Which solvent for which peptide: water, acid, base, DMSO

There is no universal peptide solvent. Water dissolves most short peptides with charged residues and fails for long or hydrophobic ones; dilute acid or base rescues many by charging them up; organic solvents dissolve the rest and then have to be diluted away. The sequence decides which, and it can be read before the vial is opened. This article gives the order of attempts, the chemistry behind each solvent, and the amounts that keep a solvent from interfering with what comes next.

Read the sequence first

Count the residues and sort them. Charged residues at neutral pH are aspartate and glutamate (negative), lysine and arginine (positive) and histidine (partly positive). Hydrophobic residues are the aliphatic and aromatic set: alanine, valine, leucine, isoleucine, methionine, phenylalanine, tryptophan, tyrosine and proline. A free N-terminus adds a positive charge and a free C-terminus a negative one; an acetylated N-terminus and an amidated C-terminus remove them.

A first solvent from the composition of the sequence

The sequence has	Net charge at pH 7	Try first	Then
Fewer than 5 residues, or charged residues at ≥ 25%	Any	Water	Rarely needed
More acidic (Asp, Glu) than basic residues	Negative	Water	Dilute ammonium bicarbonate or 0.1% ammonia, then dilute
More basic (Lys, Arg, His) than acidic residues	Positive	Water	10% acetic acid or 0.1% trifluoroacetic acid, then dilute
Charged residues balanced; net charge near zero	About zero	Buffer at least 2 pH units from the isoelectric point	Acid or base, as above
Hydrophobic residues at ≥ 50%, or a run of 5 or more	Any	DMSO, then dilute into aqueous buffer	DMF; 6 M guanidine for the worst cases
Free cysteine	Any	Degassed water or buffer, used promptly	Avoid DMSO, which oxidizes thiols

The peptide calculator gives the net charge at pH 7, the isoelectric point and the GRAVY hydropathy index for any sequence, which are the three numbers this table turns on. A GRAVY above zero means a hydrophobic peptide on average; above +0.5, expect to need an organic co-solvent.

Water

Reagent-grade water, purified by reverse osmosis and deionization or distilled, is the default solvent and the one that interferes least with anything downstream. It dissolves peptides whose charge gives them enough affinity for water to overcome the attraction between molecules. Tap water and water from an aging cartridge carry metal ions and microbial products that matter for sensitive assays. Water at pH 5–7 is slightly acidic from dissolved carbon dioxide, which helps basic peptides and hinders acidic ones at the margin.

Acid for basic peptides, base for acidic ones

A peptide dissolves when it carries charge, and pH controls charge. A basic peptide in dilute acid is fully protonated and repels its neighbors; an acidic peptide in dilute base is fully deprotonated and does the same. The amounts are small: 10% acetic acid, or 0.1% trifluoroacetic acid, for a basic peptide; 0.1% aqueous ammonia or 50 mM ammonium bicarbonate for an acidic one. The peptide is dissolved in a minimum of the acid or base and then diluted into the working buffer, which must have the capacity to bring the pH back, as pH, buffers and the charge of a peptide explains. Acid is also the standard rescue for a basic peptide that has gelled in water.

DMSO and the organic co-solvents

Dimethyl sulfoxide dissolves nearly every peptide, including the hydrophobic ones that nothing aqueous will take, at concentrations of 10–50 mM. It is used as the primary solvent for a concentrated stock that is then diluted into aqueous buffer, keeping the final DMSO at or below 0.1–1% by volume, since above that it affects proteins and cells in assays. Two cautions: DMSO slowly oxidizes free cysteine thiols and methionine, so a peptide with either should not sit in it; and DMSO is hygroscopic, so a bottle left open takes up water and its stocks drift. Dimethylformamide is the alternative for peptides DMSO will not take, with the same dilution discipline. Acetonitrile, the HPLC solvent, dissolves hydrophobic peptides in mixtures with water and evaporates cleanly, which makes it useful for preparing analytical samples rather than stocks.

Guanidine and urea

For a peptide that aggregates in everything else, 6 M guanidine hydrochloride or 8 M urea breaks the intermolecular hydrogen bonds that hold an aggregate together. They are solvents of last resort: both must be removed or diluted far below their working concentration before most uses, and urea solutions generate cyanate on standing, which carbamylates lysine and the N-terminus. A freshly made urea solution, used within hours, is the rule.

The order of attempts, and how much to risk

Test the solvent on a small portion first when a vial is precious: a tenth of the material in a tenth of the volume. Water first, then the acid or base the charge calls for, then DMSO. A peptide that goes cloudy on dilution from DMSO into buffer was never truly dissolved; it was dispersed, and the cloud is aggregate. Why a peptide will not dissolve describes each failure and what it tells you.

The preserved reagents

Bacteriostatic water and bacteriostatic saline are aqueous solvents with a preservative, and for a peptide's purposes they behave like water and like saline, respectively, with the preservative's own chemistry added. Bacteriostatic water and bacteriostatic saline as laboratory reagents covers when that chemistry matters.

Frequently asked questions

Why does the same peptide dissolve for one laboratory and not another?

pH, mostly. Unbuffered water varies between pH 5 and 7 depending on dissolved carbon dioxide, and a peptide near its isoelectric point is sensitive to a unit of pH. A defined buffer removes the variation.

How much DMSO is too much?

For most cell-based and enzyme assays, final DMSO above 1% changes the result, and 0.1% is the usual ceiling. A 10 mM DMSO stock diluted 1:1,000 lands at 0.1%.

Can I warm a solution to help it dissolve?

Gently, to 30–37 °C, for a few minutes; warming speeds dissolution but also every degradation route. Sonication for a minute or two does the same job with less risk.

Does the salt form change which solvent works?

A trifluoroacetate salt is a little more soluble in water than an acetate or chloride, because the counter-ion itself is more hydrophobic; the difference is minor beside the sequence's own effect.

Sources & Further Reading

1. Chan WC, White PD, eds. Fmoc Solid Phase Peptide Synthesis: A Practical Approach. Oxford University Press, 2000. ISBN 978-0-19-963724-9.
2. Kyte J, Doolittle RF. A simple method for displaying the hydropathic character of a protein. Journal of Molecular Biology 1982;157(1):105–132. doi:10.1016/0022-2836(82)90515-0
3. Tjernberg A, Markova N, Griffiths WJ, Hallén D. DMSO-related effects in protein characterization. Journal of Biomolecular Screening 2006;11(2):131–137. doi:10.1177/1087057105284218
4. Bjellqvist B, Hughes GJ, Pasquali C, Paquet N, Ravier F, Sanchez JC, Frutiger S, Hochstrasser D. The focusing positions of polypeptides in immobilized pH gradients can be predicted from their amino acid sequences. Electrophoresis 1993;14(10):1023–1031. doi:10.1002/elps.11501401163
5. ASTM International. ASTM D1193-06(2018), Standard Specification for Reagent Water.

Cosmic Peptides. (2026, October 30). Which solvent for which peptide: water, acid, base, DMSO. Cosmic Peptides Research Library. Retrieved October 30, 2026, from https://cosmicpeptides.com/research/methods/solvents-for-peptides

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  author       = {{Cosmic Peptides}},
  title        = {Which solvent for which peptide: water, acid, base, DMSO},
  howpublished = {Cosmic Peptides Research Library},
  year         = {2026},
  month        = oct,
  url          = {https://cosmicpeptides.com/research/methods/solvents-for-peptides},
  urldate      = {2026-10-30},
  note         = {Last reviewed 2026-10-30}
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