Molecule guides
LL-37 Explained: The Human Cathelicidin's Structure, Mechanisms and Research Use
A research reference to LL-37 — how it is cut from hCAP18, why its amphipathic helix matters, the mechanisms reported in the literature, and what to check on a certificate of analysis.
Among human host-defence peptides, LL-37 is the single cathelicidin: a cationic, amphipathic alpha helix of 37 residues, liberated from the C-terminus of the hCAP18 precursor, and the most intensively studied antimicrobial peptide in innate immunity research. The name is descriptive — a pair of leucines followed by a 37-residue chain. Its mass of 4493.33 g/mol makes it much the largest entry in this catalogue's anti-inflammatory group, and its length, charge and helicity govern both the mechanisms attributed to it and the real difficulty of making and handling it.
What follows covers where the sequence comes from, the structural traits that drive its behaviour, the mechanisms described in the literature, the fields in which it appears, and the practical details of the material itself. LL-37 ships as a lyophilized powder.
From hCAP18 to a 37-residue helix: origin and structure
Every cathelicidin pairs a conserved N-terminal cathelin domain with a highly variable antimicrobial C-terminal region. Many mammals possess a whole set of them; humans have just one gene, CAMP, encoding the 18 kDa hCAP18 protein. That protein resides in the specific granules of neutrophils and is expressed as well by keratinocytes, by airway and intestinal epithelium and by various other cell types. Enzymatic cleavage — chiefly proteinase 3 in neutrophils, and proteases such as the kallikreins in skin — frees the terminal 37 residues as the mature cathelicidin peptide.
The result is an amphipathic helix of the textbook kind. In dilute aqueous buffer the chain stays largely unfolded; contact with an anionic surface induces a helix in which arginine and lysine occupy one face and hydrophobic residues the opposite one. That separation is the mechanism in miniature: one face supplies electrostatic attraction to negatively charged bacterial membranes, the other slips into the lipid interior.
Those same features make the molecule troublesome to manufacture and to work with. The synthesis is long at 37 residues, the hydrophobic stretch tends to aggregate on the resin, and the purified peptide sticks readily to plastic and glass. Both its price relative to short sequences and the care it needs once dissolved follow from this.
Proposed mechanisms of action
Two mechanistic narratives run side by side in the literature, and they are not rivals.
Direct membrane interaction
The traditional account has the peptide associating electrostatically with the anionic phospholipids and lipopolysaccharide of a bacterial envelope, then inserting its helix and compromising membrane integrity. Carpet-like surface disruption and toroidal pores have both been proposed, with individual studies favouring one geometry or the other according to lipid composition and peptide concentration. Since the action is physicochemical rather than aimed at a discrete target, reviews often suggest it should be less vulnerable to classical resistance than enzyme-targeting antibiotics — an argument repeated in the literature rather than an established fact. Mammalian membranes carry far less negative charge on the outer leaflet and are correspondingly less affected, which is the basis of the selectivity case.
Immunomodulation
A second literature treats LL-37 as a signalling molecule instead of a membrane-active one. Described activities include recruitment of neutrophils, monocytes and T cells via formyl peptide receptor-like 1, binding and neutralisation of lipopolysaccharide with damping of the inflammatory cascade that follows, altered toll-like receptor responses, and effects on keratinocyte migration and on vessel formation in wound models. Several authors hold that at concentrations realistically achieved in tissue this immunomodulatory role predominates, with direct membrane activity a secondary consideration.
A third point, and an important one, is context dependence. LL-37 forms complexes with host DNA and RNA capable of triggering interferon responses — work that ties the peptide to research on autoinflammatory skin disorders. The molecule cast as protective in one model appears as a contributor to pathology in another, which is a strong reason to interpret any LL-37 finding strictly within the model that produced it.
Areas of published research
Antimicrobial and antibiofilm assays
The largest block of laboratory work published consists of minimum inhibitory concentration testing against Gram-positive and Gram-negative species, membrane permeabilisation assays, and studies of biofilm prevention and dispersal. One methodological theme recurs throughout: results depend heavily on the salt and serum content of the assay, because divalent cations and serum proteins compete for the peptide's cationic face.
Wound and skin models
Cell-based and rodent studies have looked at re-epithelialisation, keratinocyte migration and vascular responses, together with changes in cathelicidin expression in skin disorders.
Innate immune signalling
This heading covers chemotaxis, LPS neutralisation, macrophage phenotype and interferon induction. It is also where LL-37 is most often set against other peptides studied for immune effects such as thymosin alpha-1; the two are regularly treated as interchangeable immune peptides even though their mechanisms have nothing in common, a contrast examined in LL-37 compared with thymosin alpha-1.
Formulation and delivery research
Surface adsorption and protease sensitivity have generated a sizeable applied literature on shortened analogues, D-amino-acid variants and delivery vehicles designed to preserve activity while using less material.
All of the above is preclinical laboratory research. LL-37 has no approved application, and what human data exist concern early-stage work on derived analogues rather than the native sequence offered for research.
Available forms and sizes
We hold LL-37 as a lyophilized powder in sealed 5 mg (€115) and 10 mg (€195) vials. Compared with short peptides the molar arithmetic is harsh: 4493.33 g/mol means a 5 mg vial supplies only about 1.1 micromoles, roughly a thirteenth of the moles in a 5 mg vial of a tripeptide. Experiments needing micromolar working concentrations in workable volumes get through material fast, which is precisely why the 10 mg size is offered. Comparable material appears under anti-inflammatory peptides and immune peptides, and purchasing considerations are set out in our LL-37 buying guide.
Reconstitution and storage in the laboratory
The maths is simple: 1 mL of diluent added to a 5 mg vial yields 5 mg/mL — that is, 5,000 mcg/mL, or about 1.11 mM. As published assay concentrations usually fall in the low micromolar range, most protocols involve dilutions of several hundred-fold.
Three handling considerations are peculiar to this peptide. The first is adsorption, which is measurable rather than theoretical: low-binding tubes and tips are standard, and glass is normally preferred to untreated polypropylene for stock solutions. The second is ionic strength, which alters apparent activity, so the diluent belongs in the written method instead of being left unstated. The third is aggregation, a real risk given the long hydrophobic stretch: run the diluent slowly down the vial wall, swirl rather than shake, and hold the solution up to the light to check for haze before using it. Keep lyophilized vials at −20 °C or below, away from light and moisture; aliquot reconstituted solution and keep it cold so no single tube goes through repeated warming and chilling. The peptide storage guide gives the detail.
Purity, the COA and how to read it
Every lot is purified by reversed-phase HPLC to 99% or better, with identity confirmed by mass and a lot-matched certificate. On a peptide this long the certificate carries more information than usual and repays close reading. Deletion sequences — chains short by one residue — are the signature impurity of lengthy syntheses, and a product missing a single alanine sits only 71 Da from the full-length peptide, so mass spectrum and chromatographic resolution both count. Look for one dominant, symmetrical peak instead of a broad envelope. Net peptide content also matters more here than with short sequences: for a strongly basic peptide supplied as a trifluoroacetate salt, counter-ion plus residual water can make up a sizeable fraction of the gross vial weight, which bears directly on any molar concentration you report. Our guide to reading a peptide COA walks through these checks on genuine documents.
Regulatory position
Although LL-37 occurs naturally in the human body, the synthetic material supplied here is not an approved medicine, not a food supplement and not a compounded product. We make no therapeutic claim for it, and none of the published antimicrobial or immunomodulatory findings should be taken as evidence of clinical benefit.
Related peptides and further reading
Readers working on immune-directed questions more generally should see the peptides for immune support research overview. Thymosin alpha-1 and KPV sit nearby in the same catalogue while working through quite different mechanisms, and the institutional research listings gather host-defence material under the LL-37 family.
Questions
What exactly is LL-37?
LL-37 is the mature host-defence peptide cleaved from the C-terminal end of hCAP18, the human precursor protein encoded by the CAMP gene. Its 37 residues give a mass of 4493.33 g/mol, and the name simply records that length together with the pair of leucines at the start. In humans it is the sole representative of the cathelicidin family.
Which tissues produce LL-37?
The hCAP18 precursor is held in the specific granules of neutrophils and is additionally expressed by keratinocytes and by epithelial cells lining the airways, the intestine and other mucosal surfaces. Proteases release the active 37-residue peptide from it — proteinase 3 inside neutrophils, kallikreins and related enzymes in skin.
How does LL-37 interact with bacterial membranes?
Meeting an anionic surface, the peptide adopts an amphipathic alpha helix with basic residues gathered on one side and hydrophobic residues on the other. Electrostatics draw the cationic face toward the negatively charged phospholipids and lipopolysaccharide of a bacterial envelope, while the hydrophobic face buries itself in the lipid core; depending on conditions this is described either as carpet-like surface disruption or as pore formation.
Should LL-37 be regarded as antimicrobial or as immunomodulatory?
Both descriptions are supported and they do not contradict one another. In vitro, direct membrane activity takes over at the higher concentrations, whereas chemotaxis, LPS neutralisation and toll-like receptor modulation are often argued to dominate at the concentrations actually found in tissue. Which of the two an experiment detects depends largely on the assay conditions chosen.
Why do results with LL-37 differ so widely between laboratories?
Its activity is unusually sensitive to salt and serum. Divalent cations and serum proteins compete for the cationic face of the helix, so the same nominal concentration behaves very differently in different buffers. The diluent and assay medium therefore belong in the methods section rather than being treated as trivia.
Why does LL-37 cost more than most research peptides?
A 37-residue chain is a demanding solid-phase synthesis, and the hydrophobic segment encourages aggregation on the resin, which cuts yield and makes purification harder. Molar arithmetic adds to the expense: at 4493.33 g/mol, a 5 mg vial holds roughly 1.1 micromoles — about one-thirteenth of the moles in a 5 mg vial of a tripeptide.
Are there handling issues unique to LL-37?
It adsorbs appreciably to surfaces, so low-binding tubes and tips are routine and glass is often chosen for stocks. Aggregation is a genuine hazard: add diluent slowly against the wall of the vial and swirl instead of shaking. Check solutions for cloudiness before use, and split the stock into aliquots so that one tube is not warmed and cooled over and over.
Does LL-37 have any approved use?
None. LL-37 occurs naturally in the human body, yet the synthetic peptide supplied here holds no marketing authorisation, is not a food supplement and is not a routinely available compounded preparation.