1. What is Isoelectric Point (pI)?

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

2. How Does the Calculator Work?

The calculator uses the average of pKa values:

Where:

• \( pKa_{acidic} \) — pKa of the acidic group
• \( pKa_{basic} \) — pKa of the basic group

Explanation: The pI is calculated as the average of the pKa values immediately above and below the neutral pH in the titration curve.

3. Importance of pI Calculation

Details: Knowing the pI helps in protein purification (electrophoresis, isoelectric focusing), predicting solubility, and understanding protein behavior at different pH levels.

4. Using the Calculator

Tips: Enter the pKa values for the acidic and basic groups. The values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What's the difference between pI and pH?
A: pH is a measure of solution acidity, while pI is a property of molecules (especially amphoteric ones) indicating their charge-neutral point.

Q2: How do I find pKa values for my peptide?
A: pKa values can be found in biochemistry references or predicted using bioinformatics tools for specific amino acid sequences.

Q3: What if my peptide has multiple ionizable groups?
A: For complex peptides, the pI is calculated using more sophisticated methods that consider all ionizable groups.

Q4: Why is pI important in electrophoresis?
A: Proteins migrate to their pI in isoelectric focusing, allowing separation based on their isoelectric points.

Q5: Can pI predict protein solubility?
A: Yes, proteins are typically least soluble at their pI because there's no net charge to keep them in solution.