1. What is Peptide Molecular Weight?

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

Explanation: For each peptide bond formed, one water molecule is lost (condensation reaction). Therefore, we subtract 18 g/mol for each bond (n-1 bonds in a peptide of length n).

3. Importance of Molecular Weight Calculation

Details: Knowing a peptide's molecular weight is essential for experimental design, molarity calculations, mass spectrometry, and HPLC purification.

4. Using the Calculator

Tips: Enter the peptide sequence using standard one-letter amino acid codes (A-Z). The sequence is case-insensitive. Example: "ACDE" or "glyala".

5. Frequently Asked Questions (FAQ)

Q1: Does this calculator account for modified amino acids?
A: No, this calculator only handles standard amino acids. For modified residues, you'll need to add their specific weights manually.

Q2: What about N-terminal or C-terminal modifications?
A: This calculator assumes free N- and C-termini. For acetylated, amidated, or other terminal modifications, additional adjustments are needed.

Q3: Why subtract water molecules?
A: Peptide bond formation is a condensation reaction that releases one water molecule per bond formed.

Q4: How accurate is this calculation?
A: It provides a theoretical average molecular weight. For exact mass (including isotopic distribution), more sophisticated tools are needed.

Q5: Can I calculate molecular weight for proteins?
A: Yes, the same principle applies, though very large proteins may have additional considerations like disulfide bonds.