Expression‑Line Peptides Showdown: Pentapeptide‑18 VS Acetyl Hexapeptide‑38 VS Myristoyl Hexapeptide‑16

Expression-line peptides are compared on three documentable properties - identity, structure and specification - and never on which one performs better, because a supplier cannot substantiate that. The pentapeptide-18 vs acetyl hexapeptide-38 comparison, with Myristoyl Hexapeptide-16 beside them, is a question about cosmetic raw material documentation.

Three Expression-Line Peptides, Three Different Molecules

The three are not grades of one ingredient. Each has its own registry identity, its own molecular formula and its own declared sequence, and a purchase order that names one while the certificate names another describes two different materials.

Comparing pentapeptide-18 vs acetyl hexapeptide-38 vs myristoyl hexapeptide-16 on identity

Start with the four lines every specification should carry: registry number, molecular formula, molecular weight and sequence. Pentapeptide-18 and Acetyl Hexapeptide-38 each resolve to a single PubChem record from their CAS number, which is the cleanest way to confirm an identity before a batch is made. Myristoyl Hexapeptide-16 has no CAS number assigned in the record we checked, so the sequence and the fatty-acid chain have to carry the identification instead.

Identity table

INCI name	CAS number	Formula	Molecular weight	PubChem CID
Pentapeptide-18	64963-01-5	C29H39N5O7	569.6 g/mol	10099522
Acetyl Hexapeptide-38	1400634-44-7	C30H55N9O10	701.8 g/mol	155801571
Myristoyl Hexapeptide-16	Not assigned in the record reviewed	C47H91N9O8	910.3 g/mol	163285863

The molecular weights spread across roughly 570, 700 and 910 g/mol, and that spread is the first practical signal: the three peptides are not interchangeable at equal mass in a formula. Each is supplied as the free form named by its INCI entry, and each is synthetic rather than plant-derived, so none of the three carries the compositional variability of a botanical extract.

Where the Three Structures Diverge

The differences that matter are structural, and they can be read off the declared sequences. One is a five-residue peptide with a D-configured second residue, one is an acetylated and amidated six-residue peptide, and one is a six-residue peptide carrying a C14 fatty-acid chain on its N-terminus.

Structural features and what they change at the bench

A D-configured residue in the second position is a structural fact visible in the sequence, and it is the feature the trade literature most often links to stability in a finished formula. N-terminal acetylation and C-terminal amidation are likewise visible in the sequence. The myristoyl chain is the largest single difference in the group: it adds a lipophilic tail to a molecule that would otherwise be water-soluble, and lipophilicity is what governs partitioning into the skin's lipid phase, compatibility with an emulsion base, and the solvent needed to get the powder into solution in the first place.

Structure comparison table

Molecule	Structural feature	Declared sequence	What it changes in the formula
Pentapeptide-18	D-configured second residue	Tyr-D-Ala-Gly-Phe-Leu	Lowest molecular weight of the three; supplied for water-based leave-on formats
Acetyl Hexapeptide-38	N-acetylated, C-amidated	Ac-Ser-Val-Val-Val-Arg-Thr-NH2	neutralized chain ends; the arginine residue makes the pH window worth pinning down
Myristoyl Hexapeptide-16	C14 myristoyl chain on the N-terminus	Six residues with a tetradecanoyl group	Lipophilic; the only one of the three that is not simply dissolved in water

Read as a group, the three are alternative ways of building a topical ingredient rather than a ladder of strength. That is also why the market groups them under the loose heading of anti-wrinkle peptide, alongside Acetyl Hexapeptide-8 powder: the label describes the shelf they sit on, not a property this page is asserting for any of them.

Specification Lines Each Peptide Needs

A specification for these three is not a marketing document, and the lines that cause disputes are the same three every time: identity, sequence confirmation, and the physical behavior the formulator has to design around. Purity is the line buyers ask about first, but it is rarely the line that fails a batch.

Lines to pin down on acetyl hexapeptide-38 and the other two

Both acetyl hexapeptide-38 and pentapeptide-18 are supplied as lyophilized powders in two purity grades, and the certificate should state the assay method next to the figure. For myristoyl hexapeptide-16 the sequence line matters more than usual, because no CAS number is assigned in the record reviewed: the tetradecanoyl group has to be declared as part of the identity, not treated as an excipient. Solubility, pH window and recommended use level belong on the same sheet, because a formulator who discovers the solubility route after the batch is ordered has already lost a week.

Specification table

Specification line	What the three require	Who supplies the figure	Note
Identity	CAS where one exists, plus the INCI name on the label	Supplier	Myristoyl Hexapeptide-16 has no number assigned in the record reviewed
Sequence	Full amino-acid sequence, including D-residues and terminal modifications	Supplier	For Myristoyl Hexapeptide-16 the C14 chain is part of the identity
Assay	Purity figure with the method named beside it	Supplier	Two grades are standard in the group
Solubility route	Water-based for two, co-solvent or warm aqueous base for the lipophilic one	Supplier	The single most useful line for a formulation team
pH window	Declared range plus the optimum inside it	Supplier	Should be fixed in writing before the formula is frozen
Storage and packaging	Refrigerated short term, colder for long term, light and moisture protection	Supplier	Affects re-order quantity as much as quality

One line deliberately missing from that table is any performance figure. That is not a gap in the specification; it is the boundary of what a raw material supplier can document, and the next section explains why.

Claims and Who Has to Substantiate Them

This is where the pentapeptide-18 vs acetyl hexapeptide-38 comparison stops being a technical question and becomes a legal one, and it is the section most supplier pages leave out. Under European cosmetics law, the supplier does not carry the burden of the claim - the brand does.

What Regulation (EU) No 655/2013 requires

Commission Regulation (EU) No 655/2013 sets six common criteria for justifying cosmetic claims: legal compliance, truthfulness, evidential support, honesty, fairness and informed decision-making. Criterion 2(2) states that an ingredient claim must not imply the finished product has the same properties when it does not. Criterion 3(6) requires that a claim extrapolating ingredient properties to the finished product be supported by adequate and verifiable evidence, such as demonstrating the presence of the ingredient at an effective concentration. Under criterion 4(1), presentations of performance must not go beyond the available evidence.

Where the responsibility sits

Article 2 of the same Regulation places the duty on the responsible person: the claim wording must comply with the criteria and must be consistent with the documentation proving the claimed effect in the product information file referred to in Article 11 of Regulation (EC) No 1223/2009. In practice that means a brand cannot rely on a number printed on a supplier's page; it has to hold its own evidence, relevant to its own finished product, with a method that is valid, reliable and reproducible. Criterion 1(2) fixes the yardstick as the reasonably well-informed, observant and circumspect average end user, taking account of social, cultural and linguistic factors in the market concerned.

Claims and evidence table

Claim a brand may want	Can a supplier page carry it	Relevant criterion	What the responsible person has to hold
A wrinkle-depth reduction figure	No	Annex 3(1) and 3(6)	Studies run on the finished formula, with the ingredient shown present at an effective concentration
A statement that the product is authorized or approved	No	Annex 1(1)	Nothing; the criterion prohibits such a claim outright
A property held by every comparable peptide	No	Annex 4(2)	Evidence that the characteristic is specific to the product, or drop the wording
A comparison that disparages another ingredient	No	Annex 5(1)	Nothing; objective claims only, and legally used ingredients must not be denigrated
Purity, sequence and solubility data	Yes	No claim criterion engaged	Keep the batch certificate on file

The practical consequence is narrow and useful: a supplier can document identity, purity, sequence, solubility, pH and storage, and the brand owns everything that touches the human result. A page that blurs that line is not being generous with information; it is handing the buyer a liability it cannot support.

What a CosIng Entry Does and Does Not Mean

A CosIng entry is a naming convention, not a permission, and the European Commission says so itself. Buyers who read the presence of an INCI name as a green light are reading the database backwards, and that misreading is common enough in expression-line peptides sourcing to be worth quoting in full.

The Commission's own wording

The Commission's cosmetic ingredient database page states that an ingredient assigned an INCI name appearing in the CosIng inventory does not mean it is to be used in cosmetic products nor approved for such use, that CosIng is a non-legally binding information database, and that only Regulation (EC) No 1223/2009 and its Annexes establish whether, and under what conditions, substances may be used in cosmetic products. The same notice adds a second point that matters for peptides: CosIng may also list ingredients known to be used in medicinal products, and where a product restores, corrects or modifies physiological functions by exerting a pharmacological, immunological or metabolic action, that product is to be qualified as a medicinal product.

What follows for a specification

Three consequences follow for anyone buying the peptides in this comparison. First, a CosIng listing tells you which name goes on the label; it does not tell you the material is acceptable in your market. Second, restricted categories behave differently - colorants, preservatives and UV filters are only listed in CosIng when they are authorized in Annexes IV, V and VI respectively, so a listing in one of those categories carries a different weight from a listing in the general inventory. Third, the cosmetic or medicinal qualification is decided case by case by national competent authorities and courts, so the boundary is a product-level question that no raw material page can settle.

For the two peptides with assigned CAS numbers - CAS 64963-01-5 for Pentapeptide-18 and CAS 1400634-44-7 for Acetyl Hexapeptide-38 - the identification chain is straightforward and should be quoted on the certificate. For Myristoyl Hexapeptide-16, where the record reviewed carries no CAS number, the sequence declaration does that work instead.

Which of these three expression-line peptides should I choose: Pentapeptide-18, Acetyl Hexapeptide-38, or Myristoyl Hexapeptide-16?

How a Buyer Should Compare the Three

Comparing pentapeptide-18 vs acetyl hexapeptide-38 on paper usually resolves faster than expected, because the decision is driven by the base you already have rather than by the molecule you prefer. Start from the format, then let the specification follow. The order matters for expression-line peptides, because the three differ in solubility rather than in strength. The same specification logic applies to the other peptides on the same shelf, including Hexapeptide-9 powder.

A short decision sequence

Work through four questions. What is the base - an aqueous serum, an emulsion, or an anhydrous system? That answer eliminates one or two of the three, because the myristoyl chain changes the available solvent route. What is the pH already fixed for the rest of the formula? The declared window either fits or does not. What documentation does your regulatory team need to file? That decides whether you can accept a material with no CAS number in the identity line. What can you write on the pack? If your claim set depends on a performance figure, that figure must come from your own studies on your own formula, so the peptide decision belongs before that study is designed.

Where uncertainty should stop the order

Two situations justify pausing rather than ordering. The first is a supplier page that presents performance numbers with no study reference: you can neither use them nor verify them. The second is an identity line that names a peptide the certificate cannot confirm, by sequence or by registry number. Both are cheap to check before a purchase order and expensive after a stability batch is committed.

Next Step

If the specification lines above match what your formulation team needs, the next move is a sample and the paperwork that goes with it: certificate of analysis, sequence confirmation, solubility route and pH window in writing. Our specification sheets for the expression-line peptides in this comparison follow the structure in the table above, with the assay method named beside every purity figure and no performance figures asserted in our own voice.

Tell us the base, the target pH and the documentation your regulatory team requires, and we will send the specification sheet and a sample quote that matches it.

Frequently Asked Questions

Q: What Is The Difference Between Pentapeptide-18, Acetyl Hexapeptide-38 And Myristoyl Hexapeptide-16?

A: They are three distinct molecules, not three grades of one material. Pentapeptide-18 is a five-residue peptide with a D-configured second residue and a molecular weight of 569.6 g/mol. Acetyl Hexapeptide-38 is an acetylated, amidated six-residue peptide at 701.8 g/mol. Myristoyl Hexapeptide-16 is a six-residue peptide carrying a C14 myristoyl chain, the heaviest of the three at 910.3 g/mol.

Q: Is Pentapeptide-18 Vs Acetyl Hexapeptide-38 A Meaningful Way To Compare Them?

A: Yes, but only on identity and specification, not on outcome. The pentapeptide-18 vs acetyl hexapeptide-38 comparison works for registry numbers, sequences, molecular weights, solubility routes and pH windows, all documentable facts. It stops working once it becomes a ranking of results: no raw material supplier holds the evidence to rank finished formulas.

Q: Do These Peptides Have CAS Numbers?

A: Two of the three do. Pentapeptide-18 is registered as CAS 64963-01-5 and Acetyl Hexapeptide-38 as CAS 1400634-44-7, and each number resolves to a single PubChem record. Myristoyl Hexapeptide-16 carries no CAS number in the record reviewed, so its amino-acid sequence and its tetradecanoyl group have to serve as the identity line.

Q: Does A CosIng Listing Mean A Peptide Is Approved In The European Union?

A: No. The Commission states directly that an ingredient assigned an INCI name in the CosIng inventory does not mean it is to be used in cosmetic products nor approved for such use, and that CosIng is a non-legally binding database. Only Regulation (EC) No 1223/2009 and its Annexes establish whether, and under what conditions, a substance may be used.

Q: Can A Supplier Page State Wrinkle-Reduction Figures?

A: It should not. Under Regulation (EU) No 655/2013, evidence must be relevant to the finished product and must show the ingredient present at an effective concentration, and Article 2 places that documentation in the brand's product information file. A supplier can document identity, purity, sequence, solubility, pH and storage; anything describing the human result belongs to the brand.

Q: What Should I Ask For Before Ordering?

A: Ask for four things in writing: the certificate of analysis with the assay method named, the full sequence including any terminal modification, the solubility route, and the pH window with its optimum. Add the storage conditions and re-order implications if the material is going into a long stability program. If a supplier page cannot answer those four, the specification is not finished.

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