LL-37

What LL-37 is, structurally

LL-37 is a peptide of thirty-seven residues, its name encoding that length along with its first two residues.

No molecular formula appears in the product database, but one can be supplied with unusual confidence. The deposited record for this CAS gives C205H340N60O53, which computes to 4493.34 against the product's stated 4493.34, an exact match. Two independent statements agreeing to the second decimal is strong corroboration, and the sections below build on that formula with its external origin noted.

Read that way, the composition is emphatic. Of sixty nitrogens, thirty-seven are backbone amides, leaving twenty-three on side chains; of fifty-three oxygens, thirty-eight belong to the backbone carbonyls and C-terminal acid, leaving fifteen. Twenty-three side-chain nitrogens against fifteen oxygens across thirty-seven residues is markedly nitrogen-rich. This is a strongly cationic peptide, carrying substantial net positive charge at neutral pH and a high isoelectric point.

That property dominates the handling of this material more than any other fact about it.

There is no sulfur anywhere in the formula, worth noting on a peptide of this length. No cysteine means no disulfide to protect or scramble; no methionine means no sulfoxide route. That closes off most of the oxidative chemistry that would otherwise dominate storage.

Reconstitution and handling

Sterile water is the default solvent, and a strongly cationic peptide should dissolve readily. Where a vial resists, mildly acidic conditions are the right escalation, since lowering pH increases net positive charge and solubility.

Add diluent slowly down the inside wall of the vial, swirl gently, and let the solution clarify. Do not vortex; it introduces foam without dissolving anything faster.

Adsorptive loss is the principal practical hazard here. A peptide carrying this much positive charge binds avidly to negatively charged surfaces, and at low working concentrations the loss can be a large fraction rather than a trace. Borosilicate glass is negatively charged and is the wrong choice; low-protein-binding polypropylene is the sensible default, and minimising transfers helps more than changing tube type.

Filtration deserves specific caution for the same reason. Many membranes carry negative surface charge and can retain a cationic peptide rather than passing it, so a sterilising filtration step may remove a large share of the material. Where filtration is unavoidable, a low-binding membrane and a discarded first fraction are the usual mitigations.

No reducing agent is needed, since the formula contains no sulfur.

This is laboratory preparation chemistry, not dosing, administration, or protocol guidance of any kind.

Storage and stability

Store the lyophilized powder at -20 degrees C, sealed, desiccated and protected from light, and let a cold vial warm before opening. Reconstituted, hold at 2 to 8 degrees C, or aliquot and freeze rather than thawing repeatedly.

With no sulfur present, the oxidation routes that dominate many peptides of this size are closed, and air exposure matters correspondingly less than it would for a cysteine or methionine-containing peptide.

The hydrolytic routes remain. Thirty-seven residues will contain asparagine or glutamine positions capable of deamidation, each adding about one mass unit, and aspartate positions capable of isomerisation. On a molecule of nearly four and a half kilodaltons a one-unit change is a small proportional shift needing adequate resolution to see. Both require water, so the dry solid is well protected while solutions are not, and warmth and alkaline pH accelerate them.

The stated sequence names them: two aspartates, an asparagine and a glutamine, so both deamidation and the succinimide route have real sites here.

These are storage conditions for the material, not dosing or administration guidance.

How LL-37 is tested

Reversed-phase HPLC establishes chromatographic purity as area percent. A strongly cationic peptide retains poorly on a standard C18 column, so an ion-pairing mobile phase is effectively required rather than merely helpful, trifluoroacetic acid being conventional.

Area percent is a chromatographic measure and not peptide content by weight. Solid-phase synthesis delivers a salt, and on a peptide this cationic the counter-ion burden is substantial, since every basic side chain carries one.

Mass spectrometry confirms identity against 4493.34. At this size the average and monoisotopic figures differ appreciably, so the reporting basis should be established before comparing a result against the label. Deamidation adds about one unit, which at 4493 needs reasonable resolution to distinguish from the isotope envelope.

Detection should be assumed near 214 nm; the product database does not state whether aromatic residues are present.

These describe general methodology, not a claim about any particular batch.

Handling FAQ

Why does so little peptide seem to survive dilution? Most likely adsorption. A strongly cationic peptide binds to negatively charged surfaces, and at low concentration the loss can be a substantial fraction of the material. Low-protein-binding polypropylene rather than glass, and as few transfers as possible, are the practical mitigations.

Will sterile filtration pass this peptide through? Not necessarily in full. Many membranes carry negative surface charge and retain cationic peptides, so a filtration step can remove a large share of the material. A low-binding membrane, and discarding the first fraction through the filter, are the conventional approaches.

Full specifications for LL-37.