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 charges on the peptide population. The 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: Ala-His-Glu-Lys-Leu-Arg pl = Number 7

Explanation / Answer

pI is usually the pH at which net charge = 0,

The amino acid sequence Ala-His-Glu-Lys-Leu-Arg ,

Ala and Leu has non polar side chain , so no influcne on pI .

His has pka at 6 , Glu - 4.3 , Lys- 10.5, Arg- 12.5 ,

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

it is safe to assume that at low pHs like 4.3, 6,10.5, 12.5 .

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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