The isoelectric point (pl) of a peptide is the pH at which the peptide does not

Question

The isoelectric point (pl) 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 c pl 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 pl value of the following hexapeptide Side Acid Chain pK on the peptide population. The Arg Asp Cys Glu 12.5 3.7 8.2 4.3 6.0 10.5 10.5 8.0 Trp-His-Glu-Tyr-Gly-Asp Number Lys Tyr peptide- NH3 tide- 3.4 OH Amino and carboxy terminal values differ from the amino and carboxy values of a single amino acid

Explanation / Answer

pI = 1/2 * (pKa1+ pKa2).

for peptide - Trp-His-Glu-Tyr-Gly-Asp. (we include only middle amino acid pKa, not N and C terminal amino acid.

now carefully look at the peptide and pK values-

if we increase the pK value from 4.3, then Glu acid will take one extra positive charge so no pI above that pK. similarly, if we increase the pK value above 6, then His will take charge. but Asp is still charged.

so the pI will lies in between pK 4.3 and 3.7.

now with the formula mentioned above

pI = 1/2 * (pKa1+ pKa2) = 1/2 *(4.3+3.7) = 1/2 * 8 = 4.

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