This article is the 3rd in a series on the synthesis of peptides. It looks at solubilising peptides and peptide storage.
Peptide Synthesis: storage and solubility
Peptides are supplied as lyophilised materials, which are regarded as stable compounds for shipping purposes.
For long term storage, however, it is recommended to store them at -20°C. When taking peptide preparations out of the freezer, it is important to allow the bottles/vials to warm up to room temperature before opening the container. This is because peptides are often hygroscopic so allowing the container to warm to room temperature avoids condensation of atmospheric water on the peptide.
Peptides in solution can degrade, primarily due to oxidation of Cys, Met and Trp residues, but they are also susceptible to attack by microbes, so it is advised to store solutions at -20°C when not in use. If preferred, we can supply peptides aliquoted into smaller amounts for ease of use.
Peptide solubility is determines not only by sequence but by the peptide environment. In general, peptides with a large proportion of non-polar amino acids will be difficut to dissolve in aqueous solutions, the more polar residues that are present, the easier it will be to dissolve a peptide. Peptides that are acidic, i.e. contain more acidic amino acids than basic ones, will be more soluble at higher pH whilst peptides that are overall basic will be most soluble at lower pH.
Suggestions for dissolving peptides
We recommend the use of volatile materials such as dilute acetic acid and ammonia solutions when first dissolving a peptide of unknown solubility. This will enable the buffers to be removed by lyophilisation and the dissolution attempted again if necessary.
If the peptide is acidic, i.e. contains more Asp and Glu residues than His, Lys or Arg, dissolve the peptide in dilute ammonia solution, e.g. 0.5% ammonium hydroxide. Do not use this method if your peptide has disulphide bridges, the high pH may cause them to unfold.
If the peptide is basic, i.e. contains an excess of His, Lys and Arg groups, then 10% acetic acid may be suitable to dissolve the peptide.
Dimethyl sulphoxide (DMSO) is a very good solvent and has the advantage of being tolerated by cells. DMSO is, however, difficult to remove by drying. One method is to add a small amount of high purity grade DMSO to the stock peptide solution until it dissolves. Once dissolved, water or buffer solution can be added very slowly to dilute the DMSO content. Stop the water addition if the peptide starts to precipitate out. DMSO is not suitable for peptides containing single cysteines as it promotes disulphide bridge formation.
Gentle warming and sonication are useful tactics in getting peptides to dissolve.
Peptides originating from the trans-membrane regions of proteins are generally very difficult to dissolve.
We are happy to perform specific dissolution studies with custom peptides to provide a recommended protocol. Please contact us to discuss.