The isoelectric point is the point at which the net charge of the amino acid or protein is zero. You have learned how to determine the pI of amino acids. Finding the pI of peptides follow a similar process.

Step 1: Write out the pKa values of the amino acid from low to high

Step 2: Drop the pH below the lowest pKa value of the amino acid and determine the net charge of the amino acid

Step 3: Select a pH between the first and the second pKa value of the amino acid and determine the net charge of the amino acid

Step 4: Select a pH between the second and the third pKa value of the amino acid and determine the net charge of the amino acid

Step 5: Raise the pH above the highest pKa value and and determine the net charge of the amino acid

Step 6: Calculate the pI by averaging the two pKa values that are just before and just after the zero net charge

Example: Determine the pI of the tripeptide, Asp-Gly-Glu

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Courtney Simons
Courtney Simons
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Courtney Simons is a food science professor. He holds a BS degree in food science and a Ph.D. in cereal science from North Dakota State University.