Peptide Molecular Weight Calculator Expasy

Peptide Molecular Weight Formula:

\[ MW = \sum(MW_{aa}) - 18 \times (length - 1) \]

Molecular Weight:

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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 and proteins, we calculate it by summing the weights of 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:

\[ MW = \sum(MW_{aa}) - 18 \times (length - 1) \]

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 peptide
18 — Molecular weight of water (g/mol) lost per peptide bond

Explanation: Each peptide bond formation results in the loss of one water molecule (H₂O, MW=18.02 g/mol). For a peptide with n amino acids, there are (n-1) peptide bonds.

3. Importance of Molecular Weight Calculation

Details: Knowing a peptide's molecular weight is essential for mass spectrometry analysis, HPLC purification, molar concentration calculations, and biochemical experiments.

4. Using the Calculator

Tips: Enter the peptide sequence using standard one-letter amino acid codes (e.g., GLY-ALA-SER or GAS). The calculator automatically removes non-amino acid characters and converts to uppercase.

5. Frequently Asked Questions (FAQ)

Q1: What amino acid codes are supported?
A: Standard 20 amino acids using one-letter codes (A, R, N, D, C, Q, E, G, H, I, L, K, M, F, P, S, T, W, Y, V).

Q2: Does this include post-translational modifications?
A: No, this calculates the molecular weight of the unmodified peptide only.

Q3: How accurate is this calculation?
A: It uses average isotopic masses. For exact masses needed in mass spectrometry, use monoisotopic masses instead.

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

Q5: Why subtract water molecules?
A: Each peptide bond formation (condensation reaction) releases one water molecule, which must be accounted for in the total weight.