DSIP dissolves readily in bacteriostatic water, and that is the standard solvent for it in research. It is a short, very hydrophilic peptide that carries a net negative charge at the pH of bacteriostatic water, so it needs no acid, no organic solvent and no special technique beyond adding the water slowly and swirling.
Why DSIP dissolves so easily
The sequence Trp-Ala-Gly-Gly-Asp-Ala-Ser-Gly-Glu explains it. Of the nine residues, only the tryptophan at the front is clearly hydrophobic. The rest are glycine, alanine, serine and two acidic residues, so the chain is small, flexible and polar. PubChem’s computed XLogP for DSIP is minus 7.1, a strongly hydrophilic value (PubChem CID 68816). InvivoChem rates its DSIP acetate at about 100 mg/mL in water (InvivoChem, V64516), and Cayman gives about 10 mg/mL in PBS at pH 7.2 (Cayman Chemical, item 34426), a buffer figure rather than a ceiling for water.
Charge does the rest. Counting the ionisable groups gives two acidic side chains (Asp5, Glu9) plus the C-terminal carboxyl, against the N-terminal amine and no basic side chain at all. From that count we calculate a net charge of about minus 2 at pH 7 and an isoelectric point near pH 2.9. These are our calculations from standard pKa values, not a measured figure. A peptide is least soluble at its isoelectric point, where its charges cancel and molecules stop repelling each other. Bacteriostatic water sits far above pH 2.9, so dissolved DSIP molecules carry a like charge and repel one another.
The glycines help as well. Three glycines, two of them in a row at positions 3 and 4, keep the chain flexible and give it no surface to stack into the sheets that make some peptides gel.
Bacteriostatic water first
Bacteriostatic water is sterile water with 0.9% benzyl alcohol, which allows a refrigerated vial to be accessed more than once. For DSIP it is the solvent of choice: the peptide dissolves fully in it, and the preservative supports repeated access over the 8 to 12 week fridge window described on how long DSIP lasts. Sterile water works for a vial that will be accessed once. The difference is explained in bacteriostatic water explained.
Cayman lists DSIP as only slightly soluble in DMSO and describes organic-solvent-free solutions made by dissolving the solid directly in aqueous buffer (Cayman Chemical, item 34426). For a peptide this hydrophilic, water is the natural solvent and an organic co-solvent adds nothing.
Why not acetic acid?
Laboratories sometimes add a little dilute acetic acid to coax hydrophobic or basic peptides into solution, because lowering the pH gives them a positive charge. DSIP needs no such help, and acid works against it. Its glycine-aspartate bond at positions 4 and 5 is the arrangement that model-peptide work found most prone to hydrolysis of the bond in front of an aspartate (Oliyai and Borchardt, Pharmaceutical Research, 1994), and that hydrolysis is favoured by acid. Acidifying would also push the solution toward the calculated isoelectric point near pH 2.9, where this peptide is least soluble. Plain bacteriostatic water avoids both problems.
Saline and sodium chloride
DSIP also dissolves in 0.9% sodium chloride; the ready-mixed DSIP Nasal Spray is made in sterile saline. Salt ions screen the charges on a peptide, which can reduce solubility for peptides that rely on charge repulsion near their isoelectric point. At the pH of saline DSIP is far from that point and is hydrophilic to begin with, so this is not a practical concern at research concentrations. This is general peptide chemistry rather than a DSIP measurement. For a multi-access vial, bacteriostatic water remains the choice because of its preservative.
Concentrations that dissolve cleanly
A 5 mg vial is commonly mixed with 1 to 2.5 mL of bacteriostatic water, giving 5 mg/mL down to 2 mg/mL. With 2 mL the concentration is 2.5 mg/mL. These are the figures our DSIP reconstitution page uses for its tables, and all of them sit well below both supplier solubility figures. They dissolve to a clear, colourless solution within a minute of gentle swirling.
A larger water volume does not change the amount of peptide; it spreads it more thinly. Whatever volume is chosen, write the concentration on the vial.
Cloudiness and gelling: what causes them
Because DSIP dissolves so readily, persistent haze is uncommon and usually points to technique or storage rather than the peptide:
- Water jetted onto the cake. A strong stream can trap air and create foam, which looks cloudy. Running the water down the inside wall of the vial avoids it.
- Shaking. Vigorous agitation creates air and liquid interfaces and works oxygen into the solution, which the tryptophan does not need. Swirling or rolling is enough.
- Cold water onto cold powder. Both at fridge temperature slows dissolving. Letting the vial and the water reach room temperature first helps.
- Haze or tint appearing later in storage. This suggests the vial was kept too warm or bright, frozen and thawed, or contaminated, and the solution is discarded.
Brief cloudiness that clears within 10 to 15 minutes of mixing is normal and usually trapped air. The general causes are covered in why peptides gel or go cloudy, and the mixing method step by step is in how to reconstitute peptides with bacteriostatic water.
Related DSIP pages
How the dissolved peptide behaves over time is on the DSIP stability page, and bench technique is on the DSIP handling page.
All products supplied by Vistara Labs are intended for laboratory research use only. They are not drugs, are not approved for human or veterinary use, and are not intended to diagnose, treat, cure or prevent any condition. Nothing on this page constitutes medical advice or guidance for use in humans.
