1. What is Protein Molecular Weight?

The molecular weight (MW) of a protein or peptide is the sum of the atomic weights of all atoms in the molecule. For peptides, it's calculated by summing the molecular 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 (g/mol)
• \( MW_{aa} \) — Molecular weight of amino acids (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 (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 protein quantification, electrophoresis, mass spectrometry, and biochemical experiments.

4. Using the Calculator

Tips: Enter the amino acid sequence using either 1-letter or 3-letter codes (e.g., GLY-ALA-SER or GAS). The sequence should only contain standard amino acid codes.

5. Frequently Asked Questions (FAQ)

Q1: Does this include post-translational modifications?
A: No, this calculator only computes the molecular weight of the unmodified peptide chain.

Q2: How accurate is this calculation?
A: It provides theoretical MW based on average isotopic composition. For exact mass, isotopic distribution should be considered.

Q3: What about N-terminal and C-terminal groups?
A: The calculation assumes standard -NH2 and -COOH termini. Special modifications require manual adjustment.

Q4: Can I use this for non-standard amino acids?
A: No, only the 20 standard amino acids are included in this calculator.

Q5: Why subtract water molecules?
A: Water is eliminated during peptide bond formation (condensation reaction), reducing the total molecular weight.