ExPASy Peptide Mass Calculator

Peptide Mass Formula:

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

Peptide Molecular Weight:

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1. What is the ExPASy Peptide Mass Calculator?

The ExPASy Peptide Mass Calculator estimates the molecular weight of a peptide or protein sequence based on the sum of amino acid residue masses minus the water molecules lost during peptide bond formation.

2. How Does the Calculator Work?

The calculator uses the peptide mass 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 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 Peptide Mass Calculation

Details: Knowing the molecular weight of a peptide is essential for mass spectrometry analysis, protein identification, peptide synthesis, and biochemical experiments.

4. Using the Calculator

Tips: Enter the amino acid sequence using single-letter codes (e.g., "GAMMA"). The sequence should contain only standard amino acid letters (A-Z, excluding B,J,O,U,X,Z).

5. Frequently Asked Questions (FAQ)

Q1: Does this calculator account for modifications?
A: No, this basic calculator only computes the unmodified peptide mass. For modified peptides, specialized tools are needed.

Q2: How accurate is this calculation?
A: The calculation is theoretically accurate for unmodified peptides, but actual experimental values may vary slightly due to isotopic distributions.

Q3: What about N-terminal and C-terminal groups?
A: This calculation assumes neutral N-terminal (NH₂) and C-terminal (COOH) groups. Different terminal groups would require adjustment.

Q4: Can I calculate protein molecular weight?
A: Yes, this works for proteins as well as peptides, though very large proteins may have additional considerations.

Q5: Why subtract water molecules?
A: Water is lost during peptide bond formation (condensation reaction), so the final peptide mass is less than the sum of individual amino acids.