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 -10ºC 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.