1. What is Isoelectric Point?

The isoelectric point (pI) is the pH at which a molecule carries no net electrical charge. For amino acids and proteins, it's the pH where the number of positive charges equals the number of negative charges.

2. How Does the Calculator Work?

The calculator uses the simple average formula:

Where:

• \( pKa \) — Acid dissociation constant values
• \( \sum \) — Sum of all pKa values
• \( number\ of\ pKa\ values \) — Count of pKa values entered

Explanation: The pI is calculated as the average of relevant pKa values for the molecule.

3. Importance of pI Calculation

Details: Knowing the pI helps in protein purification, electrophoresis, and understanding protein behavior at different pH levels.

4. Using the Calculator

Tips: Enter all relevant pKa values separated by commas. For amino acids, typically use the pKa of the carboxyl group and the pKa of the amino group.

5. Frequently Asked Questions (FAQ)

Q1: What's the difference between pI and pKa?
A: pKa is the pH where a specific group is 50% protonated, while pI is the pH where the entire molecule has no net charge.

Q2: How many pKa values should I enter?
A: For amino acids, typically 2 (carboxyl and amino groups). For proteins, use all relevant pKa values of ionizable groups.

Q3: What are typical pI values for amino acids?
A: Neutral amino acids ~6, acidic ~3, basic ~9-10.

Q4: Why is pI important in electrophoresis?
A: Proteins migrate toward the electrode of opposite charge. At pI, they stop moving.

Q5: Can this calculator be used for peptides?
A: Yes, but you need to consider all ionizable groups in the peptide sequence.