Molecular Weight
Also known as: MW, Molecular mass, Formula weight, Molar mass
Molecular Weight is the sum of the atomic weights of all atoms in a molecule, typically expressed in Daltons (Da) or kilodaltons (kDa) for peptides and proteins. Molecular weight is a fundamental property that affects a peptide's solubility, membrane permeability, clearance rate, and overall pharmacokinetic behavior in drug development.
Last updated: February 1, 2026
What is Molecular Weight?
Molecular weight (MW) is the total mass of a molecule, calculated by adding up the atomic weights of every atom it contains. For peptides and proteins, molecular weight is typically expressed in Daltons (Da), where one Dalton equals approximately the mass of one hydrogen atom (1.008 atomic mass units).
Key units:
- Dalton (Da) - Standard unit for molecular mass
- Kilodalton (kDa) - 1,000 Daltons, used for larger proteins
- Atomic mass unit (amu) - Equivalent to Dalton
- g/mol - Gram per mole, numerically equal to Da
Calculating Peptide Molecular Weight
Basic Calculation
Peptide MW = Sum of amino acid residue weights - (n-1) x 18.015
The subtraction accounts for water molecules lost during peptide bond formation.
Average Amino Acid Weights
| Amino Acid | 3-Letter | MW (Da) |
|---|---|---|
| Glycine | Gly | 57.05 |
| Alanine | Ala | 71.08 |
| Serine | Ser | 87.08 |
| Proline | Pro | 97.12 |
| Valine | Val | 99.13 |
| Threonine | Thr | 101.11 |
| Cysteine | Cys | 103.14 |
| Leucine | Leu | 113.16 |
| Isoleucine | Ile | 113.16 |
| Asparagine | Asn | 114.10 |
| Aspartic acid | Asp | 115.09 |
| Glutamine | Gln | 128.13 |
| Lysine | Lys | 128.17 |
| Glutamic acid | Glu | 129.12 |
| Methionine | Met | 131.20 |
| Histidine | His | 137.14 |
| Phenylalanine | Phe | 147.18 |
| Arginine | Arg | 156.19 |
| Tyrosine | Tyr | 163.18 |
| Tryptophan | Trp | 186.21 |
Average residue weight: ~110 Da (for estimation)
Molecular Weights of Peptide Drugs
| Peptide | Amino Acids | MW (Da) | Category |
|---|---|---|---|
| Oxytocin | 9 | 1,007 | Small peptide |
| Vasopressin | 9 | 1,084 | Small peptide |
| BPC-157 | 15 | 1,419 | Medium peptide |
| Semaglutide | 31 | 4,114 | Medium peptide |
| Insulin | 51 | 5,808 | Small protein |
| Growth hormone | 191 | 22,124 | Protein |
| Antibody (IgG) | ~1,400 | ~150,000 | Large protein |
Molecular Weight and Drug Properties
Size Classifications
| Category | MW Range | Properties |
|---|---|---|
| Small molecules | Under 500 Da | Oral bioavailable, membrane permeable |
| Peptides | 500-5,000 Da | Limited oral, often injectable |
| Small proteins | 5,000-20,000 Da | Injectable, specialized delivery |
| Large proteins | 20,000+ Da | Injectable, complex pharmacokinetics |
Impact on Drug Behavior
| Property | Effect of Higher MW |
|---|---|
| Oral absorption | Decreased (limited permeability) |
| Membrane permeability | Decreased |
| Renal clearance | Decreased (glomerular filtration cutoff ~60 kDa) |
| Half-life | Generally increased |
| Tissue penetration | More limited |
| Immunogenicity | Generally increased |
The Rule of 500
Lipinski’s “Rule of 5” suggests molecules over 500 Da have poor oral bioavailability:
| Criterion | Threshold |
|---|---|
| Molecular weight | Under 500 Da |
| LogP | Under 5 |
| H-bond donors | Under 5 |
| H-bond acceptors | Under 10 |
Most peptides violate this rule, explaining why injection is the typical route of administration.
Measuring Molecular Weight
Analytical Methods
| Method | Accuracy | Application |
|---|---|---|
| Mass spectrometry (ESI, MALDI) | Very high (0.01%) | Precise MW determination |
| SDS-PAGE | Moderate | Protein size estimation |
| Size exclusion chromatography | Moderate | Native MW, aggregation |
| Analytical ultracentrifugation | High | Absolute MW in solution |
Mass Spectrometry Considerations
- Monoisotopic mass - Using most abundant isotopes (exact)
- Average mass - Using natural isotope distribution (weighted average)
- Difference - Can be significant for large proteins
Molecular Weight Modifications
PEGylation
Adding polyethylene glycol (PEG) increases MW:
| PEG Size | Added MW | Effect on Half-life |
|---|---|---|
| 5 kDa | 5,000 Da | Moderate increase |
| 20 kDa | 20,000 Da | Significant increase |
| 40 kDa | 40,000 Da | Major increase |
Fatty Acid Conjugation
Lipidation adds mass while altering distribution:
| Modification | Added MW | Example Drug |
|---|---|---|
| Palmitic acid (C16) | ~240 Da | Liraglutide |
| Stearic acid (C18) | ~268 Da | Various |
| Diacyl chains | ~500+ Da | Semaglutide |
Frequently Asked Questions
Why can’t most peptides be taken orally?
Molecular weight is a major factor. Peptides over 500 Da have poor membrane permeability and cannot easily cross the intestinal epithelium. Additionally, digestive enzymes degrade peptide bonds before absorption can occur. The combination of high MW, hydrophilicity, and enzymatic susceptibility makes oral peptide delivery extremely challenging.
How does molecular weight affect kidney clearance?
The glomeruli in kidneys filter blood based on molecular size. Small molecules (under ~5-10 kDa) are freely filtered and rapidly excreted. Larger molecules (especially over 60 kDa) are too big to pass through and remain in circulation longer. This is why PEGylation and protein carriers extend peptide half-life by increasing effective molecular weight above the filtration threshold.
What’s the difference between molecular weight and molecular formula?
Molecular formula (like C254H377N65O75S6 for insulin) tells you the exact count of each type of atom. Molecular weight is the total mass calculated from those atoms. Two molecules with the same MW could have completely different formulas and properties, while the formula uniquely defines the atomic composition.
Related Peptides
Related Terms
Disclaimer: This glossary entry is for educational purposes only and does not constitute medical advice. Always consult a qualified healthcare provider for medical questions.