α-MSH

A 13-residue peptide, N-acetylated and C-amidated, with the sequence of the first 13 residues of ACTH. It carries the His-Phe-Arg-Trp core shared by the melanocortins; KPV is its C-terminal tripeptide. 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: Ac-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH2

One-letter code: SYSMEHFRWG KPV

• 13 residues
• N-terminus: N-acetyl
• C-terminus: amide

Calculated properties

Calculated from the structure

Property	Value
Molecular formula	C77H109N21O19S
CAS Registry Number	581-05-5
Average molecular weight	1,664.91 g/mol
Monoisotopic mass	1,663.7929 Da
Net charge at pH 7	1.2
Isoelectric point	10.44
Molar absorption coefficient, 280 nm	6,990 M−1 cm−1
Hydropathy (GRAVY)	-0.92

Net charge across the pH range

pH 2	pH 4	pH 6	pH 7	pH 8	pH 10	pH 12
3.0	2.6	1.8	1.2	1.0	0.4	-1.2

Composition: Ser 2, Arg 1, Glu 1, Gly 1, His 1, Lys 1, Met 1, Phe 1, Pro 1, Trp 1, Tyr 1, Val 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]+	1,664.8002
[M+2H]2+	832.9037
[M+3H]3+	555.6049
[M+Na]+	1,686.7822
[M+K]+	1,702.7561

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+3H]3+
M (monoisotopic)	100.0	1,663.7929	1,664.8002	832.9037	555.6049
M+1	93.7	1,664.7958	1,665.8031	833.4052	555.9392
M+2	51.8	1,665.7977	1,666.8049	833.9061	556.2732
M+3	21.1	1,666.7993	1,667.8065	834.4069	556.6070
M+4	6.9	1,667.8009	1,668.8081	834.9077	556.9409
M+5	1.9	1,668.8026	1,669.8098	835.4086	557.2748

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	130.0499	65.5286	Ser	—	—	13
2	293.1132	147.0602	Tyr	1,535.7576	768.3824	12
3	380.1452	190.5763	Ser	1,372.6943	686.8508	11
4	511.1857	256.0965	Met	1,285.6623	643.3348	10
5	640.2283	320.6178	Glu	1,154.6218	577.8145	9
6	777.2872	389.1472	His	1,025.5792	513.2932	8
7	924.3556	462.6815	Phe	888.5203	444.7638	7
8	1,080.4567	540.7320	Arg	741.4519	371.2296	6
9	1,266.5361	633.7717	Trp	585.3507	293.1790	5
10	1,323.5575	662.2824	Gly	399.2714	200.1394	4
11	1,451.6525	726.3299	Lys	342.2500	171.6286	3
12	1,548.7052	774.8563	Pro	214.1550	107.5811	2
13	—	—	Val	117.1022	59.0548	1

Immonium ions (low-mass markers of the residues present): Gly 30.0338, Ser 60.0444, Pro 70.0651, Val 72.0808, Lys 101.1073, Glu 102.0550, Met 104.0528, His 110.0713, Phe 120.0808, Arg 129.1135, Tyr 136.0757, Trp 159.0917.

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 4 can oxidize to the sulfoxide, which usually elutes slightly earlier in reversed-phase HPLC.
Tryptophan oxidation	+15.995 or +31.990 Da	Tryptophan at position 9 can oxidize, particularly on exposure to light and air.
C-terminal amide	free-acid variant +0.984 Da	The C-terminus is an amide. The corresponding free acid, from synthesis or hydrolysis, differs by +0.984 Da, close to the isotope spacing, so it needs a resolved spectrum to tell apart.
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). α-MSH: peptide reference entry. Cosmic Peptides Research Library. Retrieved October 26, 2026, from https://cosmicpeptides.com/research/peptides/alpha-msh

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

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