1. What is Molecular Weight and pI?

The molecular weight (MW) of a protein is the sum of the atomic weights of all atoms in the molecule. The isoelectric point (pI) is the pH at which the protein has no net charge.

2. How Does the Calculator Work?

The calculator uses these formulas:

Where:

• \( MW_{aa} \) — Molecular weight of each amino acid (g/mol)
• 18 — Molecular weight of water (g/mol)
• length — Number of amino acids in the sequence
• pKa — Acid dissociation constants of ionizable groups

Explanation: The MW calculation accounts for water loss during peptide bond formation. The pI is estimated from the average of pKa values of ionizable side chains.

3. Importance of MW and pI

Details: MW is crucial for protein characterization and experimental design. pI helps predict solubility and isoelectric focusing behavior.

4. Using the Calculator

Tips: Enter the protein sequence in single-letter code (case insensitive). The calculator will validate the input and compute MW and pI.

5. Frequently Asked Questions (FAQ)

Q1: Why subtract water in MW calculation?
A: Each peptide bond formation releases one water molecule, so we account for this in the total MW.

Q2: How accurate is the pI calculation?
A: This is a simple estimate. More accurate methods consider protein structure and environment.

Q3: What about modified amino acids?
A: This calculator only handles standard amino acids. Post-translational modifications require specialized tools.

Q4: Why are some pKa values not considered?
A: Only side chains with significant pKa values in the physiological range are included.

Q5: Can I use sequences with non-standard characters?
A: No, the calculator only accepts the 20 standard amino acids (A-Z except B,J,O,U,X,Z).