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amino acids....pKa

sps27

Full Member
5+ Year Member
Sep 1, 2012
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    Q: All of the following amino acids have neutral side chain at pH = 7, EXCEPT.......

    1) Cysteine
    2) Histidine
    3) Lysine
    4) Tyrosine

    Given:
    Cysteine: -CH2SH pKa side chain = 8.36
    Tyrosine: -CH2C6H4OH pKa side chain = 10.07
    Histidine: -CH2C=CH-N(+)H=CH-NH- pKa side chain = 6.05
    Lysine: -CH2CH2CH2CH2NH3(+) pKa side chain = 10.38

    --------------------------------------------------------------------------------------------------------------------------------------

    I got this question wrong. And it has bothered me quite a bit. I could not understand how the ans was Lysine. But then I looked at the structures of the side chain provided in the passages and understood why. There was no charge on Cysteine and Tyrosine shown in the structure and Histidine and Lysine both had (+) charges shown. It made sense to me why Lysine was the correct ans. The basic reasoning is that....
    Cysteine side chain is neutral at pKa = 8.36 and since pH = 7, the side chain will remain protonated (ph < pKa), and therefore neutral as is shown in the structure.
    Tyrosine side chain is neutral at pKa = 10.07 and since pH = 7, the side chain will remain protonated (ph < pKa) and therefore neutral as is shown in the structure.
    Histidine side chain is (+) at pKa = 6.05 and since pH = 7, the side chain will deprotonate (ph > pKa) and will therefore become neutral. It was (+) and then lost a proton, so essentially it becomes neutral.
    Lysince side chain is (+) at pKa = 10.38 and since pH = 7, the side chain will remain protonated (ph < pKa) and so therefore it will not be neutral, instead it will be basic or (+) charged at pH = 7.

    The thing I was wondering about is, could I have solved this question with just the pKa values given for the side chain and no knowledge of the structures? Is there a simpler reasoning to get to the ans right away?

    P.S. this is TBR acid base question btw, # 90 I think....
     

    Blueprint MCAT Tutor

    MCAT Guru
    7+ Year Member
    Sep 30, 2013
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    Tucson, AZ
    nextsteptestprep.com
    1. Pre-Medical
      The thing I was wondering about is, could I have solved this question with just the pKa values given for the side chain and no knowledge of the structures?

      It looks like bjams12 answered this question correctly, but I'll just add the one word he forgot to explicitly say in responding to your question: "No".

      If all you know is pH and pKa of a thing, you can tell whether the protonated or deprotonated form dominates (or equilibrium if pH = pKa). The problem is if you don't know anything at all about what that molecule is or its structure, you won't know whether the protonated form is charged or not.
       
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