Estimate protein mass based on total sequence length (using average 110 Da per residue).
Method 2: Precise Calculation
Enter the count for each amino acid in the sequence for a precise molecular weight calculation (accounting for water loss during peptide bond formation).
Calculation Results
Mass in Daltons (Da)
0
Mass in Kilodaltons (kDa)
0
Understanding Protein Molecular Weight
The molecular weight (MW) of a protein is a critical parameter in biochemistry and molecular biology. It is defined as the total mass of a protein molecule, typically expressed in Daltons (Da) or Kilodaltons (kDa). One Dalton is approximately equal to the mass of one hydrogen atom (1.66 x 10-24 grams).
How is it Calculated?
A protein's weight is determined by the sum of the molecular weights of its constituent amino acids. However, a simple summation of individual amino acids is not enough. When two amino acids link via a peptide bond, a water molecule (H2O, molecular weight ~18.015 Da) is released. Therefore, the formula for a polypeptide chain is:
Where 'n' is the total number of amino acids in the sequence.
The "110 Da Rule"
For quick lab estimations, scientists often use the average weight of an amino acid residue, which is roughly 110 Daltons. While amino acids range from Glycine (75 Da) to Tryptophan (204 Da), the distribution in typical proteins averages out to 110 Da once water loss is accounted for.
Example Calculations
Small Peptide (10 residues): 10 × 110 Da = 1.1 kDa
Green Fluorescent Protein (GFP): 238 residues ≈ 26.2 kDa
Bovine Serum Albumin (BSA): 583 residues ≈ 66.4 kDa
Human Hemoglobin (Alpha chain): 141 residues ≈ 15.5 kDa
Factors Affecting Molecular Weight
While the calculator provides the primary sequence mass, actual biological samples may vary due to:
Post-Translational Modifications (PTMs): Addition of carbohydrates (glycosylation), phosphate groups (phosphorylation), or lipids.
Prosthetic Groups: Non-protein components like Heme in hemoglobin.
Quaternary Structure: Multiple subunits binding together (e.g., Hemoglobin is a tetramer of ~64 kDa).
function estimateProteinWeight() {
var length = document.getElementById("sequenceLength").value;
if (length && length > 0) {
var weightDa = length * 110;
displayResults(weightDa);
} else {
alert("Please enter a valid sequence length.");
}
}
function calculateDetailedMW() {
var weights = {
ala: 89.09, arg: 174.20, asn: 132.12, asp: 133.10,
cys: 121.16, gln: 146.15, glu: 147.13, gly: 75.07,
his: 155.16, ile: 131.18, leu: 131.18, lys: 146.19,
met: 149.21, phe: 165.19, pro: 115.13, ser: 105.09,
thr: 119.12, trp: 204.23, tyr: 181.19, val: 117.15
};
var totalMass = 0;
var totalCount = 0;
var aaList = ["ala", "arg", "asn", "asp", "cys", "gln", "glu", "gly", "his", "ile", "leu", "lys", "met", "phe", "pro", "ser", "thr", "trp", "tyr", "val"];
for (var i = 0; i < aaList.length; i++) {
var count = parseFloat(document.getElementById("aa_" + aaList[i]).value);
if (isNaN(count) || count < 0) count = 0;
totalMass += count * weights[aaList[i]];
totalCount += count;
}
if (totalCount === 0) {
alert("Please enter at least one amino acid count.");
return;
}
// Subtract water loss (n-1) molecules of H2O
// MW of Water = 18.01528
var netMass = totalMass – ((totalCount – 1) * 18.015);
displayResults(netMass);
}
function displayResults(da) {
var resultDiv = document.getElementById("result-display");
var resultDa = document.getElementById("result-da");
var resultKda = document.getElementById("result-kda");
resultDiv.style.display = "block";
resultDa.innerText = da.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
resultKda.innerText = (da / 1000).toLocaleString(undefined, {minimumFractionDigits: 3, maximumFractionDigits: 3});
// Scroll to result
resultDiv.scrollIntoView({ behavior: 'smooth', block: 'nearest' });
}
function resetFields() {
var inputs = document.querySelectorAll('.aa-input-group input');
for (var i = 0; i < inputs.length; i++) {
inputs[i].value = 0;
}
document.getElementById("sequenceLength").value = "";
document.getElementById("result-display").style.display = "none";
}