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

Question

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 charges on the peptide population. The 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 table at the right, estimate the pi value of the following hexapeptide: Met-Asp-Lys-Arg-Ala-Tyr *Amino and carboxy terminal values differ from the alpha amino and carboxy values of a single amino acid.

Explanation / Answer

For the given hexapeptide molecule,

Looking at the pKa values given in the table

pI is the pH at which the net charge on the peptide is zero,

pI = (8 + 10.5)/2

    = 9.25

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