What mass should an LL-37 certificate show?

PubChem gives LL-37 an average molecular weight of 4493 g/mol and a monoisotopic mass of 4490.58 Da. The raw spectrum will show many charge states because the peptide has eleven lysines and arginines, so the report should present a deconvoluted neutral mass. A result clearly lower than this suggests a shorter fragment such as FF-33 rather than LL-37.

Why is net peptide content especially important for LL-37?

LL-37 has six lysines, five arginines and a free N-terminus, and trifluoroacetate binds to positively charged groups like these during synthesis and purification. The bound counter-ion adds weight without adding peptide. A purity figure ignores it, so only a net peptide content or amount result tells you how much LL-37 the powder really contains. The lesson on net peptide content and counterions covers the general case.

Is ropocamptide the same as LL-37?

Yes. PubChem lists ropocamptide as the international nonproprietary name on the same record as LL-37, CID 16198951, with the same 37-residue sequence. A product labelled with either name should match the same formula, C205H340N60O53, and the same mass on a certificate, 4493 g/mol average.

How would a test spot FF-33 or another fragment sold as LL-37?

By mass. UniProt lists several overlapping peptides from the same cathelicidin precursor, including FALL-39, FF-33, RK-31, KS-30 and KR-20. Each is a different length and so has a different mass. An LC-MS result that matches the 37-residue mass rules these out, while a mixture appears as several distinct masses on the same run.

Why might two laboratories report different HPLC profiles for the same LL-37?

LL-37 changes shape with pH, concentration and the anions around it, becoming helical and self-associating under some conditions and disordered under others. Different mobile phases, columns and sample diluents can therefore change peak shape and retention. That does not alter the identity result, but it means purity figures are only comparable when the methods are described.

One of the 30 compound testing profiles

LL-37

LL-37 is a 37-amino-acid peptide (C205H340N60O53, average mass 4493 g/mol); a test should confirm that mass, rule out the shorter cathelicidin fragments that share its sequence, and measure net peptide content, because its eleven lysines and arginines carry a heavy counter-ion load.

LL-37 is the 37-residue peptide cleaved from the human cathelicidin antimicrobial peptide precursor, where it occupies residues 134 to 170 (UniProt P49913), and its international nonproprietary name is ropocamptide (PubChem).

Test your LL-37

Identity card

Formula: C205H340N60O53
Average mass: 4,493 g/mol
Monoisotopic mass: 4,490.58 Da
Length: 37 residues
Modifications: Free N-terminus; Free C-terminal acid
Also sold as: Ropocamptide, Cathelicidin LL-37, CAP18, hCAP18 fragment
Sequence: LLGDFFRKSKEKIGKEFKRIVQRIKDFLRNLVPRTES
Note: PubChem states the average molecular weight as 4493 g/mol without decimals and a monoisotopic mass of 4490.58 Da. In UniProt, LL-37 corresponds to residues 134 to 170 of the cathelicidin precursor P49913.

Source: LL-37 compound record (CID 16198951), Cathelicidin antimicrobial peptide (P49913).

Check a certificate's mass against these values with the mass checker, prefilled for LL-37.

What usually goes wrong with LL-37. Documented failure modes, with sources.

Self-association that changes with concentration

By circular dichroism, LL-37 was disordered at micromolar concentration in water and underwent a cooperative transition to a helical structure as concentration rose, consistent with oligomer formation (Johansson et al., 1998). The same sample can therefore behave differently at different concentrations and in different solvents, and size-based results or HPLC peak shapes may reflect self-association as well as purity. A report should state how the sample was dissolved and diluted.

Trifluoroacetate that alters both weight and structure

LL-37 carries six lysines and five arginines plus a free N-terminus, the kinds of positively charged groups that bind trifluoroacetate during synthesis and purification (Erckes et al., 2025). That counter-ion adds weight that is not peptide, and it also changes LL-37's shape: 15 mM trifluoroacetate, like bicarbonate or sulfate, drove the peptide into a helical structure (Johansson et al., 1998). Two vials with different salt forms can differ in peptide content and in solution behaviour.

Shorter cathelicidin fragments with the same core

LL-37 is one of several related peptides from the same precursor. UniProt lists FALL-39 (two residues longer at the N-terminus), LL-29, LL-23, FF-33, RK-31, KS-30 and KR-20, all overlapping the LL-37 sequence (UniProt P49913). FF-33 and SK-29, with 4 and 8 residues removed from the N-terminus, needed higher anion concentrations to become helical (Johansson et al., 1998). Each fragment has its own mass, so intact mass measurement catches a vial that holds one of them, or a mixture.

Deletion sequences from a 37-step synthesis

Deletion sequences from inefficient Fmoc removal and insertion sequences from excess reagent are core impurity classes in synthetic peptide medicines (D'Hondt et al., 2014), and a 37-residue chain has 37 couplings where they can arise. LL-37's six lysines (positions 8, 10, 12, 15, 18 and 25) give six places where a des-Lys chain could form, each one lysine residue lighter than LL-37 and named only by mass data.

Which tests matter most

LC-MSConfirms the 37-residue mass and separates LL-37 from FALL-39, FF-33, KR-20 and single-residue deletions.
Counter-ion contentEleven basic residues plus the N-terminus bind trifluoroacetate or acetate, which adds weight to the powder that is not peptide.
Amino acid analysisMeasures peptide content independently of salt form and checks the lysine and arginine counts expected for LL-37.
HPLC-UVGives purity, though peak shape for this self-associating peptide depends on method conditions that the report should state.

Reading LL-37 results

With eleven basic residues, LL-37 picks up many protons in electrospray, so the raw spectrum shows a long series of charge states at modest m/z values. A certificate should give the deconvoluted neutral mass and say whether it compares against the average (4493 g/mol) or monoisotopic (4490.58 Da) figure (PubChem), which for LL-37 are more than 2 mass units apart.

LL-37's conformation also depends on pH: helical content falls below pH 5, the peptide is entirely disordered at pH 2, and the helix is retained above pH 13 (Johansson et al., 1998). Reversed-phase HPLC commonly uses trifluoroacetic acid as an ion-pairing reagent (Erckes et al., 2025), one of the anions that pushes LL-37 toward a helix. That helps explain why different solvents and conditions can give different peak shapes or retention times for the same sample.

A label may use a precursor-family name: PubChem lists CAP18 and LL-37/hCAP18 among LL-37's synonyms (PubChem). The full precursor in UniProt is 170 residues long, with a signal peptide and a cathelin-like domain (UniProt P49913), so its mass makes any confusion with the 37-residue peptide obvious.

Public Tested.ph results

No public LL-37 records yet

Each public LL-37 record will appear here with its identity, HPLC purity and measured amount against the label, once the first vial is tested.

FIG 13Amount against the label

Questions about testing LL-37

What mass should an LL-37 certificate show?: PubChem gives LL-37 an average molecular weight of 4493 g/mol and a monoisotopic mass of 4490.58 Da. The raw spectrum will show many charge states because the peptide has eleven lysines and arginines, so the report should present a deconvoluted neutral mass. A result clearly lower than this suggests a shorter fragment such as FF-33 rather than LL-37.
Why is net peptide content especially important for LL-37?: LL-37 has six lysines, five arginines and a free N-terminus, and trifluoroacetate binds to positively charged groups like these during synthesis and purification. The bound counter-ion adds weight without adding peptide. A purity figure ignores it, so only a net peptide content or amount result tells you how much LL-37 the powder really contains. The lesson on net peptide content and counterions covers the general case.
Is ropocamptide the same as LL-37?: Yes. PubChem lists ropocamptide as the international nonproprietary name on the same record as LL-37, CID 16198951, with the same 37-residue sequence. A product labelled with either name should match the same formula, C205H340N60O53, and the same mass on a certificate, 4493 g/mol average.
How would a test spot FF-33 or another fragment sold as LL-37?: By mass. UniProt lists several overlapping peptides from the same cathelicidin precursor, including FALL-39, FF-33, RK-31, KS-30 and KR-20. Each is a different length and so has a different mass. An LC-MS result that matches the 37-residue mass rules these out, while a mixture appears as several distinct masses on the same run.
Why might two laboratories report different HPLC profiles for the same LL-37?: LL-37 changes shape with pH, concentration and the anions around it, becoming helical and self-associating under some conditions and disordered under others. Different mobile phases, columns and sample diluents can therefore change peak shape and retention. That does not alter the identity result, but it means purity figures are only comparable when the methods are described.

Sources

LL-37 compound record (CID 16198951), PubChem, US National Library of Medicine
Cathelicidin antimicrobial peptide (P49913), UniProt
Johansson J et al. Conformation-dependent antibacterial activity of the naturally occurring human peptide LL-37. J Biol Chem. 1998;273(6):3718-24, Journal of Biological Chemistry (PubMed)
Erckes V et al. Towards a consensus for the analysis and exchange of TFA as a counterion in synthetic peptides. Pharmaceuticals. 2025;18(8):1163, Pharmaceuticals (PMC)
D'Hondt M et al. Related impurities in peptide medicines. J Pharm Biomed Anal. 2014;101:2-30, Journal of Pharmaceutical and Biomedical Analysis (PubMed)

Last reviewed 6 Oct 2026.