02 PM »
Would you define what the isoelectric point (pI) is for us?
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Offline x.misosoup

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Re: Overall Charge at specific pH of Amino Acids
« Reply #4 on: October 31, 2014, 06:37:39 PM »
I can't actually offer an attempt as we all are not really sure. I would say B because there are 2 amine groups with a positive charge and the others don't have it.

That's the question, there was no isoelectric point of anything else given!

Thanks anyway :)
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Offline orgopete

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Re: Overall Charge at specific pH of Amino Acids
« Reply #5 on: November 01, 2014, 01:30:15 AM »
I thought you could solve this simply by thinking about it. Ethylamine or rather ethylammonium ion has a pKa of 10.5, so it is 50% protonated at pH 10.5, ~10% at 11.5 and ~1% at 12.5. This should tell us both amines are protonated in B at pH 3. It must either be a cation or a di-cation. Not B.

They are all alpha amino acids, so this will be in common. You can determine how the alpha-amino group affects the carboxylic acid. The other ionizable substituents are a phenol (pKa 10), a carboxylic acid (pKa ~4.85), and an imidazole (pKa of imidazolium ion 7). Which groups will be uncharged at pH 3.0?
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Offline Babcock_Hall

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Re: Overall Charge at specific pH of Amino Acids
« Reply #6 on: November 01, 2014, 09:34:33 AM »
The isoelectric point is the pH at which an amino acid has no net charge.  It is a standard exercise in Biochemistry I classes to calculate the isoelectric points of the amino acids using a table of pKa values for the carboxylic acid, the ammonium group, and any group with a dissociable proton on the side chain (not all amino acids have such a proton).  One has to be careful, in that there are three formulae, one for uncharged side chains, one for side chains that are neutral acids, and one for side chains that are cationic acids (sometimes called basic side chains).  If one identifies the side chains correctly, then calculating the pI is trivial.  The formula for amino acids with nonionizable side chains is 
 pI = {{pKa1} + {pKa2}/ 2}, where pKa1 always refers to the carboxylic acid and pKa2 always refers to the alpha-ammmonium group.  Some authors use pKR to represent the pKa of the side chain.

http://www.mhhe.com/physsci/chemistry/carey5e/Ch27/ch27-1-4.html
http://www.elcamino.edu/faculty/pdoucette/calculating-approximate-isoelectric-points.pdf
I suggest looking up the relevant pKa values in your textbook or on line and calculating the pI values of your choices and showing us your work.  That's as much of a hint as I think is warranted.
« Last Edit: November 01, 2014, 11:00:03 AM by Babcock_Hall »
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Offline zsinger

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Re: Overall Charge at specific pH of Amino Acids
« Reply #7 on: November 03, 2014, 07:38:45 AM »
As Babcock_Hall indicated.. well, He's exactly right in everything.  DO THAT! :)
              -Zack
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Offline x.misosoup

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Re: Overall Charge at specific pH of Amino Acids
« Reply #8 on: November 05, 2014, 07:32:22 AM »
Thanks everyone!!! I had my exam on Monday already and it was terrible. They changed the exam and made it so different from past exams. This question wasn't on it so it doesn't matter at all :)

Thank you again for all your help :)
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