1. What is Protein Molecular Weight?

The molecular weight of a protein is the sum of the masses of all its atoms. In practice, it's calculated by summing the weights of its amino acids and subtracting the weight of water molecules lost during peptide bond formation (18 g/mol per bond).

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( MW \) — Molecular weight of protein (g/mol)
• \( MW_{aa} \) — Molecular weight of individual amino acids (g/mol)
• \( length \) — Number of amino acids in the protein
• 18 — Molecular weight of water (g/mol)

Explanation: For each peptide bond formed, one water molecule is lost (dehydration synthesis). Therefore, we subtract 18 g/mol for each bond (which is length-1).

3. Importance of Molecular Weight Calculation

Details: Knowing a protein's molecular weight is essential for laboratory work including SDS-PAGE, protein purification, mass spectrometry, and biochemical assays.

4. Using the Calculator

Tips: Enter the protein sequence using single-letter amino acid codes (e.g., "MAEGEITTFT"). The calculator automatically removes invalid characters and calculates based on the remaining valid sequence.

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 (H₂O, MW=18 g/mol).

Q2: Are modifications considered?
A: No, this calculator provides the theoretical molecular weight of the unmodified polypeptide chain.

Q3: What about N-terminal and C-terminal groups?
A: The calculation assumes standard protonation states at physiological pH (NH₃⁺ and COO^-).

Q4: How accurate is this calculation?
A: It provides a theoretical average molecular weight. Actual experimental values may differ slightly due to isotopic distributions.

Q5: Can I use lowercase letters?
A: Yes, the calculator automatically converts input to uppercase and ignores non-amino acid characters.