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 water molecules lost during peptide bond formation. It's typically expressed in Daltons (Da) or kilodaltons (kDa).

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( \sum (AA\ masses) \) — Sum of individual amino acid masses
• \( n \) — Number of amino acids in the sequence
• \( 18.015 \) — Molecular weight of water (lost per peptide bond)

Explanation: Each peptide bond formation results in the loss of one water molecule (H₂O), hence we subtract 18.015 Da for each bond formed.

3. Importance of Molecular Weight

Details: Knowing a protein's MW is essential for gel electrophoresis, mass spectrometry, chromatography, and other biochemical techniques. It helps in protein identification, characterization, and quantification.

4. Using the Calculator

Tips: Enter the protein sequence using standard one-letter amino acid codes (A, R, N, D, etc.). The sequence should not contain spaces or other characters.

5. Frequently Asked Questions (FAQ)

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

Q2: What about N-terminal methionine cleavage?
A: The calculator assumes the sequence as entered. If the initiator methionine is removed, you should exclude it from your input sequence.

Q3: How accurate is this calculation?
A: This provides the theoretical average MW. For exact mass (monoisotopic), use specialized mass spectrometry software.

Q4: What about disulfide bonds?
A: Disulfide bonds don't affect the MW calculation as no atoms are lost or gained in their formation.

Q5: Can I calculate MW for modified amino acids?
A: This calculator only handles standard amino acids. For modified residues, you'd need to manually adjust the calculation.