Amino Side The isoelectric point (pl) of a peptide is the pH at which the peptid

Question

Amino Side 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: Acid Chain pK Arg Asp Cys Glu His Lys Tyr peptide- NH3 peptide- 12.5 3.7 8.2 4.3 6.0 10.5 10.5 8.0 Trp-His-Glu-Tyr-Gly-Asp Number pl 3.4 COOH Amino and carboxy terminal values differ from the amino and carboxy values of a single amino acid

Explanation / Answer

Amino acids

pI value is the pH at which the net charge on the peptide or amino acid is zero

For the given peptide,

Trp-His-Glu-Tyr-Gly-Asp

Looking at the pKa in the table above,

pI for the peptide = (4.3 + 3.4)/2 = 3.85

at this pH 3.85, the net charge on the peptide would be zero as needed for attaning pI.

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