PT-141 dissolves in bacteriostatic water, and that is the standard solvent for it in research. A supplier rates it at 100 mg/mL in water with ultrasound, far above the concentrations a research vial uses, so solubility is not the limiting factor for this compound. Its charge is less helpful than its supplier figure suggests, which is worth understanding.
What the supplier figure says
The MedChemExpress data sheet gives water at 100 mg/mL, noting that ultrasound is needed, and bases its stock-solution table on that figure (MedChemExpress, HY-18678). Our tables run from 2 to 10 mg/mL, an order of magnitude below it. On that figure, a clear solution at our concentrations should be straightforward with nothing more than gentle swirling and a little patience.
Why charge is not what carries it
The ring takes away the two side chains that would otherwise dominate the charge. The aspartate and the lysine are both committed to the lactam bond, so what remains ionisable is the histidine side chain, the arginine side chain and the C-terminal carboxyl, with the N-terminus acetylated and therefore uncharged. From that count we calculate a net charge of about plus 0.1 at pH 7 and an isoelectric point near pH 9.2. 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 the molecules stop repelling one another. PT-141 at the pH of bacteriostatic water is not far from being net neutral, so it has less charge repulsion to call on than a strongly basic or strongly acidic peptide. PubChem’s computed XLogP for the molecule is 0.7, close to neutral on that scale as well. That it dissolves well anyway, on the supplier’s figure, points to its polar surface rather than its charge, which is our reading. The practical consequences are small at our concentrations: add the water gently, allow time, and do not try to help it with acid.
The difference from Melanotan II
The two molecules differ at one position: PT-141 ends in a free carboxylic acid, Melanotan II in an amide. That gives PT-141 one extra negative charge at neutral pH and a lower isoelectric point, and it is the reason the two behave differently in water. Published solubility figures for Melanotan II disagree sharply with one another, as the Melanotan II solubility guide sets out. We are not going to carry that disagreement across: the figure above is for PT-141, from a sheet we opened.
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 PT-141 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 PT-141 lasts. Sterile water works for a vial that will be accessed once. The difference is explained in bacteriostatic water explained.
Why not acetic acid?
Laboratories sometimes add a little dilute acetic acid to coax hydrophobic or basic peptides into solution. PT-141 does not need it on the supplier’s water figure, and acid speeds hydrolysis of peptide bonds generally. There is also nothing to gain: with the isoelectric point near pH 9.2, moving the pH down adds positive charge only slowly, and the compound is already soluble well above our working range.
Concentrations that dissolve cleanly
A 10 mg vial is commonly mixed with 1 to 5 mL of bacteriostatic water, giving 10 mg/mL down to 2 mg/mL. With 2 mL the concentration is 5 mg/mL, the figure our PT-141 reconstitution page uses for its tables. All of these are far below the supplier’s water figure and should give a clear, colourless solution. The ready-mixed PT-141 Nasal Spray arrives already in solution.
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 PT-141 dissolves well below its supplier figure, 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 where peptides can unfold and clump. Swirling or rolling is enough for this peptide.
- Cold water onto cold powder. Both at fridge temperature slows dissolving. Letting the vial and the water reach room temperature first helps.
- Haze appearing later in storage. This suggests the vial was kept too warm, 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 PT-141 pages
How the dissolved peptide behaves over time is on the PT-141 stability page, and bench technique is on the PT-141 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.
