1. What is the Isoelectric Point?

The isoelectric point (pI) is the pH at which a molecule carries no net electrical charge. For neutral molecules, it's calculated as the average of the N-terminal (pKa_N) and C-terminal (pKa_C) pKa values.

2. How Does the Calculator Work?

The calculator uses the simple equation:

Where:

• \( pI \) — Isoelectric point (dimensionless)
• \( pKa_N \) — N-terminal pKa value (dimensionless)
• \( pKa_C \) — C-terminal pKa value (dimensionless)

Explanation: The equation calculates the midpoint between the two pKa values where the molecule is electrically neutral.

3. Importance of pI Calculation

Details: The isoelectric point is crucial for understanding protein behavior in electrophoresis, solubility, and crystallization. It helps predict how proteins will behave at different pH levels.

4. Using the Calculator

Tips: Enter both pKa values (must be positive numbers). The calculator will compute the average as the isoelectric point.

5. Frequently Asked Questions (FAQ)

Q1: What are typical pKa values for amino acids?
A: For α-amino groups (N-terminal), pKa is typically ~9.0-9.5. For α-carboxyl groups (C-terminal), pKa is typically ~2.0-2.5.

Q2: How does pI affect protein solubility?
A: Proteins are least soluble at their pI because there's no net charge to keep them in solution. They tend to precipitate at this pH.

Q3: Can this calculator be used for peptides?
A: Yes, for simple peptides with one N-terminal and one C-terminal group. For more complex molecules with multiple ionizable groups, more sophisticated methods are needed.

Q4: What's the relationship between pI and electrophoresis?
A: In isoelectric focusing, proteins migrate until they reach the pH that matches their pI, where they stop moving.

Q5: How accurate is this simple calculation?
A: It provides a good estimate for simple molecules, but for proteins with multiple ionizable side chains, more complex algorithms are needed.