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Protein Isoelectric Point:
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The isoelectric point (pI) is the pH at which a protein carries no net electrical charge. At this pH, the protein is least soluble and may precipitate out of solution. Understanding pI is crucial for protein purification and characterization.
The calculator uses the average of two pKa values:
Where:
Explanation: For most proteins, the pI is the average of the pKa values of the two charged groups that are present at neutral pH and whose protonation changes as the pH is lowered or raised.
Details: Knowing a protein's pI helps in designing purification strategies (like ion exchange chromatography), predicting solubility at different pH values, and understanding protein behavior in electrophoresis.
Tips: Enter the pKa values of the two charged groups that bracket the neutral charge state of the protein. Typical pKa values for amino acid side chains range from 3-4 for acidic groups and 9-10 for basic groups.
Q1: What if my protein has more than two charged groups?
A: For proteins with multiple charged groups, more sophisticated computational methods are needed that consider all ionizable groups.
Q2: How accurate is this simple calculation?
A: This provides a rough estimate. For precise work, consider using protein-specific algorithms that account for all ionizable groups and their microenvironment.
Q3: What are typical pI values for proteins?
A: Most proteins have pI values between 4 and 7, though some (like histones) can be very basic (pI > 9).
Q4: Can I use this for peptides?
A: Yes, this simple calculation works well for small peptides with few charged groups.
Q5: How does temperature affect pI?
A: pKa values (and thus pI) are temperature-dependent, though this effect is often small for biological temperatures.