Isoelectric Point Calculator
Calculate the pH at which molecules have zero net electrical charge
Calculate Isoelectric Point
Acid dissociation constant (-log Ka)
Base dissociation constant (-log Kb)
Formula Used
pI = (pKa + pKb) / 2
Calculation: (0 + 0) / 2 = 0.00
Results
Description: Very low isoelectric point indicating strong acidic character
Solubility: Poor solubility at neutral pH
Example Calculation
Glycine Amino Acid
Given: pKa = 2.34, pKb = 9.60
Formula: pI = (pKa + pKb) / 2
Calculation: pI = (2.34 + 9.60) / 2 = 11.94 / 2
Result: pI = 5.97
Step-by-Step Calculation
1. Add the pKa and pKb values: 2.34 + 9.60 = 11.94
2. Divide the sum by 2: 11.94 / 2 = 5.97
3. The isoelectric point is 5.97
4. At pH 5.97, glycine has zero net charge
pH Classification Guide
pI < 3
Strongly acidic
Poor solubility at neutral pH
3 ≤ pI < 5
Acidic
Negative charge at pH 7.4
5 ≤ pI < 8
Neutral
Near neutral at pH 7.4
8 ≤ pI < 11
Basic
Positive charge at pH 7.4
pI ≥ 11
Strongly basic
Poor solubility at neutral pH
Applications
Understanding Isoelectric Point
What is Isoelectric Point?
The isoelectric point (pI) is the pH value at which a molecule has zero net electrical charge. At this point, the molecule is electrically neutral, with equal numbers of positive and negative charges.
Key Concepts
- •Zero Net Charge: Equal positive and negative charges
- •pH Dependent: Charge state changes with pH
- •Solubility Minimum: Lowest solubility at pI
- •Zwitterion: Molecule with both + and - charges
Calculation Formula
pI = (pKa + pKb) / 2
- pI: Isoelectric point
- pKa: Acid dissociation constant (-log Ka)
- pKb: Base dissociation constant (-log Kb)
Note: For amino acids with ionizable side chains, the calculation involves additional pKa values and becomes more complex.
Applications and Importance
Protein Purification
Ion-exchange chromatography separates proteins based on their charge at different pH values relative to their pI.
Electrophoresis
Proteins migrate toward opposite charges in electric fields. At pI, migration stops.
Drug Development
Understanding pI helps optimize drug solubility and bioavailability at physiological pH.