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How to calculate the pI of amino acids

By vichitramicro | Updated: June 8, 2022, 2:40 p.m.

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The isoelectric point is the pH at which the an amino acid carries no net charge.

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How to calculate the pI of amino acid

Dr. Suchitra Tyagi
Isoelectric Point
The pH at which an amino acid carries no net charge is known as its isoelectric point (pI).
 pI = ½ (pKa1 + pKa2 )
The pKa value is the pH at which equimolar concentration of proton donor and proton acceptor is present.

Case 1  The pI of an amino acid that does not have an ionizable side chain such as alanine, is average of two pKa values.

STRUCTURE OF ALANINE
pI = ½ (pK1 + pK2 )
   Alanine has two pKa values i.e., 2.34 and 9.69. 
Its pI is-
   pI= ½ (2.34+9.69)
   =12.03/2      
   =6.02

Case 2. 
If an amino acid has an ionizable side chain, its pI is the average of the pKa values of similarly ionizing groups.
 Example- Lysine, Glutamic Acid, Histidine, etc
 pI = ½ (pK1 + pK2 )
   Lysine has two pKa values of the two groups that  are positively charged in their acidic form.
   pI= ½ (10.79+9.8.95)
   = 19.74/2      
   = 9.87
pI = ½ (pK1 + pK2 )
   Glutamic acid has two similar ionizing groups
   pI= ½ (2.19 + 4.25)
   = 6.44 /2      
   = 3.22

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