1. What is Protein Molecular Weight?

The molecular weight (MW) of a protein is the sum of the masses of all its amino acid residues minus the mass of water molecules lost during peptide bond formation. It's a fundamental property used in protein characterization, electrophoresis, and mass spectrometry.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( AA_{masses} \) — Masses of individual amino acid residues
• \( n \) — Number of amino acid residues in the protein
• 18 — Molecular weight of water (lost during peptide bond formation)

Explanation: Each peptide bond formation results in the loss of one water molecule (H₂O, MW=18). For a protein with n residues, there are (n-1) peptide bonds.

3. Importance of Molecular Weight Calculation

Details: Knowing a protein's MW is essential for experimental design, including gel electrophoresis, chromatography, mass spectrometry, and protein quantification.

4. Using the Calculator

Tips: Enter the protein sequence using single-letter amino acid codes (A-Z, case insensitive). The calculator will ignore any non-amino acid characters and spaces.

5. Frequently Asked Questions (FAQ)

Q1: Does this include post-translational modifications?
A: No, this calculates the theoretical MW of the unmodified polypeptide chain. PTMs like phosphorylation or glycosylation would add additional mass.

Q2: What about the N-terminal and C-terminal groups?
A: This calculation assumes standard protonated N-terminus and deprotonated C-terminus at physiological pH.

Q3: How accurate is this calculation?
A: It's accurate for theoretical MW based on amino acid composition. Actual experimental MW may vary slightly due to isotopic distribution.

Q4: What if my protein has non-standard amino acids?
A: The calculator only recognizes the 20 standard amino acids. Non-standard residues would require manual adjustment.

Q5: Does this account for disulfide bonds?
A: No, disulfide bonds between cysteines would reduce the MW by ~2 Da per bond (from loss of 2 hydrogens).