In plain English
Chemicals have many names. Water can also be called “oxidane” or “dihydrogen oxide” [4], and one peptide can have a code name, a chemical name and a trade name. To avoid confusion, chemists use ID numbers [1].
The best known is the CAS number, for example 7732-18-5 for water [1, 4]. The Chemical Abstracts Service (CAS, part of the American Chemical Society) gives each new substance its own number. The number itself carries no chemical meaning; it is handed out in order [1].
The last digit is a check digit (a digit calculated from the others). It catches many typing mistakes [2].
Other systems exist too. PubChem, a free US Government database, gives each structure a compound ID (CID). The InChIKey is a 27-character code calculated from the structure itself [3].
A small change in the substance, such as a salt instead of the free peptide, gives a different CAS number. That is why a label should be checked against more than one identifier.
The short version
- One CAS number means one substance [1].
- The check digit catches many, but not all, typing errors [2].
- Salts and stereoisomers have their own numbers [6, 7].
- Cross-check the number, formula and molar mass together.
What is a CAS number?
A CAS Registry Number is the identifier assigned to a substance when it enters CAS REGISTRY, a database of disclosed chemical substances with more than 300 million entries. Numbers are assigned in sequential order as new substances are identified, and a CAS number designates only one substance [1]. Because the numbers do not depend on any naming system, they act as a common link between the many names a substance may have [1].
Two points from CAS itself are worth keeping in mind [1]:
- CAS is the source and final authority for CAS Registry Numbers.
- Other resources may have associated an incorrect CAS number with a compound.
A CAS number is a registry-lookup identifier. The structure cannot be worked out from the number; it has to be looked up [3].
How the check digit works
A CAS number has up to 10 digits in three parts separated by hyphens. The first part has 2 to 7 digits, the second part has 2 digits, and the final part is a single check digit [2].
To calculate the check digit, take the digits before it, starting from the right. Multiply the rightmost by 1, the next by 2, the next by 3, and so on. Add the products and divide by 10. The remainder is the check digit [2].
Worked example 1: water, 7732-18-5
| Digit (right to left) | 8 | 1 | 2 | 3 | 7 | 7 |
|---|---|---|---|---|---|---|
| Multiplier | 1 | 2 | 3 | 4 | 5 | 6 |
| Product | 8 | 2 | 6 | 12 | 35 | 42 |
Sum = 105. 105 ÷ 10 leaves a remainder of 5, which matches the check digit in 7732-18-5.
Worked example 2: angiotensin II, 4474-91-3
PubChem lists 4474-91-3 for the octapeptide angiotensin II [5]. Reading 447491 from the right: (1 × 1) + (9 × 2) + (4 × 3) + (7 × 4) + (4 × 5) + (4 × 6) = 103. The remainder is 3, matching the check digit.
What the check digit can and cannot catch
- A swapped pair. Typing water’s number as 7732-81-5 gives a sum of 112 and a calculated check digit of 2, not 5, so the error shows up.
- A valid-looking wrong number. Changing the “1” in 7732-18-5 to a “6” gives 7732-68-5. Its sum is 115, so the check digit is still 5, and the error is missed. A number that passes the check is well formed; it is not proven to be the right substance.
Why one peptide can have several CAS numbers
Because a CAS number identifies one substance [1], each distinct form gets its own number.
Stereoisomers
Alanine shows the problem clearly. Its L-, D- and racemic (DL) forms share a formula and molar mass but are different substances [6]:
| Substance | CAS number (as listed in PubChem) | PubChem CID | Formula | Molar mass (g/mol) | InChIKey |
|---|---|---|---|---|---|
| L-alanine | 56-41-7 | 5950 | C3H7NO2 | 89.09 | QNAYBMKLOCPYGJ-REOHCLBHSA-N |
| D-alanine | 338-69-2 | 71080 | C3H7NO2 | 89.09 | QNAYBMKLOCPYGJ-UWTATZPHSA-N |
| DL-alanine | 302-72-7 | 602 | C3H7NO2 | 89.09 | QNAYBMKLOCPYGJ-UHFFFAOYSA-N |
A formula and molar mass check cannot tell these apart. The InChIKey can: its first block is identical because the skeleton is the same, and its second block differs because the stereochemistry differs [3]. Peptides built from D-amino acids raise the same issue. See Chirality and Racemisation.
Salt forms
Synthetic peptides are usually supplied as salts. PubChem has separate records for angiotensin II and angiotensin II acetate, and lists the free peptide as the acetate record’s parent compound [5, 7]:
| Record | PubChem CID | Formula | Molar mass (g/mol) | CAS-format numbers listed |
|---|---|---|---|---|
| Angiotensin II | 172198 | C50H71N13O12 | 1046.2 | 4474-91-3 |
| Angiotensin II acetate | 172197 | C52H75N13O14 | 1106.2 | 68521-88-0; 32044-01-2 |
The difference, 60.0 g/mol, is one molecule of acetic acid (C2H4O2). Two points follow:
- The PubChem acetate record models exactly one acetate per peptide. The acetate content of a real batch is measured, not assumed; the US Pharmacopeia has a general chapter for acetic acid in peptides [7, 16]. See Counter-Ions in Peptides.
- PubChem lists two CAS-format numbers for the acetate record. Which is correct for a given material is a question for CAS, the final authority [1].
Hydrates
Water of crystallisation changes the substance too. Copper(II) sulfate (CID 24462, CuSO4, 159.61 g/mol) is listed in PubChem under 7758-98-7, and its pentahydrate (CID 24463, 249.69 g/mol) under 7758-99-8 [9]. Weighing out one when the calculation assumes the other gives an error of more than 50% in the amount of copper sulfate.
Database identifiers are not always consistent. A 2015 study of eight chemical databases found that non-systematic identifiers (a group that includes brand names, generic names and CAS numbers) shared between databases were ambiguous, pointing to more than one structure, in 17.7–60.2% of cases (median 40.3%). Removing stereochemistry gave the largest reduction in ambiguity [10].
PubChem CIDs and SIDs
PubChem, run by the US National Center for Biotechnology Information (NCBI), has three linked databases. The Substance database holds records deposited by individual contributors, each with a substance ID (SID). The Compound database stores the unique chemical structures extracted from those deposits, each with a compound ID (CID) [8]. In its 2025 update PubChem held more than 1,000 data sources, 119 million compounds and 322 million substances [11].
One CID can gather many SIDs. On 25 October 2026, PubChem linked 144 deposited substance records to the single compound record for angiotensin II [5]. Names and CAS-format numbers in a PubChem synonym list come from many depositors [8], which is why they are a starting point for checking rather than the final word. The synonym list for water (CID 962), for example, includes entries naming deuterium oxide, a different substance [4].
InChI, InChIKey and SMILES
InChI and the InChIKey
The IUPAC International Chemical Identifier (InChI) is a non-proprietary, open-source identifier generated from a structure, developed by IUPAC and the US National Institute of Standards and Technology from 2000 [3]. The InChIKey is a compact, fixed-length, 27-character hashed version of it [3]:
- The first block of 14 characters encodes the core constitution: formula, connectivity, hydrogen positions and charge. It is always the same for the same molecular skeleton.
- The second block encodes stereochemistry, isotopes and similar features.
- A final flag character records protonation.
Because the InChIKey is a hash, two different structures can, rarely, share one (a “collision”). The designers set collision resistance at a level suited to the largest real-world databases [3]. Like a CAS number, an InChIKey is looked up rather than decoded [3].
SMILES
SMILES, introduced in 1988, writes a structure as a line of text [12]. PubChem gives L-alanine as C[C@@H](C(=O)O)N [6].
Other identifiers: UNII, ChEBI and HELM
| Identifier | Issued by | What it covers | Structure-based? |
|---|---|---|---|
| CAS Registry Number | CAS | One substance in CAS REGISTRY [1] | No (assigned in sequence) |
| PubChem CID / SID | NCBI (United States) | A unique structure (CID) or a depositor’s record (SID) [8] | CID is assigned to a standardised structure |
| InChI / InChIKey | Calculated by open-source software (IUPAC) | A defined chemical structure [3] | Yes |
| SMILES | Written from the structure | A line notation for a structure [12] | Yes |
| UNII | US FDA Global Substance Registration System | Ingredients in FDA-regulated products, from an atom to an organism [13] | Assigned; records hold scientific definitions |
| ChEBI ID | EMBL-EBI | Chemical entities of biological interest, in an ontology [14] | Assigned |
| HELM | Notation language | Complex macromolecules such as modified peptides and conjugates [15] | Yes, at the monomer level |
- UNII. The FDA created its Substance Registration System and Unique Ingredient Identifiers in 2006. The system defines substances by standardised scientific descriptions rather than names, and classifies them as chemical, protein, nucleic acid, polymer, structurally diverse or mixture under ISO 11238 [13].
- ChEBI. A freely available database and ontology of chemical entities of biological interest [14].
- HELM. The Hierarchical Editing Language for Macromolecules was designed for molecules such as peptides, proteins and oligonucleotides that are too large to represent comfortably atom by atom, yet carry non-natural modifications that a plain sequence cannot capture [15]. See Reading a Peptide Sequence and How Peptides Are Named.
How to cross-check a certificate or label
- Check the check digit. It takes seconds and catches many transcription errors [2].
- Look the number up. CAS Common Chemistry is a free CAS resource covering nearly 500,000 substances, searchable by name, SMILES, InChI or CAS number [17]. PubChem is a second source [8].
- Match the form. Compare the formula and molar mass on the document with the record for the same form: free peptide or salt, anhydrous or hydrate.
- Compare with the measured mass. For angiotensin II, PubChem gives a monoisotopic mass of 1045.535 Da [5], so the singly protonated ion [M+H]+ is expected near m/z 1046.542 (1045.535 + 1.007). See Mass Spectrometry for Peptides.
- Check the structure. Where a sequence, SMILES or InChIKey is given, confirm that it matches the record.
Worked example: a mismatched certificate
A certificate for “angiotensin II acetate” lists CAS 4474-91-3, formula C50H71N13O12 and molar mass 1106.2 g/mol.
- The check digit of 4474-91-3 is valid.
- The CAS number and formula match the free peptide record (CID 172198) [5].
- The molar mass matches the acetate record (CID 172197) [7].
The document mixes two substances. Any calculation of peptide content that relies on it needs the form confirmed first. See How to Read a Peptide COA and Safety Data Sheets and Lab Safety.
What this means in Australia
- Names and numbers in the Poisons Standard. The current Poisons Standard (October 2026) names substances, and some entries also give a CAS number; for example, it names CJC-1295 (CAS 863288-34-0) in Schedule 4 and Appendix D, clause 5 [18]. Schedule 4 means prescription-only, and Appendix D, clause 5 means possession without authority is illegal.
- Class entries. Some entries cover classes of substances rather than single compounds, and whether a class entry captures a specific compound is a legal question [18]. A CAS number on a label does not settle scheduling on its own.
- Labels. The TGA has warned that labelling of unapproved peptide products may be inaccurate or misleading, and that a “research use only” disclaimer does not make supply lawful or change a product’s regulatory status [19].
This is general information, not legal advice. Laws change; check the current Poisons Standard and TGA guidance.
Frequently asked questions
What is a CAS number?
A CAS Registry Number is a unique numeric identifier assigned by the Chemical Abstracts Service to one substance in its REGISTRY database. It has up to 10 digits in three hyphenated parts, ending in a check digit [1, 2].
How do I check that a CAS number is valid?
Multiply the digits before the check digit by 1, 2, 3 and so on from the right, add the products, and divide by 10. The remainder should equal the check digit [2]. Passing the check shows the number is well formed, not that it refers to the right substance.
Why does one peptide have more than one CAS number?
Each distinct substance gets its own number, so a free peptide, its acetate or trifluoroacetate salt, and its stereoisomers can each have a separate CAS number [1, 7].
What is the difference between a PubChem CID and SID?
A SID identifies a record deposited by one contributor. A CID identifies a unique chemical structure extracted from those deposits, so many SIDs can map to one CID [8].
What is an InChIKey used for?
It is a fixed-length, 27-character code generated from a structure. Its first 14 characters identify the molecular skeleton, which makes it useful for finding the same compound across databases [3].
References
- CAS. CAS REGISTRY. cas.org. Accessed 25 October 2026.
- CAS. Check digit verification of CAS Registry Numbers. cas.org. Accessed 25 October 2026.
- Heller SR, McNaught A, Pletnev I, Stein S, Tchekhovskoi D. InChI, the IUPAC International Chemical Identifier. J Cheminform. 2015;7:23. doi:10.1186/s13321-015-0068-4. PMID: 26136848
- National Center for Biotechnology Information. PubChem compound summary for CID 962, water. pubchem.ncbi.nlm.nih.gov. Accessed 25 October 2026.
- National Center for Biotechnology Information. PubChem compound summary for CID 172198, angiotensin II. pubchem.ncbi.nlm.nih.gov. Accessed 25 October 2026.
- National Center for Biotechnology Information. PubChem compound summaries for CID 5950 (L-alanine), CID 71080 (D-alanine) and CID 602 (DL-alanine). pubchem.ncbi.nlm.nih.gov. Accessed 25 October 2026.
- National Center for Biotechnology Information. PubChem compound summary for CID 172197, angiotensin II acetate. pubchem.ncbi.nlm.nih.gov. Accessed 25 October 2026.
- Kim S, Thiessen PA, Bolton EE, et al. PubChem Substance and Compound databases. Nucleic Acids Res. 2016;44(D1):D1202–D1213. doi:10.1093/nar/gkv951. PMID: 26400175
- National Center for Biotechnology Information. PubChem compound summaries for CID 24462 (copper(II) sulfate) and CID 24463 (copper(II) sulfate pentahydrate). pubchem.ncbi.nlm.nih.gov. Accessed 25 October 2026.
- Akhondi SA, Muresan S, Williams AJ, Kors JA. Ambiguity of non-systematic chemical identifiers within and between small-molecule databases. J Cheminform. 2015;7:54. doi:10.1186/s13321-015-0102-6. PMID: 26579214
- Kim S, Chen J, Cheng T, et al. PubChem 2025 update. Nucleic Acids Res. 2025;53(D1):D1516–D1525. doi:10.1093/nar/gkae1059. PMID: 39558165
- Weininger D. SMILES, a chemical language and information system. 1. Introduction to methodology and encoding rules. J Chem Inf Comput Sci. 1988;28(1):31–36. doi:10.1021/ci00057a005
- US Food and Drug Administration. FDA’s Global Substance Registration System. fda.gov. Accessed 25 October 2026.
- Hastings J, Owen G, Dekker A, et al. ChEBI in 2016: improved services and an expanding collection of metabolites. Nucleic Acids Res. 2016;44(D1):D1214–D1219. doi:10.1093/nar/gkv1031. PMID: 26467479
- Zhang T, Li H, Xi H, Stanton RV, Rotstein SH. HELM: a hierarchical notation language for complex biomolecule structure representation. J Chem Inf Model. 2012;52(10):2796–2806. doi:10.1021/ci3001925. PMID: 22947017
- United States Pharmacopeial Convention. 〈503〉 Acetic acid in peptides. In: United States Pharmacopeia–National Formulary. doi:10.31003/uspnf_m2427_01_01. Accessed 25 October 2026.
- CAS. CAS Common Chemistry. commonchemistry.cas.org. Accessed 25 October 2026.
- Therapeutic Goods (Poisons Standard—October 2026) Instrument 2026 (F2026L01327). Federal Register of Legislation. legislation.gov.au. Accessed 25 October 2026.
- Therapeutic Goods Administration. Understanding your responsibilities when importing, compounding and supplying unapproved peptide products. Safety advisory, 13 April 2026. tga.gov.au. Accessed 25 October 2026.
