Dalton
Also known as: Da, Unified atomic mass unit, AMU, Atomic mass unit
Dalton is a unit of mass used to express atomic and molecular weights, where one dalton (Da) is approximately equal to the mass of a single hydrogen atom or one-twelfth the mass of a carbon-12 atom. In peptide science, daltons are used to describe peptide molecular weights, typically ranging from hundreds to thousands of daltons.
Last updated: February 1, 2026
Understanding the Dalton
The dalton (symbol: Da) is the standard unit for expressing the mass of atoms, molecules, and macromolecules like peptides and proteins. One dalton is defined as exactly 1/12 the mass of a carbon-12 atom, which equals approximately:
- 1 Da = 1.66 x 10^-24 grams
- 1 Da ≈ mass of one hydrogen atom
- 1 kDa (kilodalton) = 1,000 daltons
The unit is named after English chemist John Dalton, who pioneered atomic theory in the early 1800s.
Daltons in Peptide Science
Molecular weight in daltons is a fundamental property of peptides:
| Molecule Type | Typical Size | Examples |
|---|---|---|
| Single amino acid | 75-204 Da | Glycine (75 Da), Tryptophan (204 Da) |
| Small peptide | 500-2,000 Da | BPC-157 (~1,419 Da) |
| Medium peptide | 2,000-5,000 Da | Semaglutide (~4,114 Da) |
| Large peptide | 5,000-10,000 Da | Tirzepatide (~4,814 Da) |
| Small protein | 10,000-50,000 Da | Insulin (~5,800 Da) |
| Large protein | 50,000+ Da | Antibodies (~150,000 Da) |
Why Molecular Weight Matters
The dalton measurement directly impacts peptide drug properties:
Bioavailability and Delivery
- Under 500 Da: Often orally bioavailable (small molecule rule)
- 500-5,000 Da: Typically requires injection
- Over 5,000 Da: May need specialized delivery systems
Kidney Filtration
- Under 30,000 Da: Rapidly filtered by kidneys
- Over 30,000 Da: Longer circulation time
- This affects peptide half-life and dosing frequency
Manufacturing Complexity
- Larger peptides are more difficult to synthesize
- Cost typically increases with molecular weight
- Purity verification becomes more challenging
Calculating Peptide Molecular Weight
To estimate a peptide’s molecular weight:
- Sum amino acid residue weights (average ~110 Da each)
- Add 18 Da for the terminal water molecule
- Add modifications (acetylation, amidation, PEG, etc.)
| Amino Acid | Residue Weight (Da) |
|---|---|
| Glycine | 57.02 |
| Alanine | 71.04 |
| Serine | 87.03 |
| Leucine | 113.08 |
| Phenylalanine | 147.07 |
| Tryptophan | 186.08 |
Quick estimate: Number of amino acids x 110 Da ≈ molecular weight
Frequently Asked Questions
What’s the difference between a dalton and an atomic mass unit?
They are identical. The dalton (Da) and unified atomic mass unit (u or amu) represent the same mass. “Dalton” is preferred in biochemistry and pharmaceutical contexts, while “amu” is more common in physics and chemistry.
Why do peptide drugs list molecular weight?
Molecular weight is essential for:
- Calculating molar concentrations
- Determining dosing (mcg vs. nmol)
- Predicting pharmacokinetic properties
- Quality control in manufacturing
How does molecular weight affect peptide stability?
Generally, larger peptides (higher daltons) have more complex structures that can be more susceptible to degradation. However, modifications like PEGylation add molecular weight while increasing stability by protecting the peptide core from enzymatic breakdown.
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Disclaimer: This glossary entry is for educational purposes only and does not constitute medical advice. Always consult a qualified healthcare provider for medical questions.