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Amino acids

Discussion in 'DAT Discussions' started by Awuah29, Dec 12, 2005.

  1. Awuah29

    Awuah29 Christian predent
    7+ Year Member

    Oct 13, 2003
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    Hey guys,
    quick question! How can you predict an amino acid's charge at a given PH. kaplan says as a note by using PI. Now did anyone had that question and how can you tell the PH of an amino acid.
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  3. tinman831

    tinman831 ¯\_(ツ)_/¯
    Administrator Dentist Lifetime Donor Classifieds Approved 10+ Year Member

    Dec 11, 2004
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    This is definitely not something that will be on the DAT. I think that if the PH of the environment is much lower than the PI, then anything that can be protonated will be protonated. If the PH of the environment is higher than the PI, anything that can be unprotonated, will be. These are the extreme cases and the PH (relative to the PI) will determine the degree of charge. At least I think this is right. It's been a while since I've seen this in class. :oops:
  4. elinz

    elinz Member
    5+ Year Member

    Oct 24, 2005
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    I didn't get this question , but it might be on the dat,
    basically if your PI is below the pH, you are looking or a negative charge, if the pI is above the pH, you get a positive charge....ummm okay same as what tinman said :) but another way to look at it :p
  5. howui3

    howui3 1K Member
    5+ Year Member

    Aug 8, 2005
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    It is really simple!

    You know the amino side will lose/gain charge at a pH of around 9 and the caroxy side will lose/gain charge at around pH of 2.

    Knowing that, if the pH is 4 for example, you know that it's above the IP (isoelectric point) for the carboxy side (meaning it's less acidic then the isoelectric point) so the proton will be lost from the carboxy side. On the amino end, the pH of 4 is below (more acidic) then the isoelectric point so that side will remain protinated.

    Now look at all your charges, you have a - from the deprotinated carboxy side and a + from the protinated amino side. So overall neutral.

    Now look if the pH is raised to 11. Now the carboxy will still be deprotinated and have a - charge, and the amino side is now below the pH so it will also be deprotinated and will be neutral. Now you have an overall charge of -. You have to do the same for the other side groups that could have a charge.

    Good luck

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