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. It's a fundamental property used in protein analysis, purification, and characterization.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( AA_{masses} \) — Array of amino acid masses (g/mol)
• \( n \) — Number of amino acids in the sequence
• 18 — Molecular weight of water (g/mol) lost per peptide bond

Explanation: For each peptide bond formed, one water molecule is lost. The total molecular weight is the sum of amino acid masses minus the water molecules lost.

3. Importance of Molecular Weight Calculation

Details: Knowing a protein's molecular weight is essential for SDS-PAGE analysis, mass spectrometry, protein purification, and determining molar concentrations for experiments.

4. Using the Calculator

Tips: Enter the protein sequence using single-letter amino acid codes (e.g., "MAKEPEPTIDE"). 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 include post-translational modifications?
A: No, this calculator only computes the molecular weight of the unmodified polypeptide chain.

Q2: What about N-terminal and C-terminal groups?
A: The calculation assumes standard amino and carboxyl termini. Special modifications are not included.

Q3: How accurate is this calculation?
A: It provides a theoretical average molecular weight based on isotopic averages. For exact mass, monoisotopic masses would be used.

Q4: What if my sequence contains non-standard amino acids?
A: The calculator only recognizes the 20 standard amino acids. Non-standard residues will result in an error.

Q5: Does the calculator account for disulfide bonds?
A: No, disulfide bond formation (which loses 2 hydrogens per bond) is not accounted for in this calculation.