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:

• \( AA\_masses \) — Array of amino acid masses (g/mol)
• \( n \) — Number of amino acids in the sequence
• \( 18 \) — Mass of water (g/mol) lost per peptide bond

Explanation: For each peptide bond formed, one water molecule is lost (condensation reaction). The calculator sums the amino acid masses and subtracts the appropriate number of water molecules.

3. Importance of Molecular Weight

Details: Knowing a protein's MW is essential for SDS-PAGE analysis, size-exclusion chromatography, mass spectrometry, and determining protein concentration. It also helps in predicting protein behavior during purification.

4. Using the Calculator

Tips: Enter the protein sequence using single-letter amino acid codes (A-Z). The calculator is case-insensitive. For modified amino acids or post-translational modifications, specialized calculators are needed.

5. Frequently Asked Questions (FAQ)

Q1: What type of molecular weight is calculated?
A: This calculates the monoisotopic molecular weight of the protein backbone, not including post-translational modifications.

Q2: Does this include disulfide bonds?
A: No, disulfide bonds between cysteines are not accounted for in this calculation.

Q3: How accurate is this calculator?
A: It's accurate for unmodified protein sequences. For exact mass determination, isotopic distributions must be considered.

Q4: What about N-terminal or C-terminal modifications?
A: This calculator assumes standard amino and carboxyl termini. Specialized calculators are needed for modified termini.

Q5: Can I use three-letter amino acid codes?
A: No, this calculator only accepts single-letter codes (A-Z).