Isoelectric Point Calculator

The Isoelectric Point Calculator finds the pH at which a molecule carries no net electrical charge. This pH value is called the isoelectric point (pI) and is essential in biochemistry for studying amino acids, peptides, and proteins.

$$pI = \frac{pK_a + pK_b}{2}$$

where $pK_a$ is the acid dissociation constant and $pK_b$ is the base dissociation constant of the molecule. At the isoelectric point, the molecule exists as a zwitterion with equal positive and negative charges.

Example: if $pK_a = 3.7$ and $pK_b = 9.4$, then $pI = (3.7 + 9.4) / 2 = 6.55$. Below this pH the molecule is positively charged; above it the molecule is negatively charged.

The isoelectric point affects protein solubility, electrophoresis behavior, and ion-exchange chromatography. Acidic amino acids typically have pI values between 5.0 and 7.0, while basic amino acids have higher pI values.

Frequently Asked Questions

What is the isoelectric point?

The isoelectric point (pI) is the pH at which a molecule has zero net electrical charge. At this pH, positive and negative charges balance each other.

How do I calculate pI from pKa and pKb?

Add the pKa and pKb values and divide by 2. For example, pKa = 2.2 and pKb = 7.5 gives pI = (2.2 + 7.5) / 2 = 4.85.

Can a molecule have more than one pKa value?

Yes. Molecules with multiple ionizable groups have multiple pKa values. For proteins with many ionizable residues, specialized algorithms are used instead of the simple average formula.

How does pI affect solubility?

At the isoelectric point, molecules carry no net charge and interact less with water, reducing solubility. This can cause proteins to precipitate at their pI.

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Isoelectric Point Calculator

Isoelectric Point Calculator

Frequently Asked Questions

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