Isoelectric Point Calculator
Calculate the isoelectric point (pI) of amino acids and proteins
pKa of the carboxyl group
pKa of the amino group or side chain
Understanding Isoelectric Point
The isoelectric point (pI) is the pH at which a molecule carries no net electrical charge. For amino acids and proteins, the pI is critical for understanding their behavior in solution, solubility, and separation by electrophoresis or ion-exchange chromatography.
The pI Calculation
pI = (pKa1 + pKa2) / 2
Average of relevant pKa values
Neutral Amino Acids
pI = (pKaCOOH + pKaNH3+) / 2
Acidic Amino Acids (Asp, Glu)
pI = (pKaCOOH + pKaR-COOH) / 2
Basic Amino Acids (Lys, Arg, His)
pI = (pKaNH3+ + pKaR-NH) / 2
Key Concepts
Charge States
- • pH < pI: Molecule is positively charged (cation)
- • pH = pI: Molecule has zero net charge (zwitterion)
- • pH > pI: Molecule is negatively charged (anion)
Protein Solubility
Proteins are least soluble at their pI because they lack repulsive electrostatic forces. This property is used for isoelectric precipitation and focusing.
Applications
- ⚡Electrophoresis: Separating proteins by charge in isoelectric focusing (IEF) and 2D gels
- 🔬Chromatography: Ion-exchange chromatography for protein purification
- 💧Precipitation: Isoelectric precipitation for protein purification and crystallization
- 🧪Buffer Selection: Choosing pH for optimal protein stability and solubility
- 💊Drug Formulation: Designing stable pharmaceutical formulations
- 🧬Protein Analysis: Identifying proteins based on charge properties
⚡Quick Reference
Formula:
pI = (pKa1 + pKa2) / 2
At pI:
Net charge = 0
pH < pI:
Positive charge
pH > pI:
Negative charge
Level:
College Biochemistry
🔗Related Calculators
📐Related Formulas
🎯Where It's Used
- ⚡
Electrophoresis
Isoelectric focusing
- 🔬
Protein Purification
Ion-exchange chromatography
- 💊
Drug Development
Protein formulation
- 🧬
Proteomics
2D gel electrophoresis