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 characterization, electrophoresis, and mass spectrometry.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( AA_{masses} \) — Molecular weights of individual amino acids (g/mol)
• \( n \) — 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 (condensation reaction). The calculator sums all amino acid masses and subtracts (n-1)*18 to account for these losses.

3. Importance of Molecular Weight

Details: Knowing a protein's molecular weight is essential for SDS-PAGE, size-exclusion chromatography, mass spectrometry analysis, and protein quantification methods.

4. Using the Calculator

Tips: Enter the protein sequence using single-letter amino acid codes (A-Z). The calculator automatically removes non-amino acid characters and calculates the molecular weight based on the remaining valid sequence.

5. Frequently Asked Questions (FAQ)

Q1: Does this include post-translational modifications?
A: No, this calculates the theoretical MW of the unmodified polypeptide chain. Modifications like phosphorylation or glycosylation would add to the weight.

Q2: What about disulfide bonds?
A: Disulfide bonds don't affect the molecular weight as no atoms are lost or gained during their formation.

Q3: Are the weights monoisotopic or average?
A: These are monoisotopic masses (most abundant natural isotope for each element).

Q4: How accurate is this calculation?
A: It's theoretically exact for the given sequence, but actual measurements may vary slightly due to isotopic distributions.

Q5: What about N-terminal or C-terminal modifications?
A: This assumes standard -NH2 and -COOH termini. Special modifications would need to be accounted for separately.