1. What is Protein Molecular Weight?

The molecular weight (MW) of a protein is the sum of the masses of its amino acids minus the mass of water molecules lost during peptide bond formation. It's a fundamental property used in protein analysis, purification, and characterization.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( MW \) — Molecular weight of the protein (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: For each peptide bond formed, one water molecule is lost. Therefore, for a protein with n amino acids, (n-1) water molecules are subtracted from the total.

3. Importance of Molecular Weight Calculation

Details: Knowing a protein's MW is essential for gel electrophoresis, mass spectrometry, chromatography, and other biochemical techniques. It helps in protein identification and characterization.

4. Using the Calculator

Tips: Enter the amino acid sequence in single-letter code (case insensitive). The calculator automatically removes non-amino acid characters and calculates the molecular weight based on the standard amino acid masses.

5. Frequently Asked Questions (FAQ)

Q1: Does this include post-translational modifications?
A: No, this calculator only considers the unmodified amino acid sequence. Modifications like phosphorylation or glycosylation would add additional mass.

Q2: What about N-terminal or C-terminal modifications?
A: Standard calculations assume free amino and carboxyl termini. Any modifications would need to be accounted for separately.

Q3: How accurate is this calculation?
A: It provides a theoretical average molecular weight. Actual experimental values may differ slightly due to isotopic distributions.

Q4: Does it work for sequences with non-standard amino acids?
A: No, only the 20 standard amino acids are included in this calculator.

Q5: What's the source of the amino acid molecular weights?
A: The values are based on the ExPASy biochemical constants database, representing average isotopic composition.