Protein Molecular Weight Calculator

Protein Molecular Weight Formula:

\[ MW = \sum(AA_{masses}) - (n-1) \times 18 \]

Molecular Weight:

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1. What is Protein Molecular Weight?

The molecular weight (MW) of a protein is the sum of the masses of all its amino acid residues plus the mass of one water molecule (for the N-terminal H and C-terminal OH). During protein synthesis, water molecules are lost when peptide bonds form between amino acids.

2. How Does the Calculator Work?

The calculator uses the formula:

\[ MW = \sum(AA_{masses}) - (n-1) \times 18 \]

Where:

\( AA_{masses} \) — Masses of individual amino acids (g/mol)
\( n \) — Number of amino acid residues
18 — Molecular weight of water (g/mol)

Explanation: The equation accounts for water molecules lost during peptide bond formation (n-1 bonds in a protein with n residues).

3. Importance of Molecular Weight

Details: Knowing a protein's molecular weight is essential for laboratory techniques like SDS-PAGE, mass spectrometry, chromatography, and protein quantification. It also helps in protein characterization and drug design.

4. Using the Calculator

Tips: Enter the protein sequence using single-letter amino acid codes (A, R, N, D, etc.). The sequence should not contain spaces or other characters. The calculator uses monoisotopic masses of amino acids.

5. Frequently Asked Questions (FAQ)

Q1: What are the single-letter amino acid codes?
A: A (Ala), R (Arg), N (Asn), D (Asp), C (Cys), E (Glu), Q (Gln), G (Gly), H (His), I (Ile), L (Leu), K (Lys), M (Met), F (Phe), P (Pro), S (Ser), T (Thr), W (Trp), Y (Tyr), V (Val).

Q2: Does this include post-translational modifications?
A: No, this calculator only computes the molecular weight of the unmodified polypeptide chain.

Q3: Why subtract water molecules?
A: For each peptide bond formed, one water molecule is lost (condensation reaction).

Q4: What's the difference between monoisotopic and average mass?
A: Monoisotopic mass uses the most abundant isotope for each element, while average mass accounts for natural isotope distributions. This calculator uses monoisotopic masses.

Q5: How accurate is this calculation?
A: The calculation is theoretically accurate for the unmodified sequence, but actual experimental measurements may vary slightly due to factors like isotopic distribution and protein folding.