1. What is Peptide Molecular Weight?

The molecular weight of a peptide is the sum of the atomic weights of all atoms in the molecule. For peptides, we calculate it by summing the molecular weights of the constituent 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 of peptide (g/mol)
• \( MW_{aa} \) — Molecular weight of amino acid (g/mol)
• \( length \) — Number of amino acids in peptide
• 18 — Molecular weight of water (g/mol)

Explanation: Each peptide bond formation results in the loss of one water molecule (H₂O, MW=18 g/mol). For a peptide with n amino acids, there are (n-1) peptide bonds.

3. Importance of Molecular Weight Calculation

Details: Knowing the molecular weight is essential for peptide synthesis, purification, quantification, and for preparing solutions of known concentration.

4. Using the Calculator

Tips: Enter the peptide sequence using standard 3-letter amino acid codes separated by hyphens, spaces, or commas (e.g., GLY-ALA-SER or GLY ALA SER). The sequence is case-insensitive.

5. Frequently Asked Questions (FAQ)

Q1: Why subtract water molecules?
A: During peptide bond formation, a condensation reaction occurs where one water molecule is lost per bond formed.

Q2: What about modified amino acids?
A: This calculator uses standard amino acids only. For modified residues, you would need to adjust the weights manually.

Q3: Does this include terminal groups?
A: The calculation assumes neutral termini (NH₂ and COOH). For charged termini or other end groups, additional adjustments are needed.

Q4: How accurate is this calculation?
A: It provides a theoretical average molecular weight. The actual weight of a specific isotopic form would differ slightly.

Q5: Can I calculate weight for proteins too?
A: Yes, the same principle applies, though very large proteins may have additional modifications to consider.