Cartalax Stability: How It Degrades and How to Slow It

Cartalax has very few ways to degrade. Its three residues, Ala-Glu-Asp, include no methionine, cysteine, tryptophan, histidine, tyrosine, asparagine or glutamine. That leaves general backbone hydrolysis and the aspartate chemistry that any aspartic acid can undergo, which is slow to begin with and in this sequence has no following residue to cyclise onto. Cold storage, a near-neutral pH and a dry cake keep all of it slow.

Reading the sequence

Cartalax is H-Ala-Glu-Asp-OH, a free tripeptide (PubChem CID 87815447). Against the usual degradation routes in peptide solutions (Manning et al., Pharmaceutical Research, 2010):

• No oxidation-prone residues. There is no methionine, cysteine, tryptophan, histidine or tyrosine, the residues most open to oxidation (Li et al., Biotechnology and Bioengineering, 1995).
• No deamidation. There is no asparagine or glutamine, so the Asn-Gly route does not exist.
• No pyroglutamate. The N-terminus is alanine. The glutamic acid sits in the middle, not at the start, so the ring closure that turns an N-terminal glutamic acid into pyroglutamic acid (Chelius et al., Analytical Chemistry, 2006) does not apply.
• No proline. The diketopiperazine route does not apply.
• One aspartic acid, at the C-terminus. The route worth setting out, below.

The aspartic acid: what it can and cannot do

An aspartic acid side chain can attack the backbone nitrogen of the residue after it and form a five-membered cyclic imide, a succinimide, which reopens as the normal aspartate or as isoaspartate (Geiger and Clarke, Journal of Biological Chemistry, 1987). How fast it forms depends most on the size of the residue on the C-terminal side (Oliyai and Borchardt, Pharmaceutical Research, 1994). In Cartalax there is no residue on that side: the aspartic acid ends the chain, with a free carboxyl where the next backbone nitrogen would be, so on that chemistry this route should not run here. That is our reading of the model work, not a Cartalax measurement, and our storage page says the same.

The bond before an aspartate can still be hydrolysed, and substitutions on either side barely changed that rate in model hexapeptides, because the bond is not part of the ring reaction (Oliyai and Borchardt, Pharmaceutical Research, 1994). In Cartalax that is the Glu-Asp bond, and cutting it would give Ala-Glu and free aspartic acid. pH decides which route runs: very acidic conditions favoured hydrolysis next to the aspartate, pH 4 and 5 gave both, and above pH 6 isomerisation was the only product (Oliyai and Borchardt, Pharmaceutical Research, 1993). Near-neutral bacteriostatic water sits where plain hydrolysis is least favoured.

In the model peptides an aspartate formed the ring 34 times more slowly than an asparagine in the same position, and the asparagine version lost half its starting form in 1.4 days at 37 °C and pH 7.4 (Geiger and Clarke, Journal of Biological Chemistry, 1987). Arithmetic on those two figures puts an aspartate peptide with a favourable neighbour in the weeks range at body temperature, and fridge temperature slows it much further. That is our arithmetic from model-peptide figures, not a Cartalax measurement.

Lyophilised versus reconstituted

A sealed lyophilised vial holds a dry cake with almost no water, so these reactions have very little to work with. Dry is not immune: in a lyophilised model hexapeptide, residual moisture, temperature and the bulking agent all affected how fast its aspartate degraded in the solid state (Oliyai et al., Pharmaceutical Research, 1994). Sealed powder goes in the freezer at minus 20 °C for long holds and the fridge for near-term work, boxed and dry, and a few days at ambient temperature in transit do not harm it. No chemical supplier data sheet for Cartalax was available to us, so no powder figures are given here either.

Once mixed, water is present and the slow routes start. 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 stability study covers Cartalax in bacteriostatic water, so that window is general peptide guidance. The full table is on how long Cartalax lasts.

Temperature

Peptide reactions roughly double to triple in rate for every 10 °C rise, a general rule rather than a Cartalax measurement. A mixed vial goes straight back into the main body of the fridge after each access.

Light

Cartalax has no tryptophan, tyrosine or phenylalanine, the aromatic residues that absorb ultraviolet light and start photo-oxidation. We found no light study of Cartalax, so the vial stays boxed in the dark as standard practice.

Freeze-thaw

Sealed powder can be frozen. A mixed vial that will be opened again is not, because 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. No figure for frozen Cartalax solution was found.

pH

Cartalax needs no acid to dissolve, and strong acid or base speeds backbone hydrolysis. Plain bacteriostatic water, near neutral, is the right solvent; see the Cartalax solubility guide.

What visible changes mean

Mixed Cartalax is clear and colourless. Its likely breakdown products are small and colourless too, so a clear vial is not on its own proof of full strength; the mixing date and storage history matter as much as the look. Discard the solution if it shows:

• Cloudiness that does not clear within 10 to 15 minutes of mixing, or that appears later.
• Strands, flakes or particles.
• Any yellow or other tint.
• 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 Cartalax handling page.