LL-37 and Thymosin Alpha-1 are both immune-related peptides, but they act at opposite ends of the immune system: LL-37 is a cationic antimicrobial host-defence peptide that associates directly with microbial membranes, while Thymosin Alpha-1 is an acidic thymic peptide studied for its effects on T-cell maturation and Toll-like receptor signalling. One is effector chemistry; the other is regulatory signalling. In an LL-37 vs Thymosin Alpha-1 comparison, the discriminating question is whether the assay measures direct antimicrobial or membrane activity, or the modulation of an immune response.
Both are supplied as lyophilized powder with lot-matched HPLC certificates — LL-37 in 5 and 10 mg vials, Thymosin Alpha-1 in 5, 10 and 20 mg vials. LL-37 is listed under anti-inflammatory peptides and Thymosin Alpha-1 under immune peptides. Both are research chemicals for in-vitro and preclinical laboratory work only, not for human or veterinary use.
LL-37 vs Thymosin Alpha-1 at a glance
| Attribute | LL-37 (Cathelicidin) | Thymosin Alpha-1 |
|---|---|---|
| Origin | C-terminus of human hCAP18, released by proteinase 3 cleavage | First 28 residues of prothymosin alpha; isolated from calf thymus fraction 5 |
| Family | Cathelicidin — the only human member | Thymic peptides |
| Length | 37 residues | 28 residues, N-terminally acetylated |
| Net charge character | Strongly cationic | Strongly acidic — rich in aspartate and glutamate |
| Secondary structure | Amphipathic alpha helix in membrane-mimetic conditions | Largely unstructured in solution |
| CAS number | 154947-66-7 | 62304-98-7 |
| Molecular formula | Not stated on our specification sheet | C129H215N33O55 |
| Molecular weight | 4493.33 g/mol | 3108.30 g/mol |
| Reported mechanism | Electrostatic association with anionic microbial membranes and disruption of membrane integrity; also LPS binding and receptor-mediated immune signalling | Immunomodulation via Toll-like receptor signalling and T-cell maturation pathways |
| Human evidence | Research-stage; no approved product | A synthetic version is approved outside the United States for hepatitis B and C and as a vaccine adjuvant |
| Handling difficulty | High — adsorption, aggregation, salt and serum sensitivity | Low — well-behaved, water-soluble |
| Research sizes stocked | 5 mg, 10 mg | 5, 10, 20 mg |
| Purity | ≥99% HPLC, MS identity, lot-matched COA | ≥99% HPLC, MS identity, lot-matched COA |
Effector peptide versus regulatory peptide
LL-37 is an effector molecule. Neutrophils, epithelial cells and keratinocytes express the precursor hCAP18, and proteinase 3 cleavage releases the mature 37-residue peptide at sites of infection or injury. Its defining property is geometry: a cationic, amphipathic helix that partitions into anionic bacterial membranes, with disruption of membrane integrity as the mechanism most often described. That is chemistry acting directly on a target, not signalling.
LL-37 also has a second, less tidy identity in the literature. It binds lipopolysaccharide, modulates chemotaxis, and has been reported to influence receptor-mediated immune signalling — which places it partly in the regulatory category too. This dual character is why the peptide is genuinely difficult to work with cleanly, and why its assays require more controls than most.
Thymosin Alpha-1 is purely regulatory. It carries no antimicrobial activity of its own; the published mechanism concerns immune cell behaviour — maturation and differentiation of T cells, and modulation of Toll-like receptor pathways, particularly TLR2 and TLR9 in the reported work. Its acidic, largely unstructured character is the opposite of LL-37's charged helix in every respect that matters at the bench.
Depth and type of evidence
These two compounds sit at different points on the evidence ladder, and it is worth being precise about it.
LL-37's record is laboratory-based: antimicrobial susceptibility and minimum inhibitory concentration assays across bacterial species, biofilm disruption work, wound and epithelial repair models, and cell studies on chemotaxis and LPS neutralisation. There is no approved LL-37 product and the human data are limited. It remains a research molecule in the strict sense.
Thymosin Alpha-1 is unusual in this catalogue for having a genuine clinical history. A synthetic version has been approved in a number of countries outside the United States for hepatitis B and hepatitis C and as a vaccine adjuvant, and it has appeared on the World Health Organization list of essential medicines in some editions. That means the published record includes controlled human trials alongside the usual cell and rodent work. Two qualifications apply: those trials used pharmaceutical-grade product administered under medical supervision, and they say nothing about research-grade powder purchased as a reagent. The material supplied here is not that regulated article and is not offered for any human purpose. Our Thymosin Alpha-1 primer and our LL-37 background piece cover each record in more detail.
Which to choose for which research question
Choose LL-37 for direct antimicrobial and membrane work
If the endpoint is bacterial or fungal growth inhibition, membrane permeabilisation, biofilm behaviour or LPS neutralisation, LL-37 is the only one of the two that participates at all — Thymosin Alpha-1 has no such activity and would function as a negative control rather than a comparator. LL-37 is also the appropriate probe when the question concerns host-defence peptide chemistry generally, since it is the sole human cathelicidin.
Choose Thymosin Alpha-1 for immune-modulation readouts
If the assay measures T-cell maturation markers, cytokine profiles under a defined stimulus, Toll-like receptor pathway activation or vaccine-adjuvant behaviour in an animal model, Thymosin Alpha-1 maps directly onto the published work. It is also the more practical choice for longer experiments simply because it is far easier to keep in solution reproducibly. Groups working on the thymic-peptide family more broadly often run it alongside thymalin, a thymic extract-derived preparation, as a family comparator.
Run both when host defence is being decomposed
An infection model can improve because the pathogen was directly affected or because the host response was modulated. Because LL-37 and Thymosin Alpha-1 occupy those two explanations almost cleanly, running them as separate arms is a direct way to separate them — with the caveat that LL-37 is not purely an effector and its immunomodulatory arm needs its own controls. Both are stocked in matching 5 and 10 mg sizes, which makes molar-matched parallel preparation straightforward; Thymosin Alpha-1 is additionally available as a prepared nasal solution for format-comparison work.
Handling and storage: the largest practical difference
Thymosin Alpha-1 is an easy peptide. It is acidic, water-soluble and largely unstructured, it does not aggregate readily, and a conventional protocol works: hold the sealed lyophilized vial frozen, bring it to room temperature before piercing the stopper so moisture does not condense on cold powder, run diluent down the vial wall, let it dissolve without shaking, and aliquot for single use.
LL-37 is one of the more demanding peptides in the catalogue and deserves its own protocol. Three properties drive that. First, its strong positive charge makes it adsorb to glass and to standard polypropylene, so a dilute working solution can lose a large fraction of its material to the tube wall before it ever reaches the assay — low-binding plasticware and carrier protein where the assay permits are standard countermeasures. Second, its antimicrobial activity is markedly sensitive to ionic strength and to serum: activity measured in low-salt buffer will not reproduce in physiological salt or in serum-containing medium, and results reported under one condition should never be assumed to transfer to another. Third, at higher concentrations the same amphipathic helix that disrupts bacterial membranes can affect mammalian membranes, so cytotoxicity and haemolysis controls belong in the design rather than as an afterthought.
Concentration is a laboratory calculation, not a recommendation for use: a 5 mg vial reconstituted with 2 mL of diluent gives 2.5 mg/mL, or 2,500 mcg/mL, so 0.1 mL contains 250 mcg. Because the two peptides differ in mass (4493.33 versus 3108.30 g/mol), equal mass concentrations are not equal molarities, and comparative arms should be prepared in molar terms. See our reconstitution guide and storage guide.
Purity, identity and COA checks
Request the lot-matched certificate for each vial and confirm HPLC purity with a visible chromatogram, a mass-spectrometric result matching the expected weight — 4493.33 g/mol for LL-37 and 3108.30 g/mol for Thymosin Alpha-1 — and a lot number matching the vial label. Each has a specific concern. LL-37 is long, highly charged and difficult to synthesise, which is reflected in its price; deletion sequences are the impurity class to look for, and because it binds LPS directly, endotoxin content is a legitimate question for any immune-signalling assay where an LPS contaminant would confound the readout. Thymosin Alpha-1's defining chemical feature is its N-terminal acetyl group; a des-acetyl impurity differs by 42 Da and is a different molecule for the purposes of any published comparison, so acetylation should be confirmed rather than assumed. Our COA guide covers what a complete certificate contains.
Regulatory framing
Both peptides are supplied as research chemicals for laboratory use only. LL-37 has no approved product anywhere. Thymosin Alpha-1 is approved as a prescription medicine in some countries outside the United States, but the approval attaches to a regulated pharmaceutical article, not to research-grade powder, and nothing here is offered as a medicine or for human or veterinary use. For broader context see our immune research overview.