Dissolve peptides in 0.1% aqueous acetic acid to yield a target concentration of 1-5 mg/mL. Use sonication if necessary.
Small amounts of dilute(10%) aqueous acetic acid for basic peptide antigen or aqueous ammonia for acidic peptides may help dissolution of these peptides.
If peptides are still insoluble, add acetonitrile up to 20%(v/v), and use sonication to help dissolution.
Lyophilize any remaining insoluble peptides to remove water, acetic acid and acetonitrile. When the peptides are completely dry, add neat DMF or DMSO (dropwise) until the peptides are dissolved.
Slowly dilute the solution with water to desired concentration. If precipitation occurs during dilution, add more DMF or DMSO (dropwise) to dissolve the precipitate.