Protein Molecular Weight Calculator Sigma

Protein Molecular Weight Formula:

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

Where:

\( MW \) - Molecular weight (g/mol)
\( AA_{masses} \) - Array of amino acid masses (g/mol)
\( n \) - Number of amino acids (unitless)
18 - Molecular weight of water (g/mol)

Molecular Weight:

1. What is Protein Molecular Weight?

The molecular weight of a protein is the sum of the masses of all its atoms. For practical purposes, it's calculated by adding up the masses of the amino acids and subtracting the mass of water molecules lost during peptide bond formation (18.01528 g/mol per bond).

2. How Does the Calculator Work?

The calculator uses the formula:

\[ MW = \sum(AA_{masses}) - (n-1) \times 18.01528 \]

Where:

\( MW \) — Molecular weight (g/mol)
\( AA_{masses} \) — Array of amino acid masses (g/mol)
\( n \) — Number of amino acids (unitless)
18.01528 — Molecular weight of water (g/mol)

Explanation: Each peptide bond formation results in the loss of one water molecule. 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 SDS-PAGE, mass spectrometry, protein purification, and biochemical experiments. It helps in determining protein concentration and buffer composition.

4. Using the Calculator

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

5. Frequently Asked Questions (FAQ)

Q1: Are post-translational modifications included?
A: No, this calculator provides the theoretical molecular weight of the unmodified polypeptide chain.

Q2: What about N-terminal methionine cleavage?
A: The calculator assumes the sequence exactly as entered. You should remove any cleaved residues manually.

Q3: Are disulfide bonds accounted for?
A: No, disulfide bonds would reduce the molecular weight by 2.01565 g/mol per bond (mass of two hydrogen atoms).

Q4: What mass values are used for amino acids?
A: The calculator uses monoisotopic masses of amino acid residues (without water) as commonly used in mass spectrometry.

Q5: How accurate is this calculation?
A: For most applications, this theoretical calculation is sufficient. For precise work, consider using mass spectrometry.