1. What is Peptide Molecular Weight?

The molecular weight of a peptide is the sum of the atomic masses of all atoms in the molecule. For peptides and proteins, we must account for water molecules lost during peptide bond formation between amino acids.

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 each amino acid (g/mol)
• \( length \) — 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 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:

• Preparing solutions with accurate concentrations
• Mass spectrometry analysis
• Chromatography experiments
• Determining molar ratios in reactions

4. Using the Calculator

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

5. Frequently Asked Questions (FAQ)

Q1: Why subtract water molecules?
A: During peptide bond formation, a water molecule is lost (condensation reaction). This must be accounted for in the total molecular weight.

Q2: What about modified amino acids?
A: This calculator uses standard amino acids only. For modified residues (phosphorylated, acetylated, etc.), you'll need to add their specific mass differences.

Q3: Does this account for disulfide bonds?
A: No, this calculator doesn't account for disulfide bridges or other post-translational modifications.

Q4: What's the accuracy of this calculation?
A: The calculation is theoretically exact for the given sequence, but actual measured masses may differ slightly due to isotopic distributions.

Q5: Can I use 1-letter codes?
A: No, this calculator only accepts standard 3-letter amino acid codes.