1. What is Protein Molecular Weight?

The molecular weight (MW) of a protein is the sum of the masses of all its amino acids minus the mass of water molecules lost during peptide bond formation (18 Da per bond).

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( MW \) — Molecular weight of the protein (g/mol)
• \( MW_{aa} \) — Molecular weight of each amino acid (g/mol)
• \( length \) — Number of amino acids in the sequence
• 18 — Molecular weight of water (g/mol)

Explanation: For each peptide bond formed, one water molecule is lost (condensation reaction). The formula accounts for this by subtracting 18 g/mol for each bond (length-1 bonds).

3. Importance of Molecular Weight Calculation

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 using single-letter amino acid codes (A-Z, case insensitive). The sequence should only contain the 20 standard amino acids.

5. Frequently Asked Questions (FAQ)

Q1: What about modified amino acids?
A: This calculator only handles standard amino acids. For modified residues, you'll need to account for their specific masses separately.

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

Q3: How accurate is this calculation?
A: It provides the theoretical average MW based on isotopic averages. For exact mass (monoisotopic), different values would be used.

Q4: What about N-terminal and C-terminal groups?
A: This assumes standard amino and carboxyl termini. Special modifications would require adjustment.

Q5: Can I use this for peptides?
A: Yes, the calculation works for any length of amino acid chain, from dipeptides upwards.