Met-enkephalin

Leu-enkephalin with methionine in place of leucine at position 5, which makes it susceptible to oxidation (+15.995 Da). 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-Tyr-Gly-Gly-Phe-Met-OH

One-letter code: YGGFM

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

Calculated properties

Calculated from the structure

Property	Value
Molecular formula	C27H35N5O7S
CAS Registry Number	58569-55-4
Average molecular weight	573.67 g/mol
Monoisotopic mass	573.2257 Da
Net charge at pH 7	0.0
Isoelectric point	6.09
Molar absorption coefficient, 280 nm	1,490 M−1 cm−1
Hydropathy (GRAVY)	0.52

Net charge across the pH range

pH 2	pH 4	pH 6	pH 7	pH 8	pH 10	pH 12
1.0	0.3	0.0	0.0	-0.2	-1.4	-2.0

Composition: Gly 2, Met 1, Phe 1, Tyr 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]+	574.2330
[M+2H]2+	287.6201
[M+Na]+	596.2149
[M+K]+	612.1889

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). The monoisotopic peak is the most abundant.

Isotope peaks (relative abundance, most abundant = 100)

Peak	Abundance	Neutral mass	[M+H]+	[M+2H]2+
M (monoisotopic)	100.0	573.2257	574.2330	287.6201
M+1	32.5	574.2287	575.2359	288.1216
M+2	11.0	575.2273	576.2346	288.6209
M+3	2.4	576.2283	577.2356	289.1214

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. 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+	Residue	y+	y#
1	164.0706	Tyr	—	5
2	221.0921	Gly	411.1697	4
3	278.1135	Gly	354.1482	3
4	425.1819	Phe	297.1267	2
5	—	Met	150.0583	1

Immonium ions (low-mass markers of the residues present): Gly 30.0338, Met 104.0528, Phe 120.0808, Tyr 136.0757.

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
Methionine oxidation	+15.995 Da per oxidized residue	Methionine at position 5 can oxidize to the sulfoxide, which usually elutes slightly earlier in reversed-phase HPLC.
Synthesis-related variants	minus a residue (deletion)	As for any synthetic peptide, deletion sequences and incompletely deprotected forms can occur.

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). Met-enkephalin: peptide reference entry. Cosmic Peptides Research Library. Retrieved October 26, 2026, from https://cosmicpeptides.com/research/peptides/met-enkephalin

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

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