Peptide solubility is mainly determined by polarity. Use acidic solutions to dissolve basic peptides and use basic solutions to dissolve acidic peptides. Hydrophobic and neutral peptides containing a large number of hydrophobic or polar uncharged amino acids can be dissolved in organic solvents such as DMSO, DMF, acetic acid, acetonitrile, methanol, propanol or isopropanol. The solutions should be diluted with water. If methionine and free serine are present in the peptide, DMSO should not be used as a solvent in order to avoid oxidation of the side chain.

Best practice is to test a small portion for solubility before dissolving the entire peptide. Until the best solvent is determined, several tests may be necessary. Lyophillized peptides should be briefly centrifuged beforehand to form a pellet. To enhance solubility sonication can be used.

An easy way to determine which solvent should be used for dissolving your peptide is:

1. First, the total charge of the peptide should be calculated. For this purpose, the acidic residues (Asp [D], Glu [E], and the C-terminal –COOH) are assigned a value of -1 and the basic residues (Arg [R], Lys [K], His [H], and the N-terminal -NH2) a value of +1. With these values the total charge can be calculated.
2. Overall charge is positive: the peptide is basic. Our recommendation is to try to dissolve the peptide in water. If dissolving with water does not work try to add acetic acid (total amount 10-25%). If this does not work either, a small amount of TFA (10–50 µl) should be added and can be further diluted with water to your desired concentration.
3. Over all charge is negative: the peptide is acidic. We recommend usage of PBS-buffer (pH:7,4) to dissolve the peptides. If this fails a small amount of basic solvents such as 0.1 M ammonium bicarbonate should be added and can be further diluted with water. For peptides containing free cysteine degassed acidic buffers should be used due to the possibility of oxidation of the thiol group.
4. Overall charge is 0: peptide is neutral. Neutral peptides usually dissolve in organic solvents such as acetonitrile, methanol and isopropanol. A small amount of one of these solvents should be added to dissolve the peptide. If the peptide is too hydrophobic we recommend to use a small amount of DMSO and can further be diluted with water. For peptides containing free cysteine DMF instead of DMSO should be used.
5. If none of the suggested solvents work, we suggest using trifluoroethanol (TFE). TFE may form a solvent matrix for assisted hydrophobic interactions between peptide side chains and also TFA can induce and stabilize α-helices and can induce β-turns, β-hairpins and also β-strands. However TFA can disrupt tertiary interactions in proteins by weakening non-polar interactions while preserving secondary structures. It has been found that a mixture of TFE or hexafluoroisopropanol (HFIP) with trichloromethane (TCM) or dichloromethane (DCM) also works very well to dissolve peptides. It has been shown that at a content of 10% HFIP or about 20% TFE, clathrate structure is formed.

Positively charged residues: K, R, H, and the N-terminus
Negatively charged residues: D, E, and the C-terminus
Hydrophobic uncharged residues: F, I, L, M, V, W, and Y
Uncharged residues: G, A, S, T, C, N, Q, P, acetyl, and amide

Examples: