Isoelectric Point Calculator
Calculate the pI of amino acids using their pKa values. The isoelectric point is the pH at which a molecule carries no net electrical charge.
Select an amino acid or enter custom pKa values
pKa of the carboxyl group (α-COOH)
pKa of the amino group (α-NH₃⁺)
Some amino acids have additional ionizable side chains
pKa of ionizable side chain
What is the isoelectric point (pI)?
The isoelectric point (pI) is the pH at which a molecule carries no net electrical charge. For amino acids, it is the pH where the positively charged (NH₃⁺) and negatively charged (COO⁻) groups are balanced, resulting in a net charge of zero. The pI is crucial for protein purification techniques like isoelectric focusing.
How do you calculate the pI of an amino acid?
For neutral amino acids with only α-carboxyl and α-amino groups, pI = (pKa₁ + pKa₂) / 2, where pKa₁ is the carboxyl group pKa (~2.2) and pKa₂ is the amino group pKa (~9.2). For acidic amino acids (Asp, Glu), pI = (pKa₁ + pKaR) / 2. For basic amino acids (Lys, Arg, His), pI = (pKa₂ + pKaR) / 2.
Why is pI important in protein chemistry?
At pH equal to a protein's pI, the protein has minimum solubility and maximum electrophoretic mobility is zero. This property is exploited in isoelectric focusing (IEF), a technique that separates proteins based on their pI. Additionally, understanding pI helps in predicting protein behavior in different pH environments and in designing buffer systems.
What happens to an amino acid above and below its pI?
Below the pI, amino acids carry a net positive charge (cation). Above the pI, they carry a net negative charge (anion). At the pI, amino acids exist primarily as zwitterions (dipolar ions) with both positive and negative charges but net zero. This affects how proteins interact with other molecules and their solubility in different solvents.