Expasy Molecular Weight Calculator
Protein Molecular Weight Calculator
Results
Molecular Weight Calculation Components
| Amino Acid (1-Letter Code) | Name | Average Mass (Da) | Net Charge at pH 7.4 (if applicable) |
|---|
What is Protein Molecular Weight?
Protein molecular weight, often expressed in Daltons (Da) or kilodaltons (kDa), is a fundamental physical property of a protein or peptide. It represents the total mass of the molecule, which is primarily determined by the number and type of amino acids it comprises. Understanding the molecular weight is crucial for various biological and biochemical applications, including protein purification, identification, quantification, and functional studies. The molecular weight dictates how a protein behaves during techniques like gel electrophoresis (SDS-PAGE), mass spectrometry, and size exclusion chromatography. For researchers working with biomolecules, an accurate molecular weight is the first step in characterizing a protein and ensuring its identity.
Who Should Use It: Biochemists, molecular biologists, proteomics researchers, drug discovery scientists, students in life sciences, and anyone manipulating or analyzing proteins will find an expasy molecular weight calculator indispensable. It's particularly useful when synthesizing peptides, analyzing recombinant proteins, or confirming the mass of a purified protein sample.
Common Misconceptions: A common misconception is that molecular weight is a single, fixed value. However, proteins can exist as different isotopic forms (monoisotopic vs. average mass) and their net charge can vary significantly with the surrounding pH. Furthermore, post-translational modifications (PTMs) like glycosylation or phosphorylation can substantially alter the final molecular weight, which standard calculators like this one may not account for without specific PTM input. This calculator provides a foundational calculation based on the amino acid sequence and pH.
Expasy Molecular Weight Formula and Mathematical Explanation
The calculation of a protein's molecular weight involves summing the masses of its constituent amino acids and accounting for the formation of peptide bonds.
Core Calculation: Average Mass
The average molecular weight is calculated by summing the average isotopic masses of each amino acid in the sequence and then subtracting the mass of water (H2O) for each peptide bond formed. A protein with 'n' amino acids has 'n-1' peptide bonds.
Formula: Average Molecular Weight = Σ (Average Mass of Residuei) – (n-1) * Average Mass of H2O
Where:
- Σ represents the summation over all amino acids in the sequence.
- 'i' denotes the i-th amino acid in the sequence.
- n is the total number of amino acids in the sequence.
- Average Mass of H2O is approximately 18.015 Da.
Monoisotopic Mass
The monoisotopic mass calculation uses the mass of the most abundant isotope for each atom in the amino acids (e.g., 12C, 1H, 14N, 16O, 32S). This is the precise mass of a single molecule containing only the most common isotopes.
Formula: Monoisotopic Molecular Weight = Σ (Monoisotopic Mass of Residuei) – (n-1) * Monoisotopic Mass of H2O
Net Charge Calculation
The net charge of a protein is pH-dependent. It is calculated by considering the protonation states of the N-terminus, C-terminus, and ionizable amino acid side chains. At a given pH, acidic groups (like carboxyl groups) tend to deprotonate (lose a proton, contributing a negative charge), while basic groups (like amino groups) tend to protonate (gain a proton, contributing a positive charge).
Simplified Approach (for this calculator): The net charge is approximated based on standard pKa values. The N-terminus is positively charged at neutral pH. The C-terminus is negatively charged at neutral pH. Acidic residues (Asp, Glu) are typically deprotonated (negative charge) at pH 7.4. Basic residues (Lys, Arg, His) are typically protonated (positive charge) at pH 7.4. Cysteine (Cys) and Tyrosine (Tyr) can also contribute charge depending on their specific pKa and the solution pH.
Variables Table:
| Variable | Meaning | Unit | Typical Range / Value |
|---|---|---|---|
| Amino Acid Sequence | The linear chain of amino acids. | One-letter codes | e.g., MKTAY… |
| Residue Mass (Average) | The average mass of an amino acid after water removal. | Daltons (Da) | ~57 (Gly) to ~204 (Trp) |
| Residue Mass (Monoisotopic) | The mass of an amino acid using its most abundant isotope. | Daltons (Da) | ~57 (Gly) to ~204 (Trp) |
| H2O Mass (Average) | Average mass of a water molecule. | Daltons (Da) | ~18.015 |
| H2O Mass (Monoisotopic) | Monoisotopic mass of a water molecule. | Daltons (Da) | ~18.0106 |
| pH | Measure of acidity/alkalinity. | Unitless | 0-14 |
| Net Charge | Overall charge of the protein at a given pH. | Unitless | Integer (e.g., -2, 0, +3) |
Practical Examples (Real-World Use Cases)
Example 1: Calculating the Molecular Weight of Insulin (Human)
Let's calculate the approximate molecular weight of the human insulin A-chain, which has the sequence: GL Y LL VN Q L SC G R L R A F I NC L Q C C E D P L E . (Simplified representation for calculation illustration).
A more common sequence representation is:
GIV QQ C P L R S C N C LY EL VD P Q TS V NC Y C NN .
For a full protein like Insulin, you'd sum up the A and B chains. Let's consider a smaller peptide for clarity.
Consider a synthetic peptide with the sequence: