How to Calculate the Isoelectric Point of Peptides

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Peptides are short polymer fragments composed from amino acids. Each peptide has a certain amino acid sequence denoted with a three letter or one letter code; for example the amino acid alanine is abbreviated as “Ala” or “A.” The charge of the peptides in the solution depends upon the solution acidity. The isoelectric point (pI) refers to the solution acidity value at which the peptide molecule has the net charge of zero. The solubility of the peptide is minimal at the isoelectric point. Use available web servers to calculate the pI value for a peptide amino acid sequence.

    Write the peptide sequence using the one-letter code. For example, if a peptide has an amino acid sequence Ala-Ser-Glu-Leu-Pro (Alanine—Serin—Glumatic acid—Leucine—Proline), then the one letter sequence is "ASELP." If necessary consult the three letter to one letter conversion table given in Resources.

    Use any web browser, Internet Explorer or Firefox, for example, to navigate to a server that calculates the peptide isoelectric point (pI); see Resources.

    Enter the peptide one-letter sequence—"ASELP" in our example—in the box, and click on "Compute."

    Read the isoelectric point (pI) value given in the line "Theoretical pI/Mw." In our example, the pI is 4.00. Note the server also calculates the molecular weight (Mw) of the peptide.

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