TB-500

An N-acetylated heptapeptide matching residues 17–23 of thymosin β4. 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-Leu-Lys-Lys-Thr-Glu-Thr-Gln-OH

One-letter code: LKKTETQ

• 7 residues
• N-terminus: N-acetyl
• C-terminus: free acid

Calculated properties

Calculated from the structure

Property	Value
Molecular formula	C38H68N10O14
CAS Registry Number	885340-08-9
Average molecular weight	889.02 g/mol
Monoisotopic mass	888.4916 Da
Net charge at pH 7	0.0
Isoelectric point	7.36
Molar absorption coefficient, 280 nm	No tryptophan, tyrosine or cystine: no absorbance at 280 nm; quantify at 205–220 nm instead.
Hydropathy (GRAVY)	-1.77

Net charge across the pH range

pH 2	pH 4	pH 6	pH 7	pH 8	pH 10	pH 12
2.0	0.8	0.0	0.0	0.0	-0.3	-1.9

Composition: Lys 2, Thr 2, Gln 1, Glu 1, Leu 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]+	889.4989
[M+2H]2+	445.2531
[M+Na]+	911.4809
[M+K]+	927.4548

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	888.4916	889.4989	445.2531
M+1	46.1	889.4946	890.5018	445.7546
M+2	13.3	890.4971	891.5044	446.2558
M+3	2.8	891.4996	892.5069	446.7571
M+4	0.5	892.5021	893.5094	447.2583

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	156.1019	Leu	—	7
2	284.1969	Lys	734.4043	6
3	412.2918	Lys	606.3093	5
4	513.3395	Thr	478.2144	4
5	642.3821	Glu	377.1667	3
6	743.4298	Thr	248.1241	2
7	—	Gln	147.0764	1

Immonium ions (low-mass markers of the residues present): Thr 74.0600, Leu 86.0964, Gln 101.0709, Lys 101.1073, Glu 102.0550.

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
Glutamine deamidation	+0.984 Da	Glutamine at position 7 can also deamidate, much more slowly than asparagine.
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). TB-500: peptide reference entry. Cosmic Peptides Research Library. Retrieved October 26, 2026, from https://cosmicpeptides.com/research/peptides/tb-500

@misc{cosmicpeptides_research_peptides_tb_500,
  author       = {{Cosmic Peptides}},
  title        = {TB-500: peptide reference entry},
  howpublished = {Cosmic Peptides Research Library},
  year         = {2026},
  month        = oct,
  url          = {https://cosmicpeptides.com/research/peptides/tb-500},
  urldate      = {2026-10-26},
  note         = {Last reviewed 2026-10-25}
}

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PY  - 2026
DA  - 2026/10/25
UR  - https://cosmicpeptides.com/research/peptides/tb-500
Y2  - 2026/10/26
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