Peptide Registry Records: When PubChem, CAS and a Listing Disagree

A peptide CAS number discrepancy is almost always two registry records describing two different chemical entities, or one entity entered twice, rather than two measurements of one molecule. On 2026-09-24 we re-opened the PubChem and FDA GSRS records for nine compounds in the Pepta Labs catalog that have such a disagreement: thymulin, FOXO4-DRI, 5-Amino-1MQ, Livagen, epitalon, SNAP-8, GHK-Cu, TB-500 and BPC-157. This page lists each record with its number, formula and mass, says what differs, and says which value the Pepta Labs listing uses.

Which registry records disagree for listed compounds?

The table below holds the registry disagreements checked for this page, one row per compound. The listing values are those on the product page on 2026-09-24; the registry values are those held by PubChem (CID) and the FDA Global Substance Registration System (UNII) on the same date, with CAS numbers as those two registries record them.

Why do registries disagree about a peptide?

Registries disagree about a peptide when one name is attached to more than one record. Six causes account for most rows in the table above; the epitalon and Livagen rows are differences in which CAS numbers a record or listing shows.

Is the record for the salt or the free peptide?

A synthetic peptide is isolated as a salt, usually the acetate or trifluoroacetate, while its CAS number, PubChem CID and molecular weight normally describe the free peptide. A salt can receive a record of its own, with its own number, and the salt's formula and mass include the counter-ion. The formula on a listing therefore describes the peptide, not the whole weighed solid; the TFA vs acetate guide explains the difference.

Is the record for the metal complex or the free peptide?

A peptide that binds a metal ion has one record for the complex and another for the metal-free peptide. The complex formula adds the metal atom and removes the hydrogen atoms it displaces: 1:1 GHK-Cu at 401.91 g/mol is 61.53 g/mol heavier than free GHK at 340.38, because one copper atom adds 63.55 and two displaced hydrogen atoms remove 2.02.

Is the record for a fragment or the full sequence?

A trade name or research code is sometimes attached both to a full-length peptide and to a short fragment of it. The TB-500 listing is the 43-residue N-acetylated peptide (4963.44 Da), while the fragment Ac-LKKTETQ (889.0 g/mol) matches residues 17 to 23 of that sequence; the two differ by 36 residues and about 4,074 g/mol. A figure of about 843 Da that some pages attach to the name TB-500 matches neither record: the fragment record is 889.0 g/mol, the un-acetylated heptapeptide LKKTETQ would be C36H66N10O13, 846.98 g/mol (calculated), and the full-length listing is 4963.44 Da.

Is the record for the salt or the ion of a non-peptide?

A small-molecule salt has one record for the salt as weighed and another for the charged ion alone. 5-Amino-1MQ is supplied as the iodide, 286.11 g/mol, while a mass spectrometer detects the 5-amino-1-methylquinolinium cation at m/z 159.09; the cation is about 55.6% of the salt weight.

Why is one peptide entered twice in PubChem?

PubChem builds its compound records from structures that depositors submit, so the same peptide can arrive twice under different names, CAS numbers or stereochemistry. Thymulin, FOXO4-DRI and SNAP-8 each have two PubChem records for what the listing names as one compound.

Is the figure an average or a monoisotopic mass?

Every formula has two masses: the average molecular weight, from standard atomic weights, and the monoisotopic mass, from the most abundant isotope of each element. The gap grows with size, from about 0.21 Da for epitalon (390.35 average, 390.14 monoisotopic) and 0.48 Da for thymulin (858.86 average, 858.38 monoisotopic) to about 3.2 Da for FOXO4-DRI (5358.15 average, 5354.98 monoisotopic), so a report and a listing can print different numbers for one molecule. The expected mass table lists both values and the protonated ions for every listed compound.

Which listed compounds carry two CAS numbers, and why?

Three listed compounds carry two CAS numbers for the entity or salt pair the listing names on 2026-09-24: thymulin, epitalon and 5-Amino-1MQ. In each case the reason is different, and the listing uses one of the two numbers. (PubChem also records CAS 300801-03-0 for the 2:1 GHK-Cu complex, CID 133697840, which is a different entity from the 1:1 complex on the listing.)

Thymulin: 78922-62-0 or 63958-90-7?

Thymulin is the nonapeptide pGlu-Ala-Lys-Ser-Gln-Gly-Gly-Ser-Asn-OH (pE-AKSQGGSN-OH), C33H54N12O15, 858.86 g/mol, and PubChem holds it twice. CID 71300623 carries CAS 78922-62-0 and no UNII; CID 3085284 carries CAS 63958-90-7, the INN nonathymulin and FDA UNII 9H198D04WL. Both records give the same formula, 858.9 g/mol and a monoisotopic mass of 858.38 Da. The GSRS record for UNII 9H198D04WL lists CAS 63958-90-7 and PubChem CID 3085284, and it also lists the name thymulin.

The two records differ in how the structure was drawn: CID 3085284 defines all seven stereocenters, while CID 71300623 defines six and leaves one undefined. The Pepta Labs listing uses CAS 78922-62-0 and CID 71300623. Both records describe the metal-free nonapeptide, which is what the listing supplies; a zinc(II) complex of the same nonapeptide would be a different entity with a different formula. Thymulin is also not thymosin alpha-1, a 28-residue peptide (CAS 62304-98-7, C129H215N33O55, 3108.26 Da) covered in the thymosin alpha-1 guide; the two share a name root only.

Epithalon or epitalon: one compound, two spellings

Epithalon and epitalon are two spellings of one compound name: the Pepta Labs listing uses the spelling Epithalon, and the FDA compounding page of April 22, 2026 writes the same name as Epitalon. Summing the four residues Ala, Glu, Asp and Gly and removing three waters gives C14H22N4O9 and 390.35 g/mol (calculated), the values the listing prints, so the two spellings cannot be told apart by formula or mass because they name one molecule.

Epitalon: 307297-39-8 or 64082-79-7?

Epitalon, also spelled epithalon, is the tetrapeptide H-Ala-Glu-Asp-Gly-OH (AEDG), C14H22N4O9, 390.35 g/mol. PubChem CID 219042 lists two CAS numbers for this one record, 307297-39-8 and 64082-79-7, and both pass the CAS check digit. The GSRS record for UNII O65P17785G links the same CID and lists only 307297-39-8. The Pepta Labs Epithalon listing shows 307297-39-8, the number both registries share. Its monoisotopic mass is 390.14 Da, so the expected [M+H]+ ion falls at m/z 391.15 (calculated).

5-Amino-1MQ: salt or cation?

5-Amino-1MQ is not a peptide: it is the small-molecule salt 5-amino-1-methylquinolinium iodide, C10H11IN2, 286.11 g/mol, CAS 42464-96-0, PubChem CID 66522933. The cation alone, C10H11N2+ at 159.21 g/mol, has its own CAS number (685079-15-6), CID (950107) and UNII (PMX593N4N3). GSRS assigns a separate UNII, K9G33W2TTZ, to the iodide salt. A UNII or CAS number quoted for 5-Amino-1MQ therefore belongs either to the salt or to the cation, and the Pepta Labs listing uses the iodide CAS number 42464-96-0.

Which listed compounds have more than one PubChem record?

Besides the salt and cation records of 5-Amino-1MQ and the fragment record returned for TB-500, four listed compounds have more than one PubChem record for the entity the listing names on 2026-09-24: thymulin (two, covered above), FOXO4-DRI (two), SNAP-8 (two) and GHK-Cu (four records for three copper entities).

FOXO4-DRI: CID 167312269 or CID 168431240?

FOXO4-DRI is a 46-residue D-retro-inverso peptide, H-LTLRKEPASEIAQSILEAYSQNGWANRRSGGKRPPPRRRQRRKKRG-OH with every chiral residue in the D-configuration, C228H388N86O64, 5358.15 g/mol, CAS 2460055-10-9. A PubChem name search for FOXO4-DRI returns two CIDs. Both give the same formula, a monoisotopic mass of 5354.98 Da and CAS 2460055-10-9, and neither has a UNII.

• CID 167312269 is titled FOXO4-DRI and carries the synonym "FOXO4-DRI TFA salt form", yet its formula is the free peptide, with no trifluoroacetate. It defines all 45 stereocenters and gives every chiral residue the D-configuration. This is the CID on the Pepta Labs listing.
• CID 168431240 is titled with the full three-letter sequence. The title writes residues 1 to 29 with the D- prefix, writes residues 32 to 45 without it, and marks the prolines at positions 35 and 36 as DL; the record leaves two stereocenters undefined.

Because the formula cannot show configuration, the two records have identical masses; they differ only in stereochemistry, which a mass spectrum does not distinguish. The listing's description, all chiral residues in the D-configuration, matches the fully defined record, CID 167312269. The salt label and the free-peptide formula are also a small discrepancy of their own: the gross weight of a TFA salt includes the counter-ion, which the 5358.15 figure excludes.

SNAP-8: C41H70N16O16S or C42H72N16O15S?

SNAP-8 (INCI name acetyl octapeptide-3) is the octapeptide Ac-Glu-Glu-Met-Gln-Arg-Arg-Ala-Asp-NH2, and PubChem holds two records that give it two formulas. CID 76283482, the record a search for CAS 868844-74-0 returns, gives C41H70N16O16S at 1075.2 g/mol. CID 71587832, the record GSRS links to UNII 8K14HJF88S, gives C42H72N16O15S at 1073.2 g/mol.

Summing the eight residues with the N-terminal acetyl group and the C-terminal amide gives C41H70N16O16S, the CID 76283482 formula. The systematic name on CID 71587832 draws the first residue as a 4-acetamidohex-5-enoic acid unit, with a C=CH2 where the peptide backbone has a C=O, which accounts for its one extra CH2 and one missing oxygen (a net 1.97 g/mol lighter). The Pepta Labs listing shows C42H72N16O15S and 1073.20 Da, the CID 71587832 values, while citing CID 76283482. The two formulas give different ions: [M+H]+ at m/z 1075.49 for C41H70N16O16S and 1073.52 for C42H72N16O15S (monoisotopic, calculated). The mass observed for the sampled production run is on the analytical report, which is the document to compare against.

GHK-Cu: four records, three copper entities

GHK-Cu has four PubChem records for three copper entities: the neutral 1:1 copper(II) complex C14H22CuN6O4 at 401.91 g/mol (CID 165429100 on the listing, and CID 10111685, the record GSRS links to UNII 6BJQ43T1I9 and CAS 89030-95-5), the cationic 1:1 complex at 402.92 g/mol (CID 71587328) and the 2:1 peptide:copper complex C28H48CuN12O8 at 744.3 g/mol (CID 133697840). The listing's CAS number and UNII sit on two different PubChem records: a PubChem search for CAS 89030-95-5 returns the cation record CID 71587328, which lists CAS 89030-95-5 and UNII 6BJQ43T1I9 as synonyms, while GSRS links the same UNII and CAS to the neutral record CID 10111685 (checked 2026-09-24). Copper-free GHK is CID 73587, C14H24N6O4, 340.38 g/mol. The listing uses the neutral 1:1 values; vial sizes are in the GHK-Cu buying guide.

What does it mean when a CAS number resolves to no record?

A CAS number that returns no record means one of three things: the number is mistyped, the registry searched does not hold that substance, or the number was never assigned. The CAS check digit tells the first case apart. The last digit of every CAS Registry Number is a checksum: multiply the other digits, read from right to left, by 1, 2, 3 and so on, add the products, and the last digit of the sum must equal the check digit.

The number 42088-73-1 appeared in a Pepta Labs catalog description written for 5-Amino-1MQ. It fails the checksum, so it cannot be a CAS Registry Number, and PubChem and GSRS return nothing for it; the listing's CAS field and the verified identity record give 42464-96-0. A number that fails the checksum is a transcription error, and the formula and mass on the listing are the values to match instead.

A valid number with no record in one registry is common for research compounds. GSRS holds a much narrower set of substances than PubChem: Livagen, FOXO4-DRI and thymulin under 78922-62-0 have no GSRS entry, while PubChem holds all three. The full CAS Registry is a subscription database, so a free search that returns nothing does not prove that a valid-looking number was never assigned. The Livagen listing shows no CAS number at all; PubChem CID 87919683 lists 433257-50-2 as a synonym, and the Livagen reference page gives its sequence, formula and expected mass.

How do you check which record a number belongs to?

1. Read the formula, not only the mass. Two records with the same name and different formulas describe different entities, or one of them is drawn wrongly.
2. Sum the formula from the sequence. For SNAP-8 the sum gives C41H70N16O16S, which identifies the record that matches the peptide.
3. Look for a counter-ion or metal in the formula. Iodide, trifluoroacetate or copper in a formula marks a salt or complex record.
4. Count the residues. A mass under 1,000 g/mol for a name the listing gives as a 43-residue peptide points to a fragment record.
5. Check the CAS check digit before searching, then search PubChem by CID or CAS number and GSRS by UNII or CAS number.
6. Compare an observed mass with a figure of the same kind: monoisotopic with monoisotopic, average with average.

The CAS number and molecular weight table gives the listing values for every compound in the catalog, and the sequence notation guide shows how to sum a formula from a sequence.

How does Pepta Labs decide which value a listing shows?

A Pepta Labs product page shows the CAS number, formula and molecular weight stated on the listing and, for a compound with a verified identity record, an identity section that cites the PubChem CID and, where one exists, the UNII. Where a registry records a different entity or formula, as for TB-500, GHK-Cu, 5-Amino-1MQ and SNAP-8, that section states the other value with its source. This page records those differences for the compounds listed above.

The analytical report is the document that ties a listing to a sampled run. Pepta Labs represents "Independent third-party HPLC purity analysis and mass-spectrometry identity confirmation, commissioned through the purchasing group on a sampled production run from the source. Pepta Labs does not test in-house." Purity is reported as RP-HPLC area-%, and identity is by mass spectrometry. "Vial labels identify the compound, quantity and research-use designation. They do not carry lot or batch numbers." Reports are "Sent by email on request, before you order if you want to see it first." The COA verification guide and the quality page explain how to read one, and the research peptides overview explains how listings are organized.

Frequently asked questions

Which mass should I compare a mass-spectrometry result against?

The one of the same kind as the report's figure, computed from the formula the listing states: the monoisotopic mass for a resolved high-resolution peak and the average mass for a low-resolution centroid, which differ by about 0.48 Da for thymulin at 858.86 g/mol and about 3.2 Da for FOXO4-DRI at 5358.15 g/mol.

Where can I look up a peptide's CAS number?

In PubChem, which lists CAS numbers as synonyms on each compound record, and in the FDA Global Substance Registration System, which lists them under a substance's codes; the full CAS Registry is a subscription database, and every Pepta Labs listing's CAS number is in the CAS number reference table.

Why does thymulin have two CAS numbers?

Because PubChem holds the nonapeptide C33H54N12O15 (858.86 g/mol) twice: CID 71300623 under CAS 78922-62-0, the number on the Pepta Labs listing, and CID 3085284 under CAS 63958-90-7 with the INN nonathymulin and FDA UNII 9H198D04WL; both records give the same formula and mass (checked 2026-09-24).

Is epithalon the same as epitalon?

Yes. Both spellings name the tetrapeptide H-Ala-Glu-Asp-Gly-OH (AEDG), 390.35 g/mol (formula C14H22N4O9, calculated), CAS 307297-39-8 on the Pepta Labs Epithalon listing; PubChem CID 219042 holds that number and 64082-79-7 for the same record, and the spelling changes neither the formula nor the mass.

Why does FOXO4-DRI have two PubChem records?

Because the 46-residue D-retro-inverso peptide C228H388N86O64 (5358.15 g/mol, CAS 2460055-10-9) was deposited twice: CID 167312269, the record on the Pepta Labs listing, defines all 45 stereocenters, while CID 168431240 writes residues 32 to 45 without the D- prefix and leaves two stereocenters undefined, so the records share a formula and mass and differ only in stereochemistry.

What does it mean when a CAS number resolves to no record?

It means the number is mistyped, is held by a registry other than the one searched, or was never assigned; the check digit settles the first case, as for 42088-73-1, which fails the CAS checksum and returns no PubChem or GSRS record, whereas a valid number such as Livagen's 433257-50-2 can appear in PubChem and not in GSRS.

Why can a listing CAS differ from the salt-form CAS?

Because a salt and its parent compound are separate registry entries with separate numbers: 5-Amino-1MQ iodide is CAS 42464-96-0 (UNII K9G33W2TTZ) while its cation is CAS 685079-15-6 (UNII PMX593N4N3), and a peptide's acetate or trifluoroacetate salt, where registered, has a number different from the free-peptide CAS a listing normally shows.

Sources

• PubChem compound records, re-opened 2026-09-24: CID 71300623, 3085284, 167312269, 168431240, 66522933, 950107, 87919683, 219042, 76283482, 71587832, 165429100, 10111685, 71587328, 133697840, 73587, 16132341, 62707662 and 9941957.
• FDA Global Substance Registration System, re-opened 2026-09-24: UNII 9H198D04WL, O65P17785G, K9G33W2TTZ, PMX593N4N3, 8K14HJF88S, 6BJQ43T1I9, 2D5MRE3SSY and 8ED8NXK95P; CAS searches for 42088-73-1, 78922-62-0, 2460055-10-9 and 433257-50-2 returned no record.
• CAS Registry Numbers as recorded in PubChem and GSRS.
• Calculated values: monoisotopic masses, ion m/z values, residue sums and CAS checksums on this page are calculated from each formula, sequence or number, with a proton mass of 1.007276.

All products are supplied for laboratory research use only and are not for human or animal use.