1. What is Protein Molecular Weight?

The molecular weight 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 characterization, electrophoresis, and chromatography.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( amino\_acid\_masses \) — Mass of each amino acid in the sequence (g/mol)
• \( n \) — Number of amino acid residues
• 18 — Mass of water lost per peptide bond (g/mol)

Explanation: Each peptide bond formation results in the loss of one water molecule (18 g/mol). For a protein with n residues, there are (n-1) peptide bonds.

3. Importance of Molecular Weight Calculation

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

4. Using the Calculator

Tips: Enter the protein sequence using single-letter amino acid codes (e.g., "MAKER"). 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 considers the unmodified polypeptide chain. Modifications like phosphorylation or glycosylation would add additional mass.

Q2: How accurate is this calculation?
A: It provides a theoretical average molecular weight. For exact mass (including isotopic distribution), mass spectrometry is needed.

Q3: What about N-terminal or C-terminal modifications?
A: This assumes standard -NH2 and -COOH termini. Modified termini would require additional mass adjustments.

Q4: Why subtract water for peptide bonds?
A: Each peptide bond forms via condensation, losing one water molecule (H2O = 18 g/mol).

Q5: Can I use lowercase letters?
A: Yes, the calculator automatically converts to uppercase, but only standard amino acid letters are valid.