Most peptides in the catalog dissolve readily in water or bacteriostatic water at the concentrations used for research stocks. Sequence composition predicts the exceptions.
A quick classification
- Net charge at neutral pH. Count aspartate and glutamate as −1 and lysine, arginine and histidine (partly) as +1, then add +1 for a free N-terminus and −1 for a free C-terminus. A peptide with a net charge of ±2 or more is usually water-soluble.
- Hydrophobic fraction. If more than about half the residues are hydrophobic (Leu, Ile, Val, Phe, Trp, Met, Ala, Pro), expect poorer water solubility, especially when the net charge is near zero.
- Acylated peptides. Fatty-diacid chains (semaglutide, tirzepatide, retatrutide) make the molecule amphipathic; these peptides dissolve in water near neutral pH but form micelles and can stick to plastic. Use glass and avoid vortexing.
- Cyclic and disulfide-bridged peptides (PT-141, AOD-9604) are generally water-soluble at research concentrations.
When a peptide does not dissolve
- Give it time. Many peptides take several minutes of gentle swirling to go into solution.
- For basic peptides (net positive charge), add a small volume of dilute acetic acid (10%) before bringing to volume with water.
- For acidic peptides (net negative charge), use dilute ammonium bicarbonate or a few microlitres of dilute ammonium hydroxide.
- For neutral, hydrophobic peptides, dissolve first in a minimal volume of DMSO and then dilute into aqueous buffer; keep the final DMSO fraction below what the assay tolerates.
- Brief sonication in a bath sonicator can help; heating is a last resort and is unsuitable for oxidation-prone sequences.
Record whatever was added. A stock that contains acetic acid or DMSO behaves differently in downstream assays from one made in water alone.