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

Question

The 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: Phe-Lys-Asp-Cys-Thr-Tyr

STRATEGY: Step 1: Determine the total positive charge on the peptide (n) when all acidic and basic groups are fully protonated Step 2: List the pK values of all acidic and basic groups in order from lowest (pK1) to highest. Step 3: Calculate the pl as the average of the values for pKn, the proton dissociation forming a neutral species from a +1 species, and ph the proton dissociation forming a-1 species from the neutral species. Step 1: Determine the total positive charge on the peptide (n) when all acidic and basic groups are fully protonated. (Enter as a whole number without the sign.) Number n 5.81

Explanation / Answer

For the given peptide

Phe-Lys-Arg-Cys-Thr-Tyr

PI is the pH at which the net charge on the peptide is zero.

PI for the peptide = (3.86 + 10.53)/2 = 6.965

at this pH the net charge on peptide comes out to be zero

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