The solubility of a peptide in a solvent or buffer is determined mainly by its polarity, which results from the amino acid composition, and its conformation.
Solubility in water can be expected for peptides containing a large proportion of charged amino acids. As the peptides are usually produced as trifluoroacetate salts, the basic amino acids (Arg, Lys, His) and the N-terminal amino group will be protonated. The aqueous solubility of peptides containing Asp and/or Glu can be improved by adding diluted ammonia, which will generate the ammonium salts.
Polar amino acids as Asn, Gln, Gly, Ser, Thr facilitate reconstitution in water.
DMSO (dimethyl sulfoxide) is a highly suitable solvent for dissolving non-polar peptides (peptides containing a large proportion of Ile, Leu, Met, Phe, Pro, Trp, Val; most fluorophoric and chromophoric peptide substrates), it will also disrupt aggregates. Addition of
DMSO (or DMF, acetonitrile, or other highly polar water-miscible solvents) may help to reconstitute a peptide, which is scarcely soluble in water. DMSO should not be used in combination with strong acids as trifluoroacetic acid. Most peptides will dissolve in acetic acid. Cys- and Met-containing peptides require special attention to prevent oxidation of the sulfur.
Reconstitution of peptides, especially long peptides or inner salts, may take time. Gentle warming or short repetitive sonication to accelerate dissolution is usually tolerated.
The intended use may limit the choice of solvents.
We advise against dissolving peptides directly in assay buffer, except if they show a high water-solubility.
How to dissolve peptides containing free cysteines?
Peptides containing a single free cysteine will be oxidized at pH>7 yielding dimers. This oxidation can be reverted. Peptides containing two or more thiol moieties yield a mixture of products upon oxidation. pH 7.5-8 is the pH optimum for disulfide bond formation.
Hence, peptides containing free cysteines are best dissolved in degassed solvents, e.g. buffers pH<7, diluted acetic acid, 0.1% trifluoroacetic acid in aqueous acetonitrile.
DMSO should be avoided, especially with peptide trifluoroacetates.
How to dissolve peptides containing disulfide bridges?
Basic buffers should be avoided.
The peptide I’ve dissolved is rapidly losing its activity. Why? Can I prevent it?
There could be many reasons for loss of activity, though oxidation of Met yielding the sulfoxide is a common one. The rate of sulfoxide formation is sequence-dependent. The problem can be circumvented by replacing Met with its stable isostere Nle. Bachem offers Nle analogs of a number of Met-containing peptides. If the Nle peptide you require is not available, please ask for a quote.
Sulfotyrosine-containing peptides may loose their activity due to desulfation, Gla-containg peptides can decarboxylate.