Lyophilized peptides are usually stored dry at −20 °C. Dissolve only what you will use. Sequence-dependent solubility and freeze–thaw damage are covered in peptide solubility and storage and stability.
Moisture, oxygen, light and freeze–thaw cycles are the usual reasons a certificate no longer matches the vial six months later. Aliquot solid peptide on arrival if the campaign will last. DMSO stocks need tight caps because DMSO is hygroscopic. Reduced cysteine peptides oxidize in air; fluorescent peptides fade in light.
Solubility is sequence-dependent. Test a small aliquot before you commit the lot to a single solvent. Acid, base, acetonitrile and sonication are tools, not a universal recipe. Handling notes on this site are for research materials and are not a pharmaceutical stability program.