1. What is Protein Molecular Weight?

The molecular weight (MW) of a protein is the sum of the masses of all its atoms. For practical purposes, it's calculated by adding the weights of the amino acids and subtracting the weight of water molecules lost during peptide bond formation.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( MW \) — Molecular weight (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: Each peptide bond formation results in the loss of one water molecule (18 g/mol), hence we subtract 18×(n-1) where n is the number of amino acids.

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). The calculator is case-insensitive and ignores non-letter characters.

5. Frequently Asked Questions (FAQ)

Q1: Does this 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: The calculator assumes the sequence exactly as entered. You should remove any cleaved residues before calculation.

Q3: How accurate is this calculation?
A: It's accurate for theoretical MW but actual experimental values may differ slightly due to isotopic distribution and other factors.

Q4: What if my sequence contains non-standard amino acids?
A: The calculator only recognizes the 20 standard amino acids. Non-standard residues will be flagged as errors.

Q5: Does this account for disulfide bonds?
A: No, disulfide bond formation would slightly decrease the MW (by 2 g/mol per bond) due to hydrogen loss.