1. What is the Protein Molecular Weight Formula?

The protein molecular weight formula calculates the mass of a protein in kilodaltons (kDa) based on the sum of its amino acid molecular weights and accounting for water loss during peptide bond formation.

2. How Does the Calculator Work?

The calculator uses the protein molecular weight formula:

Where:

• \( MW_{aa} \) — Sum of amino acid molecular weights (g/mol)
• \( length \) — Number of amino acids in the protein
• 18 — Molecular weight of water (g/mol)
• 1000 — Conversion factor from daltons to kilodaltons

Explanation: The formula accounts for the loss of one water molecule for each peptide bond formed (length-1 bonds).

3. Importance of Molecular Weight Calculation

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

4. Using the Calculator

Tips: Enter the sum of all amino acid molecular weights in g/mol and the protein length in amino acids. Both values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: Why subtract 18*(length-1)?
A: For each peptide bond formed, one water molecule is lost (18 g/mol). A protein with N amino acids has N-1 peptide bonds.

Q2: What's the difference between kDa and g/mol?
A: 1 kDa = 1000 g/mol. kDa is more convenient for protein sizes.

Q3: How do I get the sum of amino acid MW?
A: Add up the molecular weights of all amino acids in the protein sequence (available in standard tables).

Q4: Does this account for post-translational modifications?
A: No, this calculates only the unmodified polypeptide chain. Add any modification weights separately.

Q5: What about disulfide bonds?
A: Disulfide bonds don't affect molecular weight as no atoms are lost (just electrons rearranged).