909 AMINOSPeptide Identifiers: Sequence, Formula, Molecular Weight and CAS Number
A product name alone doesn't pin down a compound. Sequence, molecular formula, molecular weight and CAS number are the standard identifiers laboratories use to confirm which compound a material is supposed to be.
- Names vary between suppliers; standard identifiers state which molecule is meant.
- Average molecular weight (g/mol) is used for weighing; monoisotopic mass is used in high-resolution mass spectrometry.
- A free peptide and its salts can have different weights and sometimes different CAS numbers, so check which form a value refers to.
- Identifiers state what a compound should be. Whether a sample is consistent with it is a question for identity testing.
A product name alone doesn't pin down a compound. Names vary between suppliers, and similar names can refer to different molecules. Laboratories use a small set of standard identifiers to confirm exactly which compound a material is supposed to be. This page explains each one and how to check it.
1The identifiers
| Identifier | What it tells you | Where to check |
|---|---|---|
| Sequence | The amino acids in order, in one-letter or three-letter code, plus any end groups or modifications | UniProt (natural peptides), PubChem, the literature |
| Molecular formula | The number of each atom in one molecule | PubChem |
| Molecular weight (average mass) | The mass of one molecule averaged over the natural mix of isotopes, in g/mol; used for weighing and amounts | PubChem |
| Monoisotopic mass | The mass calculated using only the lightest stable isotope of each element (carbon-12, hydrogen-1, nitrogen-14 and so on) | PubChem |
| CAS Registry Number | A unique number assigned by CAS to a specific substance | PubChem, CAS SciFinder |
| PubChem CID / UNII | Database identifiers that link to the records above | PubChem, the FDA substance registry |
A supplier's own product code (for example, 909's "909A-" codes) is an internal catalog code, not a chemical identifier.
2Reading a sequence
M = methionine, R = arginine, W = tryptophan, K = lysine, and so on.
"H-…-OH" means a free N-terminal amine and a free C-terminal acid. "-NH2" at the end means a C-terminal amide. These change the formula and the weight.
Modifications such as an added acyl group at the N-terminus are part of the compound's identity and are included in its formula and CAS number.
3Two worked examples
| MOTS-c | Tesamorelin | |
|---|---|---|
| Form | Free peptide, free C-terminal acid (H-…-OH), neutral; no salt | Free peptide, C-terminal amide, neutral; no salt |
| Sequence | MRWQEMGYIFYPRKLR (16 amino acids) | trans-3-hexenoyl- (GHRH 1-44 with an N-terminal acyl group and C-terminal amide) |
| Molecular formula | C101H152N28O22S2 | C221H366N72O67S |
| Molecular weight (average) | 2174.6 g/mol | 5136 g/mol |
| Monoisotopic mass | 2173.108 Da | 5132.717 Da |
| CAS Registry Number (as listed in PubChem) | 1627580-64-6 | 218949-48-5 |
| PubChem CID | 146675088 | 16137828 |
Sources: PubChem records for each CID; MOTS-c sequence from UniProt A0A0C5B5G6; GHRH 1-44 residues from UniProt P01286.
The tesamorelin sequence is checked by formula: the GHRH 1-44 residues plus the hexenoyl group and C-terminal amide give exactly C221H366N72O67S.
Values are for the free peptide, not a salt. CAS numbers are as listed in PubChem and have not been confirmed directly with CAS.
4Free peptide vs. salt
Databases often have separate entries for a peptide and its salts (for example, a trifluoroacetate or acetate salt), each with its own molecular weight and sometimes its own CAS number.
When you compare a molecular weight, check which form it refers to. See Acetate vs. TFA: What a Peptide's Salt Form Means.
5How identifiers relate to identity testing
Identifiers state what a compound should be. Identity testing checks whether a sample is consistent with it:
Compared with a reference standard (the method on 909's KMD certificates), these give supporting evidence of identity.
Measures mass-to-charge ratios. A peptide usually appears at several charge states, so the result is first converted back to a neutral mass, then compared with the value calculated from the formula.
Which calculated value is used depends on the instrument's resolution:
When individual isotope peaks are resolved, the observed monoisotopic peak is compared with the monoisotopic mass.
When they aren't (more common for larger peptides), the average mass is compared.
A useful MS result states the observed mass, the theoretical mass and which type it is, and the tolerance.
See Identity Testing: RT + UV vs. Mass Spectrometry for what each method establishes.
6Checklist
- Does the documentation give a sequence (with end groups) or a structure, not just a name?
- Do the formula and molecular weight match a public database entry?
- Is a CAS number given, and does it belong to the same form (free peptide or salt)?
- Does the identity method on the certificate test against that compound?
?Frequently asked questions
Is a CAS number proof that a material is that compound?
No. A CAS number identifies which substance a material is claimed to be. Whether a sample is consistent with it is a question for identity testing.
Why do two sources list slightly different molecular weights?
They may refer to different forms (free peptide or salt), give average versus monoisotopic mass (for MOTS-c, 2174.6 vs. 2173.108), or round differently.
Where can I check these identifiers for free?
PubChem (pubchem.ncbi.nlm.nih.gov) lists formula, weights, synonyms and CAS numbers. UniProt lists sequences for naturally occurring peptides.
Sources
- PubChem CID 146675088 (MOTS-c)
- PubChem CID 16137828 (Tesamorelin)
- UniProt A0A0C5B5G6 (MOTS-c sequence)
- UniProt P01286 (GHRH precursor)
Sources are cited for reference. Their publishers have not reviewed or endorsed this guide.
For laboratory research use only. Not for human or veterinary use. Not FDA approved.