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ochem question

Discussion in 'MCAT Discussions' started by swiftaqwer, May 6, 2007.

  1. swiftaqwer

    swiftaqwer Guest

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    Apr 23, 2007
    Hope you are well. I have some questions regarding ochem

    At ph=9.5, tyrosine exists with a net charge of -1. The answer explanation is that the amino terminal and side chain are neutral. While the carboxyl is negative. I understand the negative thing, but why the neutral. Why is it that some amino acid is neutral (though still depronated or protonated), while others carry charge? On the exam, how do I know if it is neutral or charged?

    Thank you,
    swift
     
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  3. bluesTank

    bluesTank Zombie 5+ Year Member

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    Feb 2, 2007
    College Station, TX
    You have to look at the R group side chains on the amino acids. Some of them have acidic/basic properties also.
     
  4. swiftaqwer

    swiftaqwer Guest

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    Apr 23, 2007
    could you elaborate? Tyrosine is acidic, so why is it neutral? Why no charge?
     
  5. PseudomonasPAO1

    PseudomonasPAO1 New Member 7+ Year Member

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    Aug 14, 2006
    Miami, FL
    You have to know the pKas, which are normally given. For tyrosine:

    http://www.biology.arizona.edu/biochemistry/problem_sets/aa/Tyrosine.html

    That page seems to indicate tyrosine would be neutrally charged at pH 9.5.
     
  6. swiftaqwer

    swiftaqwer Guest

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    Apr 23, 2007
    the pKa of the group is 10. The pH is 9.5. Since pI> pH, the R group is protonated.... so I had assumed that it would be positive. Yet, the anwers is neutral. Within what range of pH difference would we still consider the protonated R group (on in some other cases, depronated R group) to be neutral? Within 1 pH difference?

    Thank you!
     
  7. swiftaqwer

    swiftaqwer Guest

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    Apr 23, 2007
    can someone help with this question.

    Thank you!
     
  8. MSTPbound

    MSTPbound student Moderator Emeritus 2+ Year Member

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    Oct 1, 2006
    I don't follow your reasoning. If the pKa of the R group is 10, and the pH is 9.5, how does that get you to "pI>pH"?

    pI of Tyrosine seems to be about 5.7 from my resources. So pI<pH. Does that affect your prediction?

    Also, the R group of tyrosine contains a phenolic proton; so if it's protonated, then it woud be neutral - so why would you assume that it is positively charged?

    Maybe the answer to your question has to do with the nature of the R-group; I don't recall having to work very hard to figure this out (usually the structure was given along with a titration curve), but I suppose that if you wanted know whether a proton-bearing group carries charge or not, you would calculate the formal charge. An -OH substituent has a formal charge of 0, so it is acidic, but neutrally charged. An -NH3 substituent is also acidic, but has a formal charge of +1.

    Is any of this helping? If not, please try to clarify your question. Thanks.
     
  9. meddoc1

    meddoc1 2+ Year Member

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    Apr 10, 2007
    When the pH is at 9.5, you have to consider the three groups on tyrosine. COOH (pK ~2), -NH3+ (pK~9.1) and the basic -R group for tyrosine is OH(pK~10). So at pH 9.5. the COOH had already depotonated so, it is COO-. Now, the NH3+ depotonate at pK~9.1 so at pH=9.5, It depotonated it H+ so -NH3+ becomes -NH2 which explain why it is neutral. Finally, because the Tyrosine R gp's pK is 10, it is not depotonated so it remains at OH group(neutral)... therefore the overall charges is (-1). Having the COO- (-1), the NH2(0) and on the R group(0). So there you go.
     
  10. meddoc1

    meddoc1 2+ Year Member

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    Apr 10, 2007
    Do we need to remember the the structure of Amino Acid?
    Do we need to know the different groups of Amino Acid?
    So we need to know how to calculate pI for Amino Acid with R group that can be charged?
     

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