1. What is Protein Molecular Weight?

The molecular weight of a protein is the sum of the atomic weights of all atoms in the molecule. For a protein, it's calculated by summing the weights of its amino acids and subtracting the weight of water molecules lost during peptide bond formation.

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: Each peptide bond formation results in the loss of one water molecule (H₂O, MW=18 g/mol). For a protein with n amino acids, there are (n-1) peptide bonds.

3. Importance of Molecular Weight Calculation

Details: Knowing a protein's molecular weight is essential for protein characterization, gel electrophoresis, mass spectrometry, and biochemical experiments.

4. Using the Calculator

Tips: Enter the protein sequence using single-letter amino acid codes (A, R, N, D, C, E, Q, G, H, I, L, K, M, F, P, S, T, W, Y, V). The sequence should not contain spaces or numbers.

5. Frequently Asked Questions (FAQ)

Q1: Why subtract water molecules?
A: During protein synthesis, each peptide bond formation results in the loss of one water molecule (condensation reaction).

Q2: Does this include post-translational modifications?
A: No, this calculator only considers the unmodified polypeptide chain. PTMs would add additional mass.

Q3: What about N-terminal and C-terminal groups?
A: This calculation assumes standard NH₂ and COOH termini. Special modifications would change the weight.

Q4: How accurate is this calculation?
A: It provides a theoretical average molecular weight. Actual isotopic distribution would show multiple peaks.

Q5: Can I use lowercase letters?
A: Yes, the calculator automatically converts to uppercase, but only standard amino acid letters are accepted.