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 acid residues 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 (g/mol)
• \( MW_{aa} \) — Amino acid residue weight (g/mol)
• \( length \) — Number of amino acids in peptide

Explanation: Each peptide bond formation removes one water molecule (18.01528 g/mol). The calculation sums the weights of all amino acid residues and subtracts the weight of (n-1) water molecules.

3. Importance of Molecular Weight

Details: Knowing a peptide's MW is essential for:

• Calculating molar concentrations
• Mass spectrometry analysis
• HPLC purification
• Determining synthesis scale

4. Using the Calculator

Tips:

• Enter the peptide sequence in single-letter code (uppercase or lowercase)
• No spaces or special characters allowed
• Standard 20 amino acids are supported

5. Frequently Asked Questions (FAQ)

Q1: What molecular weight standard is used?
A: The calculator uses monoisotopic masses based on the most abundant isotope of each element.

Q2: Are modified amino acids supported?
A: No, this calculator only handles standard amino acids. For modifications, use specialized software.

Q3: How accurate is this calculation?
A: The calculation is theoretically accurate but doesn't account for isotopic distribution or post-translational modifications.

Q4: What about N-terminal or C-terminal modifications?
A: This calculator assumes free N-terminus (NH2) and C-terminus (COOH). For other termini, manual adjustment is needed.

Q5: Why subtract water molecules?
A: Each peptide bond formation (condensation reaction) removes one water molecule (H2O).