LL-37

A 37-residue cationic, amphipathic peptide, the C-terminal domain of the human cathelicidin protein hCAP18. Every value on this page is calculated from the structure shown, by the method and constants in the sources below; measured values for a particular lot are on its certificate of analysis.

Structure

Three-letter code: H-Leu-Leu-Gly-Asp-Phe-Phe-Arg-Lys-Ser-Lys-Glu-Lys-Ile-Gly-Lys-Glu-Phe-Lys-Arg-Ile-Val-Gln-Arg-Ile-Lys-Asp-Phe-Leu-Arg-Asn-Leu-Val-Pro-Arg-Thr-Glu-Ser-OH

One-letter code: LLGDFFRKSK EKIGKEFKRI VQRIKDFLRN LVPRTES

• 37 residues
• N-terminus: free amine
• C-terminus: free acid

Calculated properties

Calculated from the structure

Property	Value
Molecular formula	C205H340N60O53
CAS Registry Number	597562-32-8
Average molecular weight	4,493.34 g/mol
Monoisotopic mass	4,490.5754 Da
Net charge at pH 7	6.0
Isoelectric point	11.35
Molar absorption coefficient, 280 nm	No tryptophan, tyrosine or cystine: no absorbance at 280 nm; quantify at 205–220 nm instead.
Hydropathy (GRAVY)	-0.72

Net charge across the pH range

pH 2	pH 4	pH 6	pH 7	pH 8	pH 10	pH 12
11.9	8.8	6.1	6.0	5.8	4.2	-1.8

Composition: Lys 6, Arg 5, Leu 4, Phe 4, Glu 3, Ile 3, Asp 2, Gly 2, Ser 2, Val 2, Asn 1, Gln 1, Pro 1, Thr 1.

Expected mass spectrometry ions

The protonated ions expected in positive-mode electrospray, from the monoisotopic mass, and the common sodium and potassium adducts. MALDI spectra show mainly [M+H]⁺.

Expected ions (monoisotopic)

Ion	m/z
[M+H]+	4,491.5827
[M+2H]2+	2,246.2950
[M+3H]3+	1,497.8658
[M+4H]4+	1,123.6511
[M+5H]5+	899.1224
[M+6H]6+	749.4365
[M+Na]+	4,513.5646
[M+K]+	4,529.5386

Isotope pattern

Natural carbon-13, nitrogen-15, oxygen-18, sulfur-34 and deuterium spread each ion over a cluster of peaks about 1 Da apart (1/z on the m/z scale). For a molecule this size the monoisotopic peak is not the tallest: the most abundant is M+2. A spectrum read at its tallest peak, or a deconvoluted mass from it, sits about 2.01 Da above the monoisotopic mass.

Isotope peaks (relative abundance, most abundant = 100)

Peak	Abundance	Neutral mass	[M+H]+	[M+2H]2+	[M+3H]3+	[M+4H]4+	[M+5H]5+	[M+6H]6+
M (monoisotopic)	31.1	4,490.5754	4,491.5827	2,246.2950	1,497.8658	1,123.6511	899.1224	749.4365
M+1	77.7	4,491.5783	4,492.5856	2,246.7964	1,498.2000	1,123.9018	899.3229	749.6037
M+2	100.0	4,492.5811	4,493.5884	2,247.2978	1,498.5343	1,124.1525	899.5235	749.7708
M+3	88.2	4,493.5838	4,494.5911	2,247.7992	1,498.8686	1,124.4032	899.7240	749.9379
M+4	59.9	4,494.5866	4,495.5938	2,248.3006	1,499.2028	1,124.6539	899.9246	750.1050
M+5	33.3	4,495.5892	4,496.5965	2,248.8019	1,499.5370	1,124.9046	900.1251	750.2722
M+6	15.8	4,496.5919	4,497.5992	2,249.3032	1,499.8712	1,125.1553	900.3257	750.4393
M+7	6.5	4,497.5946	4,498.6018	2,249.8046	1,500.2055	1,125.4059	900.5262	750.6064
M+8	2.4	4,498.5972	4,499.6044	2,250.3059	1,500.5397	1,125.6566	900.7267	750.7735
M+9	0.8	4,499.5998	4,500.6070	2,250.8072	1,500.8739	1,125.9072	900.9272	750.9406

Fragment ions (MS/MS)

Collision-induced fragmentation breaks peptide bonds along the backbone. Fragments that keep the N-terminus are b ions; those that keep the C-terminus are y ions. Reading the ladder confirms the sequence residue by residue. Singly charged monoisotopic values, with doubly charged values beside them. The b ion containing every residue is rarely seen and the y ion containing every residue is the precursor itself, so neither is listed.

b and y ions

b#	b+	b2+	Residue	y+	y2+	y#
1	114.0913	57.5493	Leu	—	—	37
2	227.1754	114.0913	Leu	4,378.4986	2,189.7530	36
3	284.1969	142.6021	Gly	4,265.4146	2,133.2109	35
4	399.2238	200.1155	Asp	4,208.3931	2,104.7002	34
5	546.2922	273.6498	Phe	4,093.3662	2,047.1867	33
6	693.3606	347.1840	Phe	3,946.2978	1,973.6525	32
7	849.4618	425.2345	Arg	3,799.2293	1,900.1183	31
8	977.5567	489.2820	Lys	3,643.1282	1,822.0678	30
9	1,064.5887	532.7980	Ser	3,515.0333	1,758.0203	29
10	1,192.6837	596.8455	Lys	3,428.0012	1,714.5043	28
11	1,321.7263	661.3668	Glu	3,299.9063	1,650.4568	27
12	1,449.8213	725.4143	Lys	3,170.8637	1,585.9355	26
13	1,562.9053	781.9563	Ile	3,042.7687	1,521.8880	25
14	1,619.9268	810.4670	Gly	2,929.6847	1,465.3460	24
15	1,748.0218	874.5145	Lys	2,872.6632	1,436.8352	23
16	1,877.0643	939.0358	Glu	2,744.5682	1,372.7878	22
17	2,024.1328	1,012.5700	Phe	2,615.5256	1,308.2665	21
18	2,152.2277	1,076.6175	Lys	2,468.4572	1,234.7322	20
19	2,308.3288	1,154.6681	Arg	2,340.3623	1,170.6848	19
20	2,421.4129	1,211.2101	Ile	2,184.2611	1,092.6342	18
21	2,520.4813	1,260.7443	Val	2,071.1771	1,036.0922	17
22	2,648.5399	1,324.7736	Gln	1,972.1087	986.5580	16
23	2,804.6410	1,402.8241	Arg	1,844.0501	922.5287	15
24	2,917.7251	1,459.3662	Ile	1,687.9490	844.4781	14
25	3,045.8200	1,523.4137	Lys	1,574.8649	787.9361	13
26	3,160.8470	1,580.9271	Asp	1,446.7700	723.8886	12
27	3,307.9154	1,654.4613	Phe	1,331.7430	666.3751	11
28	3,420.9994	1,711.0034	Leu	1,184.6746	592.8409	10
29	3,577.1006	1,789.0539	Arg	1,071.5905	536.2989	9
30	3,691.1435	1,846.0754	Asn	915.4894	458.2483	8
31	3,804.2275	1,902.6174	Leu	801.4465	401.2269	7
32	3,903.2960	1,952.1516	Val	688.3624	344.6849	6
33	4,000.3487	2,000.6780	Pro	589.2940	295.1506	5
34	4,156.4498	2,078.7286	Arg	492.2413	246.6243	4
35	4,257.4975	2,129.2524	Thr	336.1401	168.5737	3
36	4,386.5401	2,193.7737	Glu	235.0925	118.0499	2
37	—	—	Ser	106.0499	53.5286	1

Immonium ions (low-mass markers of the residues present): Gly 30.0338, Ser 60.0444, Pro 70.0651, Val 72.0808, Thr 74.0600, Leu 86.0964, Ile 86.0964, Asn 87.0553, Asp 88.0393, Gln 101.0709, Lys 101.1073, Glu 102.0550, Phe 120.0808, Arg 129.1135.

What to watch for analytically

Variants this sequence can give, from its chemistry. They are what an HPLC or mass spectrometry result for this peptide is read against; none is a statement about a particular lot. Where synthesis variants come from is explained in How synthetic peptides are made.

Sequence-specific variants

Variant	Mass change	Why
Asparagine deamidation	+0.984 Da	Asparagine at position 30 can deamidate to aspartic or isoaspartic acid through a succinimide intermediate. Moisture, higher pH and warmth speed it up.
Glutamine deamidation	+0.984 Da	Glutamine at position 22 can also deamidate, much more slowly than asparagine.
Synthesis-related variants	e.g. +252.082 Da (Pbf), minus a residue (deletion)	As for any synthetic peptide, deletion sequences and incompletely deprotected forms can occur; on arginine, a residual Pbf protecting group adds 252.082 Da.

Sources & Further Reading

1. Prohaska T, Irrgeher J, Benefield J, Böhlke JK, Chesson LA, Coplen TB, et al. Standard atomic weights of the elements 2021 (IUPAC Technical Report). Pure and Applied Chemistry 2022;94(5):573–600. doi:10.1515/pac-2019-0603
2. Wang M, Huang WJ, Kondev FG, Audi G, Naimi S. The AME 2020 atomic mass evaluation (II). Tables, graphs and references. Chinese Physics C 2021;45(3):030003. doi:10.1088/1674-1137/abddaf
3. Rice P, Longden I, Bleasby A. EMBOSS: the European Molecular Biology Open Software Suite. Trends in Genetics 2000;16(6):276–277. doi:10.1016/S0168-9525(00)02024-2
4. Pace CN, Vajdos F, Fee L, Grimsley G, Gray T. How to measure and predict the molar absorption coefficient of a protein. Protein Science 1995;4(11):2411–2423. doi:10.1002/pro.5560041120
5. Kyte J, Doolittle RF. A simple method for displaying the hydropathic character of a protein. Journal of Molecular Biology 1982;157(1):105–132. doi:10.1016/0022-2836(82)90515-0
6. Meija J, Coplen TB, Berglund M, Brand WA, De Bièvre P, Gröning M, et al. Isotopic compositions of the elements 2013 (IUPAC Technical Report). Pure and Applied Chemistry 2016;88(3):293–306. doi:10.1515/pac-2015-0503
7. Roepstorff P, Fohlman J. Letter to the editors: proposal for a common nomenclature for sequence ions in mass spectra of peptides. Biomedical Mass Spectrometry 1984;11(11):601. doi:10.1002/bms.1200111109
8. Biemann K. Contributions of mass spectrometry to peptide and protein structure. Biomedical and Environmental Mass Spectrometry 1988;16(1–12):99–111. doi:10.1002/bms.1200160119
9. Geiger T, Clarke S. Deamidation, isomerization, and racemization at asparaginyl and aspartyl residues in peptides. Succinimide-linked reactions that contribute to protein degradation. Journal of Biological Chemistry 1987;262(2):785–794. doi:10.1016/S0021-9258(19)75855-4
10. Manning MC, Chou DK, Murphy BM, Payne RW, Katayama DS. Stability of protein pharmaceuticals: an update. Pharmaceutical Research 2010;27(4):544–575. doi:10.1007/s11095-009-0045-6
11. D'Hondt M, Bracke N, Taevernier L, Gevaert B, Verbeke F, Wynendaele E, et al. Related impurities in peptide medicines. Journal of Pharmaceutical and Biomedical Analysis 2014;101:2–30. doi:10.1016/j.jpba.2014.06.012

Cosmic Peptides. (2026, October 25). LL-37: peptide reference entry. Cosmic Peptides Research Library. Retrieved October 26, 2026, from https://cosmicpeptides.com/research/peptides/ll-37

@misc{cosmicpeptides_research_peptides_ll_37,
  author       = {{Cosmic Peptides}},
  title        = {LL-37: peptide reference entry},
  howpublished = {Cosmic Peptides Research Library},
  year         = {2026},
  month        = oct,
  url          = {https://cosmicpeptides.com/research/peptides/ll-37},
  urldate      = {2026-10-26},
  note         = {Last reviewed 2026-10-25}
}

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AU  - Cosmic Peptides
TI  - LL-37: peptide reference entry
T2  - Cosmic Peptides Research Library
PB  - Cosmic Peptides
PY  - 2026
DA  - 2026/10/25
UR  - https://cosmicpeptides.com/research/peptides/ll-37
Y2  - 2026/10/26
LA  - en
ER  -