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 amino acids and subtracting the mass of water molecules lost during peptide bond formation.

2. How Does the Calculator Work?

The calculator uses the standard formula for protein molecular weight:

Where:

• \( AA\_masses \) — Masses of individual amino acids (g/mol)
• \( n \) — 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 (H₂O, MW=18). For a protein with n amino acids, there are (n-1) peptide bonds.

3. Importance of Molecular Weight Calculation

Details: Knowing a protein's molecular weight is essential for laboratory techniques like SDS-PAGE, mass spectrometry, chromatography, and protein quantification. It also helps in determining molar concentrations for experiments.

4. Using the Calculator

Tips: Enter the protein sequence in single-letter code (case insensitive). Non-amino acid characters are automatically removed. The calculator uses standard amino acid masses from Expasy.

5. Frequently Asked Questions (FAQ)

Q1: Does this include post-translational modifications?
A: No, this calculator only computes the molecular weight of the unmodified polypeptide chain. PTMs like phosphorylation or glycosylation would add additional mass.

Q2: How accurate is this calculation?
A: It's accurate for theoretical calculations but actual experimental values may differ slightly due to isotopic distributions and other factors.

Q3: What about N-terminal methionine cleavage?
A: This calculator assumes the sequence as entered. If the initiating methionine is removed, you should exclude it from your input sequence.

Q4: Does it account for disulfide bonds?
A: No, disulfide bond formation would reduce the mass by ~2 Da per bond (from loss of hydrogen atoms).

Q5: What if my sequence contains non-standard amino acids?
A: Non-standard amino acids will be ignored in the calculation. Only the 20 standard amino acids are supported.