A Strongly Cationic Helix Fights the Column
Two properties drive nearly every complication that arises when LL-37 is handled or analyzed in the laboratory. The 37-residue peptide carries a heavy net positive charge, and it folds into a helix whose extent of folding shifts with the solvent it sits in. Almost everything else follows from those two starting points.
Where the sequence comes from
LL-37 reads LLGDFFRKSKEKIGKEFKRIVQRIKDFLRNLVPRTES, the final 37 residues at the C-terminus of hCAP18, the human cathelicidin precursor protein. Its label encodes the opening pair of leucines plus the chain length. The formula is C205H340N60O53, average mass roughly 4493.3 Da, CAS 154947-66-7.
Because this is an excised piece of a human protein and not a designed construct, published numbering is not always consistent: some reports index residues against hCAP18 rather than against the fragment. A reference to residue 33 may therefore point either to the thirty-third position of LL-37 or to a position in the precursor.
Charge, and the two problems it creates
Counting the basic residues gives five arginines and six lysines; the acidic side totals two aspartates and three glutamates. The net at neutral pH sits near plus six.
The first consequence is chromatographic. Protonated guanidinium and ammonium groups latch onto residual silanols on a silica-based reversed-phase column, and the result is pronounced tailing unless an ion-pairing additive intervenes. Trifluoroacetic acid suppresses the interaction and is the usual choice here. Methods that substitute formic acid for mass-spectrometry compatibility typically pay for it in visibly worse peak shape.
The second consequence is loss. From dilute solution the same charge drives adsorption onto glass and onto certain plastics. Material disappears onto container walls at low concentration, and the symptom is poor recovery rather than anything you can see in the vial.
Detection with no strong chromophore
Four phenylalanines are present; tryptophan and tyrosine are not. Phenylalanine absorbs only weakly near 257 nm and contributes essentially nothing at 280 nm, so detection defaults to 214 nm, as it does for other aromatic-poor sequences.
That carries the usual trade-off of a 214 nm method. Every species containing peptide bonds registers, truncated fragments included, and those fragments would go unseen at 280 nm. An area percentage taken at 214 nm is therefore both more conservative and more useful than a 280 nm number on the same lot.
Structure is a function of what it is dissolved in
Put the peptide in plain water at low ionic strength and it remains largely disordered. Add salt, move to a buffer at physiological ionic strength, or introduce anionic surfaces, and it takes up an amphipathic alpha-helix with hydrophobic and charged residues sorted onto opposing faces.
The distinction has practical weight. Helical content feeds into aggregation, adsorption and retention, so a single lot can behave one way in one solvent and differently in another. Circular dichroism is the technique that quantifies helicity, and it occasionally shows up on characterization paperwork for this peptide, where for a small linear sequence it would serve no purpose.
Self-association and how to avoid it
The same amphipathic face arrangement that builds the helix also promotes self-association once concentration climbs. Stock solutions made up concentrated may go turbid, or may form oligomers that survive a visual inspection while still altering the chromatogram, usually as a broadened main peak or an early-eluting shoulder.
A better route is to dissolve in mildly acidic water first and dilute into buffer afterward, rather than going straight into buffer, since the peptide is both more soluble and less helical at low pH.
Which degradation pathways are open
Cysteine and methionine are both absent, removing the standard oxidation targets. The liabilities sit elsewhere. Asparagine at position 30 is a deamidation site. Either aspartate can cyclize to a succinimide and go on to isomerize to isoaspartate. And the arginine count makes the peptide an easy substrate for trypsin-like contamination whenever the sample is not clean. None of these announce themselves by mass: deamidation shifts one dalton or less and isomerization shifts nothing at all, so chromatography rather than mass measurement is what exposes them.
Reading the charge envelope
A peptide this basic ionizes without difficulty and spreads across a wide charge envelope, four to eight protons being typical. The five-plus species lands near m/z 899.7, the six-plus near m/z 749.9. Deconvolution back to a neutral mass is routine, and the resulting number belongs alongside the calculated average mass rather than the monoisotopic value, since at 4493 daltons the monoisotopic peak is only a small slice of the isotope cluster.
The short list worth checking
A deconvoluted average mass close to 4493.3 Da; a 214 nm chromatogram acquired with trifluoroacetic acid as the ion-pairing additive; and a clear statement of the solvent used for sample preparation. Purity determined on an aggregated sample is not a purity figure at all, only a description of the aggregate.
Further material
What is described above stems from charge and conformation generally, not from anything unique to this sequence. The underlying cases are laid out in the piece on isoelectric point and solubility and in the discussion of aggregation and solution behavior.