Peptides can have widely varying solubility properties, depending largely on their primary sequence. While many peptides dissolve easily in water, some, especially those containing multiple hydrophobic amino acid residues, may not readily dissolve. As a general procedure, we recommend first attempting to reconstitute peptides in sterile, distilled water, with sonication if necessary. If solutability is still a problem, addition of a small amount of dilute (approximately 10%) aqueous acetic acid (for basic peptides) or aqueous ammonia (for acidic peptides) may facilitate dissolution of the peptide. It may be desirable, initially, to determine solutability on an aliquot of the total sample. The buffer of choice for your experiments should only be added after the peptide is fully in solution, since salts may promote aggregation and therefore create solubility problems.
The long-term storage of peptide samples presents a somewhat different problem. Lyophilized peptides generally have excellent stabilities (in most cases, lyophilizates can be stored for years at -10°C or lower temperatures with little or no degradation), but, in solution, they generally have much more limited stabilities. Since peptides are susceptible to degradation by proteases of bacterial or microbial origin, the first rule is to prepare sterile solutions, either by reconstitution in sterile, distilled water, or by sterile filtration after reconstitution. For peptides which contain methionine, cystenic or tryptophan residues, oxidation may generate impurities with or without concomitant biological inactivation. To avoid this, peptides containing these amino acids should be dissolved in oxygen-free solvents.
As far as the pH of the solution is concerned, the majority of peptides are most stable under acidic conditions. We therefore generally recommend keeping solutions in the range of pH 3–6. It is also recommended that solutions of peptides be stored frozen in aliquots in order to minimize freeze-thaw cycles. For maximum stability, however, we recommend that reconstituted peptides be relyophilized.
We hope that these general comments will prove useful as guidelines for your work with our peptides. Of course, if you have any specific problems or questions, please contact us.