1. What is Molecular Weight?

Molecular weight (MW) is the sum of the atomic weights of all atoms in a molecule. For proteins, it's calculated by summing the masses of all amino acids and subtracting the mass of water molecules lost during peptide bond formation.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \(\sum(\text{amino acid masses})\) — Sum of masses of all amino acids in the sequence
• \(n\) — Number of amino acids in the sequence
• 18.015 — Molecular weight of water (lost during peptide bond formation)

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

3. Importance of Molecular Weight

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

4. Using the Calculator

Tips: Enter the protein sequence in single-letter code (case insensitive). The sequence should only contain standard amino acid letters (A-Z except B,J,O,U,X,Z).

5. Frequently Asked Questions (FAQ)

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

Q2: What about N-terminal methionine cleavage?
A: This calculator assumes the full sequence as entered. You must manually remove the N-terminal methionine if applicable.

Q3: How accurate is this calculation?
A: It provides the theoretical average molecular weight based on amino acid composition. Actual experimental values may differ slightly.

Q4: Can I use this for nucleic acids?
A: No, this calculator is specifically for proteins. Different formulas apply for DNA/RNA.

Q5: What about disulfide bonds?
A: Disulfide bonds don't affect molecular weight (just mass of 2H), but they do affect migration in SDS-PAGE under non-reducing conditions.