Calculation of pI for an Amino Acid.

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The pI is the pH where the molecule exists in an uncharged state. Here are some practice problems for calculating the pI of an amino acid.

1. What is the pI of histidine?

Answer:
For histidine pKa₁=2.3, pKa₂=6.0 and pKa₃=9.6.

At the pI the net charge of the molecule is zero. To find the pI, average the two pKa values on either side of the neutral form of histidine.

(pKa₂ + pKa₃)/2 = pI
(6 + 9.6)/2 = 7.8

Draw out the titration curve for histidine to convince yourself that the uncharged form is predominant at this pH.

For more practice:

What is the pI of E?
What is the pI of N?

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This page was written by Koni Stone, please send comments to koni@chem.csustan.edu