1. What is Protein Molecular Weight?

The molecular weight (MW) of a protein is the sum of the masses of its amino acids minus the mass of water molecules lost during peptide bond formation. This calculator uses amino acid masses from Expasy's ProtParam tool.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( AA_{masses} \) — Masses of individual amino acids (g/mol)
• \( n \) — Number of amino acids in the sequence
• 18 — Mass of one water molecule (g/mol)

Explanation: For each peptide bond formed, one water molecule is lost. Thus, for a protein with n amino acids, (n-1) water molecules are subtracted from the total mass.

3. Importance of Molecular Weight

Details: Knowing a protein's MW is essential for gel electrophoresis, mass spectrometry, protein purification, and biochemical calculations.

4. Using the Calculator

Tips: Enter the protein sequence using standard one-letter amino acid codes. The sequence should only contain letters A-Z (case insensitive) with no spaces or numbers.

5. Frequently Asked Questions (FAQ)

Q1: Does this include post-translational modifications?
A: No, this calculates the theoretical MW of the unmodified polypeptide chain.

Q2: What about N-terminal methionine cleavage?
A: The calculator assumes the sequence exactly as entered. Remove the initial Met if it's cleaved in your protein.

Q3: How are selenocysteine (U) and pyrrolysine (O) handled?
A: These unusual amino acids are included with their standard masses.

Q4: Does this account for disulfide bonds?
A: No, disulfide bonds between cysteines would reduce the MW by 2 Da per bond.

Q5: Why is my experimental MW different?
A: Experimental MW may differ due to PTMs, protein folding, buffer interactions, or measurement techniques.