(Solved) Question regarding isoelectric point (pI)

vcunningham22

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9 years ago

Question regarding isoelectric point (pI)

Hey guys! I'm having trouble with this particular question. Could you also tell me how you obtained the answer you got? Obviously the answer in the picture is not correct. Thanks so much.

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bio_man

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9 years ago

Hey vcunningham22, could you write out the question word for word? You can leave the chart as an attachment.

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vcunningham22 Author

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9 years ago

Yes! Here it is:

he isoelectric point (pI) of a peptide is the pH at which the peptide does not migrate in an electric field. Since the peptide is zwitterionic, there are the same number of positive charges as negative charges on the peptide population. The pI can be estimated fairly accurately (within 0.1 or 0.2 pH units) from the pK values of all the proton dissociable groups in the peptide. Using pK values from the table at the right, estimate the pI value of the following hexapeptide:
Ser-Arg-Asp-Arg-Val-Lys

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rsb

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9 years ago

pI is pH at which net charge = 0,

first let us check sequence Leu-His-Glu-Lys-Met-Asp ,

Leu and Met has non polar side chain , so influcne on pI .

His has pka at 6 , Glu - 4.3 , Lys- 10.5, Asp- 3.7 ,

now we need to select pI such that net charge is balanced and equal to zero.

pka values in order are 3.4, 3.7 ,4.3 , 6 , 8 , 10.5 .

clearly select pH range in between 4.3 -6 we observe total negative charge = total positive charge

pI = ( 4.3+6)/2 = 5.15 ,

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rsb

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9 years ago

i have selected range because at this intermediate range we get 3 positive charges ( due to NH3+)and 3 negative charges ( due to COO-) thus total net charge will be zero,

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bio_man

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9 years ago

See if this helps...

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vcunningham22 Author

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9 years ago Edited: 9 years ago, vcunningham22

I tried 5.15 prior to posting and it wasn't correct. Thats why I'm stumped.

Post Merge: 9 years ago

Quote from: bio_man (9 years ago)

See if this helps...

I'm having trouble understanding the problem and what I'm looking at in the picture :/

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Answer accepted by topic starter

Alexx

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Question regarding isoelectric point (pI)

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