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. It's a fundamental property used in protein analysis, purification, and characterization.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( AA_{masses} \) — Array of amino acid masses (g/mol)
• \( n \) — Number of amino acids in the sequence
• 18 — Mass of water (H₂O) lost per peptide bond (g/mol)

Explanation: Each peptide bond formation results in the loss of one water molecule (18.01528 g/mol). The total mass is the sum of amino acid residues minus the water lost.

3. Importance of Protein Mass Calculation

Details: Knowing a protein's molecular weight is essential for SDS-PAGE, mass spectrometry, chromatography, and determining protein concentration.

4. Using the Calculator

Tips: Enter the protein sequence using single-letter amino acid codes (e.g., "GAVLIPFYWSTCMNQDEKRH"). The calculator automatically removes invalid characters.

5. Frequently Asked Questions (FAQ)

Q1: Are these monoisotopic or average masses?
A: This calculator uses monoisotopic masses (most abundant natural isotope of each element).

Q2: Does this include post-translational modifications?
A: No, this calculates the theoretical mass of the unmodified polypeptide chain.

Q3: What about N-terminal or C-terminal modifications?
A: The calculator assumes standard amino and carboxyl termini. Special modifications must be added manually.

Q4: How accurate is this calculation?
A: The theoretical mass is typically within 0.01% of the actual mass for unmodified proteins.

Q5: What if my protein has non-standard amino acids?
A: This calculator only handles the 20 standard amino acids. Special residues require manual adjustment.