kDa (Kilodalton) Calculators

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A kilodalton (kDa) is a unit of molecular weight equal to 1000 daltons (Da), where 1 dalton equals 1/12 the mass of a carbon-12 atom (approximately 1.66 × 10⁻²⁷ kg). kDa is the standard unit for expressing the molecular mass of proteins, nucleic acids, and other biological macromolecules. The average molecular weight of an amino acid is approximately 110 Da, so a protein of 200 amino acids has a molecular weight of ~22 kDa. In gel electrophoresis, molecular weight ladders (markers) calibrate the gel and are reported in kDa.

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Dalton and kDa Conversions

  • 1 Da = 1.66054 × 10⁻²⁷ kg = 1 g/mol
  • 1 kDa = 1000 Da = 1 kg/mol
  • 1 MDa = 1000 kDa = 1000 kg/mol

Estimating protein MW: average amino acid MW ≈ 110 Da. A 300 amino acid protein ≈ 300 × 110 = 33,000 Da = 33 kDa. More precisely, calculate from the actual amino acid sequence using ExPASy ProtParam.

Typical Molecular Weights

  • Insulin: 5.8 kDa (51 amino acids)
  • Ubiquitin: 8.5 kDa (76 amino acids)
  • GFP: 26.9 kDa (239 amino acids)
  • Antibody IgG: ~150 kDa (4 chains: 2×50 + 2×25 kDa)
  • Ribosome (E. coli 70S): ~2500 kDa
  • DNA (1 kb): ~660 kDa (660 Da/bp × 1000 bp)

SDS-PAGE and Molecular Weight

SDS-PAGE separates proteins by mass: SDS denatures proteins and gives them uniform negative charge proportional to size; migration distance through polyacrylamide gel is inversely proportional to log(MW). Molecular weight markers (ladders) are run alongside samples. Protein MW is estimated by comparing migration to ladder band positions: plot log(MW) vs. migration distance → linear regression → read off unknown protein MW.

Glossary

Kilodalton (kDa)
Unit of molecular mass: 1 kDa = 1000 Da = 1 g/mol; average amino acid ≈ 110 Da; used to express protein and nucleic acid molecular weights in biochemistry and molecular biology.
SDS-PAGE
Sodium dodecyl sulfate polyacrylamide gel electrophoresis; separates proteins by molecular weight after SDS denaturation; migration compared to kDa ladder bands estimates protein MW.
Molecular Weight Ladder
A mixture of proteins of known molecular weights run alongside samples in SDS-PAGE; provides a calibration curve (log MW vs. migration distance) for estimating unknown protein sizes.

Frequently Asked Questions

A kilodalton (kDa) is a unit of molecular mass: 1 kDa = 1000 Da = 1 g/mol. It is used to express the molecular weight of proteins, nucleic acids, and other macromolecules. Average amino acid mass ≈ 110 Da (0.110 kDa), so a 100 amino acid protein ≈ 11 kDa. In SDS-PAGE, protein bands are sized relative to molecular weight ladder bands reported in kDa. For nucleic acids: 1 bp of dsDNA ≈ 660 Da; so 1 kb DNA ≈ 660 kDa.

Multiply the number of amino acids by the average amino acid molecular weight (~110 Da or 0.110 kDa). Example: a 450 amino acid protein ≈ 450 × 110 = 49,500 Da ≈ 49.5 kDa. For an accurate calculation, use ExPASy ProtParam (web.expasy.org/protparam) — input the amino acid sequence and it calculates exact MW, pI, amino acid composition, and other properties. Note: post-translational modifications (glycosylation, phosphorylation) add mass above the theoretical value, which is why SDS-PAGE bands sometimes appear at higher MW than predicted from sequence.

SDS-PAGE separates proteins by mass: SDS denatures and uniformly coats proteins with negative charge proportional to size; smaller proteins migrate faster through the polyacrylamide matrix. Run molecular weight markers (pre-stained or unstained protein ladders with bands at known kDa values) in adjacent lanes. After electrophoresis: measure migration distance from the well for each ladder band and unknown protein. Plot log(MW) vs. relative migration (Rf) for ladder bands → linear regression. Read off the unknown protein MW from the regression equation: MW = 10^(regression equation for Rf of unknown).

Several factors cause apparent MW discrepancy: (1) Post-translational modifications — glycosylation adds carbohydrate mass (N-linked glycosylation can add 2–30 kDa); phosphorylation, ubiquitination, and other modifications add mass. (2) Anomalous SDS binding — some proteins (membrane proteins, highly charged proteins) bind SDS disproportionately, affecting migration. (3) Incomplete denaturation — disulfide bonds not fully reduced migrate differently. (4) Highly acidic or basic proteins migrate aberrantly. Always compare to the appropriate control: run in reducing (+ β-mercaptoethanol or DTT) vs. non-reducing conditions to identify disulfide-linked oligomers.