How can I tell if the tryptophan in my DSIP has oxidised?

Only by mass spectrometry or a separation that resolves oxidised forms. Oxidation products such as kynurenine have a different mass from intact DSIP. A report should show the intact mass and say whether oxidised forms were found. Because tryptophan oxidises easily during handling, the report should also say how the sample was prepared, since the test itself can create a small oxidised peak.

What mass should a DSIP certificate list?

PubChem gives DSIP, also called delta sleep-inducing peptide, an average molecular weight of 848.8 g/mol and a monoisotopic mass of 848.33 Da. A high-resolution report will usually compare against the monoisotopic value. A main mass much lower than this may mean the N-terminal tryptophan or a glycine is missing from the chain.

Why would free tryptophan show up in a DSIP sample?

The bond holding tryptophan to the rest of the chain is DSIP's best-known weak point. Researchers studying its breakdown found that losing the N-terminal tryptophan is the first and most important step, and they needed a dedicated method to separate free tryptophan from intact peptide. In a vial, free tryptophan suggests some of the peptide has been cut.

Can HPLC purity alone confirm a DSIP vial is good?

Not fully. HPLC shows how much of the detected material is in the main peak, but DSIP has impurities that sit close to that peak, such as chains missing or carrying an extra glycine, and isoaspartyl forms. Pairing HPLC with LC-MS identity, and adding an amount test, gives a much clearer answer about what the vial holds.

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DSIP

DSIP is a 9-amino-acid peptide (C35H48N10O15, average mass 848.8 g/mol); a test should confirm that mass, check that the N-terminal tryptophan is still attached and unoxidised, and look for glycine deletion or insertion products from its glycine-rich sequence.

DSIP (delta sleep-inducing peptide) is a nonapeptide with the sequence Trp-Ala-Gly-Gly-Asp-Ala-Ser-Gly-Glu, described in PubChem as a nonapeptide found in neurons, peripheral organs and plasma (PubChem).

Test your DSIP

Identity card

Formula: C35H48N10O15
Average mass: 848.8 g/mol
Monoisotopic mass: 848.33 Da
Length: 9 residues
Modifications: Free N-terminus; Free C-terminal acid
Also sold as: Delta sleep-inducing peptide, WAGGDASGE, Trp-Ala-Gly-Gly-Asp-Ala-Ser-Gly-Glu
Sequence: WAGGDASGE
Note: PubChem lists the line notation H-Trp-Ala-Gly-Gly-Asp-Ala-Ser-Gly-Glu-OH, with a free amine at Trp1 and a free acid at Glu9.

Source: Delta Sleep-Inducing Peptide compound record (CID 68816).

Check a certificate's mass against these values with the mass checker, prefilled for DSIP.

What usually goes wrong with DSIP. Documented failure modes, with sources.

Oxidation of the N-terminal tryptophan

Trp1 is a known weak point in DSIP. A study of short insect peptides that carry tryptophan concluded that Trp oxidation is known to occur easily during sample handling, and identified oxidised Trp forms such as kynurenine by high-resolution LC-MS (König et al., 2023). A 2024 anti-doping study of kisspeptin-10, another Trp-containing peptide, attributed a degradation product to probable oxidation of tryptophan to kynurenine (Colpaert et al., 2024). Oxidised DSIP has a different mass from the intact nonapeptide, so LC-MS can see it. The König authors judged their oxidised forms most likely to be handling artefacts, so a small oxidised peak is not automatically a product fault; a report that says how and how quickly the sample was prepared makes it interpretable.

Loss of the tryptophan from the chain

DSIP's N-terminal tryptophan can also be cut off entirely. In work on DSIP breakdown in plasma and serum, cleavage of the N-terminal Trp was described as the first and most important step, and separating free tryptophan from intact DSIP was a recognised analytical obstacle that required a dedicated method (Graf et al., 1986). In a vial, the result would be free tryptophan plus an octapeptide that starts at Ala2. Both fragments are lighter than DSIP and have different retention times, so an LC-MS run that looks beyond the main peak will find them.

Glycine deletions and insertions

Three of DSIP's nine residues are glycine, including the Gly3-Gly4 pair. A review of impurities in peptide medicines identifies deletion sequences, linked to inefficient Fmoc removal, and insertion sequences, linked to excess amino acid reagent, as core synthesis-related impurities (D'Hondt et al., 2014). In a repetitive stretch such as Gly-Gly, a chain with one glycine too few or too many differs from DSIP by a single glycine residue and can sit close to the main HPLC peak, which is why a purity figure needs mass data beside it.

Aspartimide chemistry at Asp5

DSIP contains one aspartic acid, at position 5. Aspartimide formation at aspartic acid is a notorious side reaction in Fmoc solid-phase synthesis, its by-products appear 18 mass units below the target, and glycine after aspartic acid is especially prone (Samson et al., 2019). In DSIP, alanine follows Asp5. The same study notes that hydrolysis reopens the aspartimide ring to give linear alpha- and beta-Asp peptides, and a beta-linked Asp bond is an isoaspartyl site (Reissner and Aswad, 2003).

Which tests matter most

LC-MSDetects oxidised Trp1 forms, the des-Trp octapeptide and single-glycine deletions or insertions by their masses.
HPLC-UVSeparates DSIP from free tryptophan and from isoaspartyl forms, giving the purity figure.
Amino acid analysisChecks the glycine count against the expected three and gives a second, separate measure of peptide amount.
HRMSAccurate mass near 848.33 Da helps separate DSIP from closely related chains and from oxidised forms.

Reading DSIP results

PubChem lists DSIP at 848.8 g/mol average and 848.33 Da monoisotopic (PubChem). A main mass lower by about one tryptophan residue means the N-terminal residue is gone; lower by one glycine means the chain is short. How charge states convert back to a neutral mass is covered in reading a mass spectrum.

DSIP is acidic: besides the C-terminal carboxyl it carries Asp5 and the Glu9 side chain, and its only basic group is the free N-terminus. Trifluoroacetate counter-ions pair with positively charged groups, and the counter-ion count tracked the number of positive charges in the peptides studied (Erckes et al., 2025). The salt load for DSIP should be measured, not assumed from figures for basic peptides; net peptide content is the figure to set against the label.

Public Tested.ph results

No public DSIP records yet

Each public DSIP record will appear here with its identity, HPLC purity and measured amount against the label, once the first vial is tested.

FIG 13Amount against the label

Questions about testing DSIP

How can I tell if the tryptophan in my DSIP has oxidised?: Only by mass spectrometry or a separation that resolves oxidised forms. Oxidation products such as kynurenine have a different mass from intact DSIP. A report should show the intact mass and say whether oxidised forms were found. Because tryptophan oxidises easily during handling, the report should also say how the sample was prepared, since the test itself can create a small oxidised peak.
What mass should a DSIP certificate list?: PubChem gives DSIP, also called delta sleep-inducing peptide, an average molecular weight of 848.8 g/mol and a monoisotopic mass of 848.33 Da. A high-resolution report will usually compare against the monoisotopic value. A main mass much lower than this may mean the N-terminal tryptophan or a glycine is missing from the chain.
Why would free tryptophan show up in a DSIP sample?: The bond holding tryptophan to the rest of the chain is DSIP's best-known weak point. Researchers studying its breakdown found that losing the N-terminal tryptophan is the first and most important step, and they needed a dedicated method to separate free tryptophan from intact peptide. In a vial, free tryptophan suggests some of the peptide has been cut.
Can HPLC purity alone confirm a DSIP vial is good?: Not fully. HPLC shows how much of the detected material is in the main peak, but DSIP has impurities that sit close to that peak, such as chains missing or carrying an extra glycine, and isoaspartyl forms. Pairing HPLC with LC-MS identity, and adding an amount test, gives a much clearer answer about what the vial holds.

Sources

Delta Sleep-Inducing Peptide compound record (CID 68816), PubChem, US National Library of Medicine
Graf MV et al. Separation of tryptophan from DSIP, a Trp-nonapeptide, by adsorption to aluminum oxide. Anal Biochem. 1986;157(2):295-9, Analytical Biochemistry (PubMed)
König S et al. Oxidation products of tryptophan and proline in adipokinetic hormones. Life (Basel). 2023;13(12):2315, Life (PubMed)
Colpaert T et al. Investigating the detection of kisspeptin-10 in urine using LC-HRMS. Biomed Chromatogr. 2024;38(9):e5946, Biomedical Chromatography (PubMed)
D'Hondt M et al. Related impurities in peptide medicines. J Pharm Biomed Anal. 2014;101:2-30, Journal of Pharmaceutical and Biomedical Analysis (PubMed)
Samson D et al. The aspartimide problem persists. J Pept Sci. 2019;25(7):e3193, Journal of Peptide Science (PMC)
Reissner KJ, Aswad DW. Deamidation and isoaspartate formation in proteins. Cell Mol Life Sci. 2003;60(7):1281-95, Cellular and Molecular Life Sciences (PubMed)
Erckes V et al. Towards a consensus for the analysis and exchange of TFA as a counterion in synthetic peptides. Pharmaceuticals. 2025;18(8):1163, Pharmaceuticals (PMC)

Last reviewed 6 Oct 2026.