What is Dalton?
Also called: Da, kDa, Unified atomic mass unit, u, amu
The unit of molecular mass: one dalton (Da) is one-twelfth the mass of a carbon-12 atom, about 1.66 × 10⁻²⁴ g, so a molecule of X Da has a molar mass of X g/mol.
By the APL Research Team · Updated
The dalton (Da) is the unit for the mass of a single molecule. It is defined as one-twelfth of the mass of a carbon-12 atom, 1.66054 × 10⁻²⁷ kg, and is the same quantity as the unified atomic mass unit (u); "amu" is an older name for it. Biochemists generally use daltons and kilodaltons (1 kDa = 1,000 Da).
Daltons and grams per mole
A mass in daltons and a molar mass in g/mol are numerically equal, to within parts per billion since the 2019 redefinition of the mole. A peptide whose molecules weigh 1419.54 Da has a molar mass of 1419.54 g/mol. The two units answer different questions (the mass of one molecule versus the mass of a mole of them), but because the number is the same, certificates of analysis and catalogues use them interchangeably. For arithmetic, g/mol is the unit to carry: milligrams divided by daltons is not a meaningful quantity, whereas milligrams divided by g/mol gives millimoles.
Worked example: counting molecules
1 mg of BPC-157 (1419.54 Da) contains 1 × 10⁻³ g ÷ 1419.54 g/mol × 6.022 × 10²³ mol⁻¹ ≈ 4.24 × 10¹⁷ molecules. Going the other way, a single molecule weighs 1419.54 × 1.66054 × 10⁻²⁴ g ≈ 2.36 × 10⁻²¹ g.
Where peptides sit on the scale
| Molecule | Approximate mass |
|---|---|
| Glycine (free amino acid) | 75 Da |
| GHK (3 residues) | 340 Da |
| Selank (7 residues) | 752 Da |
| BPC-157 (15 residues) | 1.42 kDa |
| MOTS-c (16 residues) | 2.17 kDa |
| Semaglutide (31 residues plus a fatty-acid side chain) | 4.11 kDa |
| Human insulin (51 residues in two chains) | 5.8 kDa |
| Serum albumin | about 66 kDa |
Most research peptides fall between about 0.3 and 5 kDa, one to two orders of magnitude below typical proteins. That gap shows up in practice in two places.
Daltons at the bench
Filters and membranes. Ultrafiltration and dialysis membranes are rated by a nominal molecular-weight cut-off (MWCO) in kDa. A 3 kDa device is designed to retain molecules larger than that, so it will not reliably hold back a 1.4 kDa peptide such as BPC-157, and peptides can pass through or be lost on membranes chosen with protein work in mind. Cut-off ratings are nominal and depend on a molecule's shape as well as its mass.
Mass spectra. The x-axis of a mass spectrum is m/z, mass divided by charge, not mass in daltons. Semaglutide (4.11 kDa) carrying four extra protons appears near m/z 1029, so a peak's position equals the molecular mass only when the charge is one. Mass spectrometry explains how charge states are converted back to a mass.
Common slips
- Quoting protein masses as if they were exact. Large protein masses are often rounded to the nearest kDa, whereas a peptide's mass calculated from its sequence is given to two decimal places, and the two kinds of figure should not be mixed in a calculation.
- Forgetting that the mass of a salt or hydrate differs from the peptide's own mass; see molecular weight.
The peptide molecular weight calculator gives the mass of any sequence in daltons, and the molarity entry turns it into concentrations.