SIINFEKL carries no net charge near neutral pH and four of its eight residues are hydrophobic, so plain water is a poor first solvent. Standard laboratory practice is a concentrated stock in DMSO, diluted into aqueous buffer with mixing, with the organic carryover kept low enough for the assay.
- SIINFEKL is ovalbumin 257 to 264, an octapeptide of average mass 963.1 Da presented by the murine class I molecule H-2Kb.
- Its net charge near neutral pH is approximately zero and four of its eight residues are hydrophobic, which is why plain water is a poor first solvent.
- The standard route is a concentrated DMSO stock diluted into aqueous buffer, with organic carryover held to what the assay and its vehicle control support.
- The sequence contains no tryptophan, tyrosine or cysteine, so absorbance at 280 nanometres cannot establish concentration.
- Net peptide content on the certificate, commonly seventy to ninety percent, must be applied to the gross vial weight or every derived concentration is overstated.
- No compendial monograph covers this peptide, so catalog storage windows are conventions rather than measurements on a specific lot.
What the material is
SIINFEKL is residues 257 to 264 of chicken ovalbumin, an octapeptide that binds the murine class I molecule H-2Kb and is recognised by the OT-I transgenic T cell receptor. That is the whole reason it sits in so many freezer boxes, and why the solubility question comes up more often for this sequence than for compounds ten times its price: the assays that consume it are cell-culture assays, and cell culture is unforgiving about whatever solvent carries a ligand into the well.
Nothing about the molecule is exotic. Eight residues, no disulfide, no lipid conjugate, no post-synthetic modification. What makes it awkward in water is the particular set of side chains it happens to carry.
| Property | Value |
|---|---|
| Sequence | Ser-Ile-Ile-Asn-Phe-Glu-Lys-Leu |
| Origin | Chicken ovalbumin, residues 257 to 264 |
| Molecular formula, free acid | C45H74N10O13 |
| Average mass | 963.1 Da |
| Monoisotopic mass | 962.54 Da, singly protonated at m/z 963.55 |
| Ionisable groups | N-terminus, C-terminus, Glu6, Lys7 |
| Net charge at pH 7.0 | Approximately zero |
| Computed isoelectric point | Near pH 6 |
| Absorptivity at 280 nm | Zero: no Trp, Tyr or Cys |
Why water alone rarely works
Aqueous solubility of a short peptide is governed, to a first approximation, by two things: net charge, and how much hydrophobic surface the sequence exposes. Charged groups pull water in. Aliphatic and aromatic side chains push it away and pull the molecules toward one another.
Count them here. Isoleucine twice, then leucine and phenylalanine. Half the sequence is strongly hydrophobic. Against that, the ionisable inventory is one glutamate, one lysine and the two termini, which near pH 7.4 cancel almost exactly. No net charge plus a large hydrophobic fraction is the textbook description of a peptide that will sit at the bottom of the tube looking like it has not noticed the water.
The consequence is not that SIINFEKL is insoluble. It dissolves readily in dimethyl sulfoxide and holds in aqueous buffer once it is already in solution. What fails is the intuitive route: buffer onto dry powder, then waiting.
Raising the pH a unit above the isoelectric point moves the molecule off zero net charge and helps a little. It is a small lever here. Both acidic groups are deprotonated by pH 6, and the next ionisation of consequence is the lysine ammonium above pH 10, which is not somewhere a working solution should go. Most laboratories skip the pH route and reach for organic solvent.
Solvent options, and what each costs
The decision rule that most custom-synthesis technical notes converge on is short enough to hold in your head. Work out the net charge at pH 7. Where the answer is zero or close to it, dissolve in a minimum volume of organic solvent first and take the material into aqueous buffer second.
| Solvent | Behaviour from dry powder | What it costs |
|---|---|---|
| Dimethyl sulfoxide | Dissolves readily at stock concentrations; the default route | Carryover into the assay, hygroscopicity, solid below 18.5 degrees Celsius |
| Water or PBS at pH 7.4 | Slow and often incomplete; material can look wetted rather than dissolved | Partial dissolution is hard to see and reads later as a weak lot |
| Dilute aqueous base, such as ammonium bicarbonate near pH 8 | Modest improvement over neutral water | Small effect on balanced charges; alkaline pH accelerates deamidation |
| Acetonitrile and water mixtures | Effective, and familiar from the purification the material came off | Volatile, and incompatible with most cell assays unless removed |
DMSO earns its default status and brings two nuisances with it. It freezes at 18.5 degrees Celsius, so a stock held cold is a solid block that has to come fully back to room temperature before anything is drawn. It is also strongly hygroscopic. An open bottle takes up atmospheric water within minutes, and a stock thawed and reopened a dozen times is a partly aqueous one whose peptide has been sitting in water throughout.
The carryover ceiling is the other half of the conversation. Common cell-culture practice holds final DMSO at or below 0.1 percent by volume and runs a vehicle-only control at the same level. Immortalised lines often tolerate more and primary cells less, and the control is the only thing that says which case you are in.
Precipitation on dilution, and the order things go in
A concentrated organic stock dropped into a large volume of aqueous buffer passes briefly through a region where the local solvent fraction is falling fast and the peptide is above its aqueous solubility. Hydrophobic sequences crash out there. The precipitate is frequently fine enough to miss by eye, and whatever stays in solution is at a concentration nobody knows.
Two habits prevent most of it. Add the stock into the buffer rather than the buffer into the stock, so the peptide always meets an excess of the receiving phase. Keep that phase moving while you do it. Where the dilution factor runs into the thousands, step it: an aqueous intermediate, mixed, then the final dilution into medium.
Losses to surfaces are the quieter version of the same problem. A hydrophobic octapeptide at nanomolar concentration adsorbs to tube walls and filter membranes in amounts that matter. Low-binding polypropylene is the usual answer for tubes; for sterile filtration, PVDF and PES bind less peptide than cellulose acetate or nylon, and pre-rinsing the membrane takes the edge off the first-pass loss. Where the assay tolerates it, a carrier protein at 0.1 percent occupies the sites that would otherwise take the ligand.
Quantitation without a UV handle
SIINFEKL contains no tryptophan, no tyrosine and no cysteine. Its absorptivity at 280 nanometres is zero. Phenylalanine absorbs weakly nearer 257 nanometres and is no practical basis for quantitation at these scales. The routine protein-concentration measurement therefore does not apply here at all, and a laboratory that reaches for it is reading the buffer.
Three routes remain, in descending order of convenience and ascending order of trustworthiness. Gravimetric preparation corrected for net peptide content is what most laboratories do. Quantitative HPLC against a characterised standard sits in the middle. Amino acid analysis is the reference method and the one to reach for when a number has to defend itself.
Net peptide content is the field people skip. A vial labelled one milligram holds one milligram of lyophilised solid, and that solid is peptide plus counterion plus residual water. Material purified by reversed-phase HPLC in trifluoroacetic acid arrives as the TFA salt, and the certificate should state what fraction of the weight is peptide. Figures in the seventy to ninety percent range are ordinary. Preparing from the gross weight biases every concentration derived from that vial in the same direction, and the bias persists quietly until the vial runs out.
The counterion has a second consequence. Residual trifluoroacetate is cytotoxic to some primary cells at concentrations a straightforward dilution can reach, which is why acetate salt exchange is offered and occasionally worth paying for. When a sensitive assay behaves oddly for reasons the ligand does not explain, the counterion belongs on the list of suspects.
Worked example: one vial to a ten micromolar working solution
Start from a vial labelled 1 mg gross, with a certificate stating 82 percent net peptide content.
1 mg × 0.82 = 0.82 mg of peptide
0.82 mg ÷ 963.1 g/mol = 0.851 µmol
For a 10 mM stock, that quantity calls for:
0.851 µmol ÷ 10 µmol/mL = 0.0851 mL, or 85 µL of DMSO
Diluting that stock one part in a thousand gives a 10 µM working solution carrying 0.1 percent DMSO, which lands exactly on the conventional ceiling. Anything further down should be reached through a second dilution from an aqueous intermediate, never by pipetting a smaller volume of organic stock.
Now the version that skips the correction. Assuming the vial holds a full milligram of peptide, the same target calls for 104 microlitres of solvent. The stock that comes out is labelled 10 mM and is really 8.2 mM: the stated figure is overstated: the true concentration sits 18 percent below the nominal label, so every figure derived from the nominal value overstates reality by about 22 percent. The vial concentration calculator handles the arithmetic for other vial sizes and volumes, though the correction factor still has to be read off the certificate by a person.
Stability once it is in solution
The dry solid is the stable state. Held sealed, desiccated and cold, an unmodified octapeptide of this composition is undramatic. There is no cysteine to oxidise, no methionine, no tryptophan, and no lipid chain to drive aggregation at interfaces, which makes it an easier starting position than most catalog peptides occupy. Vigorous mixing that would be reckless with a lipidated compound is tolerable here.
In solution the picture narrows to two routes worth tracking. Backbone hydrolysis runs on water, temperature and pH as it does everywhere. Deamidation of the asparagine at position four is the sequence-specific one: it proceeds through a cyclic succinimide, and the rate depends heavily on the residue immediately following, fastest when that neighbour is glycine and slower when it is bulky. Here the neighbour is phenylalanine, about as unhelpful to the reaction as a neighbour gets. Alkaline conditions still accelerate it, which is the argument against parking working solutions at pH 9 for convenience.
A deamidated peptide is one dalton heavier and chromatographically close to its parent. It does not announce itself. Where a stock has sat in aqueous buffer for weeks and a binding result starts to drift, re-running the material by LC-MS costs less than the experiment it would otherwise be blamed on. The general degradation routes, and the condensation problem that dominates dry storage, are covered in the storage and stability guide.
As of this 2026 review no USP or Ph. Eur. monograph covers this sequence, and no published shelf life applies to a specific supplier lot. Storage windows quoted on catalog pages are conventions carried across the industry and should be read that way.
What the record should carry
Solution preparation that is not written down cannot be reconstructed later, and this is a compound where three separate numbers can each be quietly wrong. The record for a prepared stock should name the lot identifier, the net peptide content from the certificate and the gross mass it was applied to, the solvent and its lot, the volume added, the resulting molar concentration, the date, and whoever prepared it. Aliquots inherit all of that through a shared identifier written on the tube in something that survives a freezer.
On the incoming side, the certificate should carry an identity method and a purity method that are genuinely separate from each other, a mass result matching 963.1 daltons within the instrument's stated tolerance, a net peptide content figure, and a lot number that matches the vial rather than the order. Purity by RP-HPLC area percent describes the fraction of what eluted, which says nothing on its own about whether the peak is the right molecule. The documentation standard sets out what a complete package looks like, and the guide to aqueous diluents covers preserved water for anyone building solutions that will be entered more than once.
FOR LABORATORY AND IN-VITRO RESEARCH USE ONLY. NOT FOR HUMAN OR ANIMAL CONSUMPTION. NOT FOR PERSONAL, MEDICAL, DIAGNOSTIC, THERAPEUTIC, OR RECREATIONAL USE.
Common questions
Can SIINFEKL be dissolved directly in water?
How much DMSO can reach the assay?
Why can concentration not be measured at 280 nanometres?
What is net peptide content and why does it change the arithmetic?
Does the TFA counterion matter?
How should stock solutions be stored?
Sources
- PubChem record for the SIINFEKL octapeptide. Supports the molecular formula for the free acid and the average and monoisotopic masses quoted in the identity table.
- Solubility guidance published in the technical notes of established custom peptide synthesis houses. Source of the net-charge-at-pH-7 decision rule and the organic-solvent-first route for neutral, hydrophobic sequences; described generically because the guidance is common to several vendors rather than one primary document.
- Peer-reviewed literature on non-enzymatic deamidation of asparagine in peptides. Succinimide-mediated mechanism and the strong dependence of rate on the following residue, fastest at Asn-Gly and slower with a bulky neighbour.
- Immunology literature establishing SIINFEKL as ovalbumin residues 257 to 264 presented by H-2Kb. Provenance of the sequence and its standing as the OT-I model antigen; cited by authority rather than by a single paper title.