Daltons Calculators

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The Dalton (Da), also called the unified atomic mass unit (u or amu), is the unit of atomic and molecular mass. One Dalton is defined as exactly 1/12 the mass of a carbon-12 atom: 1 Da = 1.66054 × 10⁻²⁴ g. The numerical value of a molecule's mass in Daltons equals its molar mass in grams per mole (g/mol) — this convenient equivalence comes from the definition of Avogadro's number. In biochemistry, molecular masses are often expressed in kilodaltons (kDa = 1,000 Da): a typical protein is 10–500 kDa; a ribosome is ~2.5 MDa (megadaltons).

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Dalton Definition and Conversions

1 Da = 1.66054 × 10⁻²⁴ g = 1/12 mass of ¹²C. 1 kDa = 1,000 Da. 1 MDa (megadalton) = 10⁶ Da. Molar mass (g/mol) = molecular mass (Da) numerically. Example: water H₂O = (2×1.008 + 15.999) Da = 18.015 Da = 18.015 g/mol.

Typical Molecular Masses in Biochemistry

  • ATP: 507 Da
  • Glucose: 180 Da
  • Insulin: ~5.8 kDa
  • Lysozyme: ~14 kDa
  • Albumin (BSA): ~66 kDa
  • IgG antibody: ~150 kDa
  • Ribosome (bacterial 70S): ~2.5 MDa
  • Ribosome (eukaryotic 80S): ~4 MDa
  • DNA (1 kb): ~650 kDa

Dalton in SDS-PAGE

Molecular weight markers (ladders) for SDS-PAGE are labeled in kDa. During electrophoresis: smaller proteins (lower kDa) migrate faster; larger proteins slower. Estimate MW of unknown protein by comparing migration to ladder bands on a log(MW) vs. distance regression.

Da in Mass Spectrometry

Mass spectrometry measures the mass-to-charge ratio (m/z) of ionized molecules. m/z in Da/charge. For singly charged ions (z=1): m/z = molecular mass in Da + 1 (for the added proton). High-resolution mass spec: mass accuracy < 5 ppm → identify molecular formula from exact mass.

Glossary

Dalton (Da)
1/12 mass of ¹²C atom = 1.66054×10⁻²⁴ g; molecular mass in Da equals molar mass in g/mol numerically; 1 kDa = 1,000 Da; used for protein MW in SDS-PAGE and biochemistry.
kDa (Kilodalton)
1,000 Daltons; common unit for protein molecular weight: BSA ≈ 66 kDa; IgG ≈ 150 kDa; ribosome ≈ 2,500 kDa (2.5 MDa); used to label molecular weight markers in SDS-PAGE.
Molar Mass
Mass per mole (g/mol); numerically equal to molecular mass in Da; used to convert between mass (grams) and amount (moles): n = mass/molar mass; enables calculating molar concentration.

Frequently Asked Questions

A Dalton (Da) = 1/12 the mass of a carbon-12 atom = 1.66054 × 10⁻²⁴ g. It is the standard unit for atomic and molecular mass in chemistry and biochemistry. Why used: atoms and molecules have incredibly small masses — a typical protein weighs around 10⁻²⁰ g; expressing this as 30,000 Da (30 kDa) is far more convenient. Convenient property: a molecule's mass in Da equals its molar mass in g/mol numerically (due to Avogadro's number definition). So a 30,000 Da protein has a molar mass of 30,000 g/mol = 30 g per mmol. This makes converting between individual molecule masses (Da) and laboratory-scale quantities (grams, moles) straightforward.

In SDS-PAGE, molecular weight markers (protein ladders) contain proteins of known masses (in kDa). After running the gel: measure migration distance of each ladder band from the top of the resolving gel. Plot log(MW in kDa) on y-axis vs. migration distance on x-axis → linear regression. Read the MW of unknown protein bands from the regression. Example: a band migrating 4.2 cm when the ladder bands at 25, 50, 75, 100, 150, 250 kDa are plotted gives log(MW) from regression → antilog gives estimated MW in kDa. Report as 'apparent molecular weight' — SDS-PAGE gives approximate MW; anomalous migration occurs for membrane proteins, glycoproteins, and proteins with unusual charge or shape.

The numerical value of molecular mass in Daltons equals molar mass in grams per mole (g/mol). This equivalence comes from the definition of Avogadro's number (Nₐ = 6.022 × 10²³ mol⁻¹): 1 Da × Nₐ = 1.66054 × 10⁻²⁴ g × 6.022 × 10²³ mol⁻¹ = 1.00000 g/mol. So: H₂O = 18.015 Da → molar mass = 18.015 g/mol → 1 mole of H₂O weighs 18.015 grams. BSA = 66,430 Da = 66,430 g/mol = 66.430 g/mmol → 1 mg BSA = 1/66.430 × 10⁻³ mol = 15.1 nmol = 15.1 μM if dissolved in 1 mL. This equivalence makes it easy to calculate molar concentrations from mass measurements.

Mass spectrometry measures the mass-to-charge ratio (m/z) of ions in Daltons per elementary charge (Da/e or Th, Thomson). For ESI (electrospray ionization) of proteins: proteins acquire multiple charges (z = +5 to +50 for large proteins) → appear at lower m/z. The molecular mass (M) is calculated from multiple charge state peaks: M = z × (m/z) − z × 1.0073 (mass of proton). For MALDI (matrix-assisted laser desorption ionization): usually singly charged (z = 1) → m/z ≈ M + 1.0073. Mass accuracy: low-resolution instruments: ±1 Da (integers). High-resolution (Orbitrap, QTOF): ±0.001 Da (< 5 ppm) → determine molecular formula from exact mass. Protein identification: digest with trypsin → measure peptide masses → search against protein database → identify protein (peptide mass fingerprinting).