1. What is Protein Molecular Weight?

The molecular weight (MW) of a protein is the sum of the atomic weights of all atoms in the molecule. This calculator uses the monoisotopic masses of amino acid residues to compute the theoretical molecular weight of a protein sequence.

2. How Does the Calculator Work?

The calculator uses the following formula:

Where:

• Each amino acid has a specific molecular weight
• The calculation includes the weight of water (H₂O) for the terminal groups
• The sequence should contain only standard amino acid letters (A-Z, except B,J,O,U,X,Z)

3. Importance of Molecular Weight Calculation

Details: Knowing a protein's molecular weight is essential for protein characterization, gel electrophoresis, mass spectrometry, and many other biochemical applications.

4. Using the Calculator

Tips: Enter the protein sequence using single-letter amino acid codes (case insensitive). The sequence should not contain spaces or numbers. Non-standard characters will be flagged as errors.

5. Frequently Asked Questions (FAQ)

Q1: What molecular weight standard does this calculator use?
A: It uses monoisotopic masses of amino acid residues as implemented in the ExPASy Compute pI/Mw tool.

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

Q3: How accurate is this calculation?
A: It's accurate for theoretical calculations, but actual measured MW may differ due to factors like isotopic distribution and protein folding.

Q4: What about selenocysteine (U) or pyrrolysine (O)?
A: These rare amino acids are not included in this calculator. Sequences containing them will show errors.

Q5: Why is water added to the calculation?
A: The water accounts for the hydroxyl (-OH) and hydrogen (-H) groups at the protein's N- and C-termini.