Peptides are sold by degree of hydrolysis, and degree of hydrolysis is a claim about molecular weight. That makes Pea Peptide one of the few protein-derived raw materials where the specification can be checked against arithmetic rather than taken on trust. The relevant numbers are small, well-defined and publicly available.
The building blocks you can actually look up
L-Leucine is CAS 61-90-5, C6H13NO2, 131.17 g/mol, CID 6106, XLogP -1.5. L-Lysine is CAS 56-87-1, C6H14N2O2, 146.19 g/mol, CID 5962, XLogP -3.0. L-Arginine is CAS 74-79-3, C6H14N4O2, 174.20 g/mol, CID 6322, XLogP -4.2. Three residues, three different masses, and a 43.03 g/mol spread from leucine to arginine that explains most of the charge behaviour buyers care about.
When two amino acids join, one water molecule leaves. Water is 18.02 g/mol. So a dipeptide of leucine and lysine weighs 131.17 + 146.19 – 18.02 = 259.34 g/mol, not 277.36 g/mol. That 18.02 g/mol correction applies once per peptide bond, and it is the single most common arithmetic error in peptide documentation.
Residue reference for specification writing
| Amino acid | CAS | Formula | MW (g/mol) | CID | XLogP |
|---|---|---|---|---|---|
| L-Leucine | 61-90-5 | C6H13NO2 | 131.17 | 6106 | -1.5 |
| L-Lysine | 56-87-1 | C6H14N2O2 | 146.19 | 5962 | -3.0 |
| L-Arginine | 74-79-3 | C6H14N4O2 | 174.20 | 6322 | -4.2 |
| Water (per bond) | 7732-18-5 | H2O | 18.02 | 962 | — |
| Leu-Lys dipeptide (calc.) | — | C12H25N3O3 | 259.34 | — | — |
What the molecular weight cut-off actually means
| Fraction | Approx. residues | Mass scale | Typical QC method |
|---|---|---|---|
| Free amino acid | 1 | 131.17 to 174.20 g/mol | Amino acid analyser |
| Dipeptide / tripeptide | 2 to 3 | ~259.34 to ~430 g/mol | SEC-HPLC, LC-MS |
| Oligopeptide | 4 to 10 | ~500 to ~1,200 g/mol | SEC with calibration |
| Polypeptide / protein | >10 | >1,200 g/mol | SDS-PAGE, SEC-MALS |
A supplier quoting “average molecular weight 500 Da” is describing material around four residues. A supplier quoting 2,000 Da is selling something closer to a partially hydrolysed protein. The same plant source, two completely different products.
Procurement checklist
- Ask for the SEC calibration standard. Molecular weight distributions are method-defined; without the standard the number is not reproducible.
- Request the free amino acid fraction. At 131.17 to 174.20 g/mol, free residues behave very differently from peptides in taste and osmolality.
- Check the nitrogen conversion factor. Protein content derived from nitrogen will differ from peptide content derived from SEC.
- Request batch-specific data. Assay, moisture, particle size, packaging and shelf life are issued per batch on the COA and are not published as generic values.
Frequently asked questions
Does a lower average molecular weight always mean better?
No. It means smaller, more soluble and usually more bitter. Suitability depends on the application, not on the number alone.
Why is the dipeptide mass less than the sum of its parts?
Condensation. One water molecule at 18.02 g/mol is lost per peptide bond formed.
Is pea peptide the same as pea protein?
No. Pea protein is largely intact storage protein well above 1,200 g/mol; a peptide fraction is the hydrolysed material below that.
Facts in this article are taken from PubChem (US National Library of Medicine) records for CIDs 6106, 5962, 6322 and 962. Grade-specific data is issued per batch on the COA and is not published here.