LL-37 is named for what it is: a 37-residue peptide that begins with two leucines. It is the only cathelicidin peptide humans produce, released from the C-terminus of a precursor protein called hCAP18, and the catalogue lists it in 5 mg and 10 mg vials under the synonyms cathelicidin antimicrobial peptide and hCAP-18(134-170).
It is also one of the most cationic peptides in a research catalogue, and that single property governs almost everything about handling it: what it sticks to, how it behaves in salt, and why recovery from a dilute solution can be lower than the arithmetic predicts. This guide covers the structure, the literature in its models, and the handling and certificate points that follow from the charge. The material is supplied strictly for laboratory research.
| LL-37 | Catalog specification |
|---|---|
| Vial strengths | 5 mg, 10 mg |
| Sequence | LLGDFFRKSKEKIGKEFKRIVQRIKDFLRNLVPRTES |
| Molecular formula | C205H340N60O53 |
| Molecular weight | 4493.33 g/mol |
| Amino acid count | 37 |
| CAS number | 154947-66-7 |
From the Restate Health catalog record. Lot-specific values are on the certificate of analysis.
From FALL-39 to LL-37
The gene was described before the peptide. Agerberth and colleagues reported in 1995 a putative human peptide antibiotic they called FALL-39, predicted from cloned cDNA, cysteine-free and expressed in bone marrow and testis (PNAS 1995). The following year Gudmundsson and colleagues isolated the peptide actually released in granulocytes and found it two residues shorter than the prediction, beginning at leucine-leucine rather than phenylalanine-alanine, hence LL-37 (Eur J Biochem 1996).
The precursor, hCAP18, is a cathelicidin: a protein with a conserved cathelin-like domain followed by a variable C-terminal antimicrobial region. Sørensen and colleagues showed in 2001 that neutrophils release hCAP18 and that the enzyme proteinase 3 cleaves it extracellularly to produce LL-37 (Blood 2001). The catalogue synonym hCAP-18(134-170) records where the 37 residues sit in the precursor.
The sequence, and what the charge count says
The catalogue record writes the sequence in one-letter code: LLGDFFRKSKEKIGKEFKRIVQRIKDFLRNLVPRTES. Counting along it gives six lysines and five arginines against three glutamates and two aspartates. At neutral pH that is eleven positive side chains against five negative ones, which leaves a net side-chain charge of +6. The formula, C205H340N60O53, and the molecular weight, 4493.33 g/mol, are the catalogue figures; PubChem carries the same formula at CID 16198951 with a monoisotopic mass of 4490.58.
| Residue class | Positions | Count | Why it matters |
|---|---|---|---|
| Basic | Lys 8, 10, 12, 15, 18, 25; Arg 7, 19, 23, 29, 34 | 11 | Positive charge; counterion sites; adsorption to negatively charged surfaces |
| Acidic | Asp 4, 26; Glu 11, 16, 36 | 5 | Partly offsets the positive charge |
| Hydrophobic (Leu, Ile, Val, Phe) | Leu 1, 2, 28, 31; Ile 13, 20, 24; Val 21, 32; Phe 5, 6, 17, 27 | 13 | The hydrophobic face of the helix; aggregation and surface binding |
| Cysteine | None | 0 | No disulfide; nothing to oxidise or mis-pair |
| Net side-chain charge | 11 basic less 5 acidic | +6 | Governs solubility, salt sensitivity and surface loss |
Counts are taken from the sequence as written in the catalogue record, numbering from the N-terminal leucine as 1. Net charge is the simple side-chain count at neutral pH and leaves out the terminal amine and carboxylate.
The absence of cysteine is worth noting because the 1995 paper made a point of it: most antimicrobial peptides known at the time were disulfide-stabilised, and FALL-39 was not. For a buyer it means no oxidation-dependent folding step and no disulfide dimer to look for on a certificate, which is one fewer thing to check.
An amphipathic helix, but only in the right environment
In water alone LL-37 is largely unstructured. In membrane-mimicking environments, detergent micelles or lipid vesicles, it folds into an alpha-helix in which the basic residues line up along one face and the hydrophobic residues along the other. Dürr, Sudheendra and Ramamoorthy's 2006 review assembles the biophysical evidence for that amphipathic helical structure (Biochim Biophys Acta 2006), and Wang determined a high-resolution structure of the peptide in lipid micelles in 2008 (J Biol Chem 2008).
Johansson and colleagues showed in 1998 that the helical content depends on the solution: it rises with ionic strength and with certain anions, and the peptide oligomerises as it becomes helical, with antibacterial activity in their assays tracking helicity (J Biol Chem 1998). That paper is the reason salt matters for this compound. The same peptide behaves differently in water, in buffered saline and in culture medium.
The practical reading is that LL-37 does not have one shape. It has a shape per environment, and a result obtained in one buffer may not transfer to another.
What the literature describes, and in which models
- Antibacterial activity in microbial culture assays, across Gram-negative and Gram-positive organisms, in the 1996 and 1998 papers already cited; the 1998 paper also examines its effect on eukaryotic cells in culture.
- Binding of lipopolysaccharide and signalling in immune cells, summarised in the 2006 review, which the catalogue about line describes as broader immunomodulatory signalling.
- Wound-healing models in culture, which the 2006 review lists among the settings in which the peptide has been examined.
All of this is model work, and the models are named in the papers. A certificate addresses none of it.
Why a +6 peptide sticks to everything
Glass surfaces carry a negative charge in water at neutral pH, from ionised silanol groups. Many plastics acquire a negative surface charge too, and polystyrene and untreated polypropylene also offer hydrophobic surfaces to which the helix's hydrophobic face can bind. A peptide carrying +6 and a hydrophobic face has two separate reasons to leave solution for the wall of its container.
Goebel-Stengel and colleagues measured this for several peptides in 2011 and found that recovery depended on the tube material and differed between peptides, with low-binding plasticware retaining more peptide in solution than standard tubes (Anal Biochem 2011). The loss is proportionally worst at low concentration, because the surface area is fixed and the amount of peptide is not.
- Prepare and store concentrated stocks rather than dilute ones, and dilute immediately before use.
- Use low-protein-binding tubes and pipette tips for dilute solutions.
- Expect a gap between the concentration calculated from the vial and the concentration actually present in a dilute working solution, and measure where the number matters.
Salt, aggregation and solvent choice
The 1998 finding that LL-37 folds and oligomerises as ionic strength rises cuts both ways for handling. In low-salt water the peptide stays monomeric and unstructured and dissolves readily. In saline or buffer it becomes helical and self-associates, which is the state the activity assays describe but also the state most prone to aggregation at high concentration and to adsorption.
The catalogue record for LL-37 carries no solubility or storage line of its own, unlike most records in the catalogue, so the certificate of analysis for the lot is the document to consult for the solvent the laboratory used. Where no specific instruction is given, the general approach in why some peptides resist dissolving applies: dissolve a strongly basic peptide first in water, at a concentration well above the working one, and add salt or buffer afterwards. Reconstitution solvents compared covers the choice of diluent.
Once in solution, divide the stock into single-use portions and freeze them. Each thaw of a cationic, aggregation-prone peptide is an opportunity for loss, and freeze-thaw cycles and aliquoting covers why one portion per use is the rule.
Certificate considerations for a cationic 37-mer
- Identity. The observed mass should reconcile with 4493.33 g/mol on the average scale or 4490.58 on the monoisotopic scale; the two differ by nearly 3 Da at this size, so the certificate must state which it reports. A 37-residue peptide is long enough that a fragment pattern adds real confidence over an intact mass alone.
- Purity by HPLC. A strongly basic, hydrophobic peptide tends to tail on reversed-phase columns and to show late-eluting aggregates. A clean main peak at a high percentage is meaningful; a broad one is a reason to ask about the method and the column.
- Counterion and net content. Eleven basic side chains plus the N-terminal amine give twelve sites for trifluoroacetate. Fully occupied, that is 1368 Da of trifluoroacetate on a 4493 Da peptide, so the TFA salt could be as little as 77% peptide by mass before water is counted. This is the compound for which net peptide content is most likely to differ sharply from the label, and the salt form should be asked for if it is not stated.
- Endotoxin. LL-37 binds lipopolysaccharide, which is to say endotoxin. A lot used in immune-cell work needs an endotoxin result for the same reason any reagent in such work does, and the peptide's own affinity for endotoxin makes contamination harder to separate from a result.
In short
A 37-residue, cysteine-free, +6 peptide that is unstructured in water and helical in membranes and salt. Store it concentrated, in low-binding containers, in single-use portions; expect counterions to take a larger share of the solid than usual; and read the certificate for a stated mass basis and a stated salt form. What it does in any model is in the papers below, not on the certificate.