Isoelectric Point: Protein pI and Amino Acids
The isoelectric point (pI) is the pH at which a molecule has zero net charge. For amino acids: simple pI = (pKa₁ + pKa₂)/2; acidic side chain pI = (pKa₁ + pKa₃)/2; basic side chain pI = (pKa₂ + pKa₃)/2. Essential for protein purification, electrophoresis, and solubility prediction.
Why This Chemistry Calculation Matters
Why: pI determines protein solubility, electrophoresis mobility, and purification strategies. At pI, proteins precipitate; away from pI they are charged and soluble.
How: Select amino acid from database or enter custom pKa values. Simple AA: pI = (pKa₁+pKa₂)/2. Acidic (Asp, Glu): pI = (pKa₁+pKa₃)/2. Basic (Lys, Arg, His): pI = (pKa₂+pKa₃)/2.
- ●At pI, net charge = 0; minimum solubility.
- ●Isoelectric focusing separates proteins by pI.
- ●pI affects drug solubility and membrane permeability.
Compact Examples
Inputs
⚠️For educational and informational purposes only. Verify with a qualified professional.
🔬 Chemistry Facts
Glycine pI = 5.97; Glutamate pI = 3.22; Lysine pI = 9.74.
— Biochemistry
At pI, proteins have minimum solubility—used for precipitation.
— Protein chem
Isoelectric focusing separates proteins by pI in a pH gradient.
— Electrophoresis
Zwitterion: NH₃⁺-R-COO⁻ at pI; charge = 0.
— IUPAC
📋 Key Takeaways
- • pI = pH at which net charge = 0 (zwitterion)
- • Simple AA: pI = (pKa₁ + pKa₂) / 2
- • Acidic side chain: pI = (pKa₁ + pKa₃) / 2
- • Basic side chain: pI = (pKa₂ + pKa₃) / 2
Did You Know?
Glycine pI = 5.97; Glutamate pI = 3.22; Lysine pI = 9.74.
Source: Biochemistry
At pI, proteins have minimum solubility—used for precipitation.
Source: Protein chem
Isoelectric focusing separates proteins by pI in a pH gradient.
Source: Electrophoresis
Zwitterion: NH₃⁺-R-COO⁻ at pI; charge = 0.
Source: IUPAC
pI affects drug solubility and membrane permeability.
Source: Pharma
pI is temperature-dependent; values often at 25°C.
Source: CRC Handbook
How the Isoelectric Point Calculator Works
Select an amino acid from the database or enter custom pKa values. pI is the average of the two pKa values that bracket the neutral charge state.
Simple
pI = (pKa₁ + pKa₂) / 2
Charged
Acidic: (pKa₁+pKa₃)/2; Basic: (pKa₂+pKa₃)/2
Expert Tips
Database
Use 20 standard amino acids with built-in pKa values.
Acidic vs Basic
pKa₃ < 7: acidic; pKa₃ > 7: basic.
IEF
Isoelectric focusing separates by pI.
Solubility
Minimum solubility at pI.
Amino Acid pI Reference
| AA | Code | pI | Type |
|---|---|---|---|
| Glycine | G | 5.97 | nonpolar |
| Alanine | A | 6.01 | nonpolar |
| Valine | V | 5.97 | nonpolar |
| Leucine | L | 5.98 | nonpolar |
| Isoleucine | I | 6.02 | nonpolar |
| Methionine | M | 5.75 | nonpolar |
| Proline | P | 6.29 | special |
| Phenylalanine | F | 5.48 | aromatic |
Frequently Asked Questions
What is pI?
pH at which net charge = 0. Zwitterion form.
How to calculate pI for Glycine?
pI = (2.34 + 9.60) / 2 = 5.97.
Acidic vs basic side chain?
Acidic (Asp, Glu): pI = (pKa₁+pKa₃)/2. Basic (Lys, Arg): pI = (pKa₂+pKa₃)/2.
What is zwitterion?
NH₃⁺-R-COO⁻; both + and - charges, net 0.
Why minimum solubility at pI?
No net charge ⇒ no electrostatic repulsion ⇒ aggregation.
Isoelectric focusing?
Separates proteins by pI in pH gradient.
pI vs pKa?
pKa = single group; pI = whole molecule at zero charge.
Key Numbers
📚 Official Sources
⚠️ Disclaimer: pI values are temperature-dependent. For proteins, pI depends on all ionizable groups. Consult IUPAC and ExPASy ProtParam for authoritative data.