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The Expasy Protein Molecular Weight Calculator estimates the molecular weight of a protein from its amino acid sequence using standard amino acid masses. This tool follows the algorithm used by the Expasy bioinformatics resource portal.
The calculator uses the formula:
Where:
Explanation: The calculation sums the masses of all amino acids in the sequence plus the mass of one water molecule (H₂O), which accounts for the hydrolysis of the peptide bonds.
Details: Knowing a protein's molecular weight is essential for various applications including gel electrophoresis, mass spectrometry, protein purification, and biochemical characterization.
Tips: Enter the protein amino acid sequence using single-letter codes. The sequence can be entered in any case as it will be converted to uppercase automatically. Non-amino acid characters will result in an error.
Q1: What amino acid codes are supported?
A: The calculator supports standard 20 amino acids plus selenocysteine (U), pyrrolysine (O), and ambiguous residues (B, Z).
Q2: Does the calculator consider post-translational modifications?
A: No, this calculator provides the theoretical molecular weight based on the amino acid sequence only, without considering any modifications.
Q3: Why is water mass added to the calculation?
A: The water mass accounts for the hydrolysis of peptide bonds, as each peptide bond formation releases one water molecule.
Q4: How accurate is this calculation?
A: This provides a theoretical molecular weight based on average isotopic masses. For precise measurements, experimental techniques like mass spectrometry are required.
Q5: Can I calculate molecular weight for modified amino acids?
A: This calculator uses standard amino acid masses only. For modified residues, specialized tools or manual calculation would be needed.