DSIP Stability: How It Degrades and How to Slow It

DSIP is a simple research peptide in most respects: its nine residues, Trp-Ala-Gly-Gly-Asp-Ala-Ser-Gly-Glu, include no methionine, cysteine, histidine, tyrosine, asparagine or glutamine. Its two sensitive points are the tryptophan at the front of the chain, which light and oxygen can oxidise, and the single aspartic acid at position 5. Cold, dark storage, a near-neutral pH and a dry cake slow both.

Reading the sequence

DSIP is H-Trp-Ala-Gly-Gly-Asp-Ala-Ser-Gly-Glu-OH, a free nonapeptide (PubChem CID 68816). Against the usual degradation routes in peptide solutions (Manning et al., Pharmaceutical Research, 2010):

• One oxidation-prone residue. Tryptophan 1. There is no methionine, cysteine, histidine or tyrosine, the other residues most open to oxidation (Li et al., Biotechnology and Bioengineering, 1995).
• No deamidation. There is no asparagine or glutamine.
• No pyroglutamate. The N-terminus is tryptophan and the glutamic acid sits at the C-terminal end, so no ring closure of a leading glutamate is possible.
• One aspartate. Asp5, with glycine before it and alanine after it. This is the slow chemical route.
• Nothing to aggregate. A short, flexible, charged chain with no hydrophobic stretch to stack.

The tryptophan: light and oxygen

Tryptophan is among the residues most reactive toward oxygen species, and its oxidation in stored proteins and peptides can be driven by light, catalysed by trace metal ions and influenced by pH and temperature (Li et al., 1995). Its photo-oxidation products include N-formylkynurenine and kynurenine, which are yellow chromophores; in proteins exposed to light they are what turns the material yellow (Dyer et al., Photochemical and Photobiological Sciences, 2006). That is why a tint is a warning sign for DSIP in particular.

Cayman advises purging the stock solvent with an inert gas (Cayman Chemical, item 34426), and InvivoChem asks for sealed storage, for example under nitrogen, away from moisture and light (InvivoChem, V64516). For a research vial that means a tight stopper, a box and the dark of the fridge.

The aspartate: the slow chemical route

An aspartic acid side chain can attack the backbone nitrogen of the next residue and form a five-membered cyclic imide, which reopens as normal aspartate or as isoaspartate (Geiger and Clarke, Journal of Biological Chemistry, 1987). How fast that ring forms depends most on the size of the residue after the aspartate (Oliyai and Borchardt, Pharmaceutical Research, 1994). In DSIP that residue is alanine, small but not the smallest; glycine, which gives the fastest rate, is not in that position, so isomerisation is slow.

The bond on the other side matters in acid. In the same model peptides, putting glycine in front of the aspartate instead of proline greatly increased hydrolysis of that preceding bond (Oliyai and Borchardt, 1994). DSIP has exactly that arrangement, Gly4-Asp5, at the end of its Gly-Gly stretch. Cleavage there would split DSIP into Trp-Ala-Gly-Gly and Asp-Ala-Ser-Gly-Glu, both colourless. That is our reading of model-peptide work, not a DSIP measurement.

Lyophilised versus reconstituted

A sealed lyophilised cake has almost no water, so both routes have little to work with. Cayman rates the solid at minus 20 °C at 4 years or more (Cayman Chemical, item 34426). InvivoChem gives its powder 3 years at minus 20 °C and 2 years at 4 °C, and notes that it is stable at ambient temperature for a few days during ordinary shipping (InvivoChem, V64516). Days at room temperature in transit do not harm it.

Under our handling guidance a vial mixed with bacteriostatic water lasts 8 to 12 weeks at 2 to 8 °C in the dark, with quality slowly fading from around week 6 to 8. No published study covers DSIP in a sterile solution, so that window is general peptide guidance. Cayman’s sheet says not to store an aqueous solution for more than one day; that is a cautious figure for unpreserved buffer stock made for a single experiment, not a measured breakdown time for a cold, preserved vial. InvivoChem’s frozen-solution figures, 6 months at minus 80 °C and 1 month at minus 20 °C, apply to stock frozen once and kept sealed. The full table is on how long DSIP lasts.

The one published breakdown study measured DSIP in blood, where enzymes released its tryptophan (Graf et al., Peptides, 1987); in a sterile vial enzymes arrive only with contamination.

Temperature

Oxidation and aspartate reactions both speed up with warmth; peptide reactions roughly double to triple in rate for every 10 °C rise, a general rule rather than a DSIP measurement. A mixed vial goes straight back into the main body of the fridge after each access.

Freeze-thaw

Sealed powder can be frozen. A mixed vial that will be opened again is not, because ice formation concentrates peptide and salts at the crystal boundaries and repeated cycles promote aggregation (Manning et al., 2010), and each thaw warms the solution. Laboratories that need frozen solution split it into single-use aliquots and thaw each once.

pH

Because acid speeds hydrolysis at the Gly-Asp bond, DSIP is not dissolved in acetic acid, and it needs no acid to dissolve; see the DSIP solubility guide.

What visible changes mean

Mixed DSIP is clear and colourless. Isoaspartate and cleavage fragments are also clear, so a clear vial is not proof of full strength; the mixing date and storage history matter as much. Discard the solution if it shows:

• Any yellow or other tint, the typical sign of tryptophan oxidation.
• Cloudiness that does not clear within 10 to 15 minutes of mixing, or that appears later.
• Strands, flakes or particles.
• A history of being frozen and thawed, or left out overnight.

The general causes of haze and gel are covered in why peptides gel or go cloudy. Bench steps that protect stability are on the DSIP handling page.