Lab method note
Peptide solubility: predicting it from sequence charge and hydrophobicity
By Vitality Chems · Published · updated · vendor-authored explanatory note
Short answer
A peptide's solubility in water is set mostly by its charge and its hydrophobic content. Count the charged groups at pH 7 (Asp, Glu and the C-terminal carboxyl are −1; Lys, Arg and the N-terminal amine are +1): peptides with a clear net charge usually dissolve in water, with a little dilute acid helping basic sequences and dilute ammonium bicarbonate helping acidic ones, while sequences with few charges or more than about half hydrophobic residues usually need a small volume of organic co-solvent first. Solubility is generally lowest near the isoelectric point, so work at a pH well away from the pI.
Catalog materials discussed on this page
| Sequence profile | Typical first solvent | Cautions |
|---|---|---|
| Net positive (basic) | Water; if needed, a small volume of dilute acetic acid, then dilute with water or buffer | Check the final pH suits the assay |
| Net negative (acidic) | Water; if needed, a small volume of 0.1 M ammonium bicarbonate, then dilute | Avoid basic conditions with free Cys (thiol oxidation) |
| Neutral, or under ~25% charged residues | Small volume of organic solvent (DMSO, DMF or acetonitrile), then add aqueous buffer slowly | DMSO is not advised for Cys- or Met-containing sequences |
| Over ~50% hydrophobic residues | Dissolve fully in the organic solvent first, then dilute dropwise | Precipitation on dilution means the target concentration is too high for that mixture |
| Gelling or aggregating sequences | Chaotropes such as guanidine hydrochloride or urea, where the assay tolerates them | Chaotropes interfere with many biological assays |
Step 1: count the charges
Supplier technical guidelines use the same simple scoring. Assign −1 to each Asp (D), Glu (E) and the free C-terminal carboxyl; +1 to each Lys (K), Arg (R) and the free N-terminal amine; and +1 to each His (H) only below about pH 6. The sum is the approximate net charge at pH 7; the number of charged groups divided by the residue count is the charged share (Sigma-Aldrich; GenScript).
Terminal modifications matter. An acetylated N-terminus (Ac-) removes the +1, and a C-terminal amide (-NH2) removes the −1. Many research peptides carry one or both, so read the full sequence notation on the certificate.
For a more exact figure, the net charge calculator on this site applies the Henderson–Hasselbalch equation to every ionizable group with the EMBOSS pKa set, and estimates the isoelectric point. Sequence-based pI estimates are typically within about a quarter of a pH unit for peptides with modern pKa sets (Kozlowski, 2016).
Step 2: check the hydrophobic share
Charge rules break down for very hydrophobic sequences. Guidelines classify peptides with roughly 50% or more hydrophobic residues (such as W, L, I, F, M, V, Y, P and A), and neutral peptides with fewer than about 25% charged residues, as candidates for an organic co-solvent rather than water alone (Sigma-Aldrich; SB-Peptide). Hydropathy scales such as Kyte and Doolittle's (1982) give a residue-by-residue view when a sequence has a long hydrophobic stretch.
Not all hydrophilic residues are equal. In a systematic study of all 20 amino acids at one exposed position of ribonuclease Sa, Asp, Glu and Ser contributed more favorably to solubility than the other hydrophilic residues, especially at high net charge (Trevino, Scholtz and Pace, 2007).
Step 3: choose conditions away from the pI
Solubility is often lowest at the isoelectric point, where positive and negative charges balance and intermolecular repulsion is weakest, so molecules tend to aggregate (EMBOSS iep documentation). Choosing a buffer at least one to two pH units away from the estimated pI is a common starting point for solution work and for ion-exchange or HPLC method development.
Always test a small portion of the material first rather than the whole vial, and dissolve the peptide completely in the initial solvent before adding water or buffer. Dissolution is usually faster in the pure initial solvent than in a premixed water and solvent blend, and starting with a blend can force you to use far more organic solvent than needed (Sigma-Aldrich).
Solvent cautions for sensitive residues
DMSO oxidizes free thiols: it is used deliberately as a reagent to form disulfide bonds in peptides (Tam et al., 1991). Supplier guidelines therefore advise against DMSO for sequences with Cys or Met and suggest DMF instead. Basic conditions also promote thiol oxidation in Cys-containing sequences.
Counter-ions change the picture too. Peptides purified by reversed-phase HPLC are usually isolated as TFA or acetate salts, and the salt form and residual water affect both apparent solubility and the true peptide mass in a weighed sample. Correct for net peptide content when you calculate a concentration.
Record the solvent, volumes, pH, date and lot number for every solution you prepare, so results can be traced back to the material and its certificate.
Related on Vitality Chems
- Peptide net charge and pI calculator — Step 1 and Step 3 done for you.
- Peptide molarity calculator — concentration corrected for net peptide content.
- TFA vs acetate peptide salts — how the counter-ion changes the weighed mass.
- Peptide degradation pathways — oxidation, deamidation and aggregation.
- Selank lot documentation — a basic sequence (TKPRPGP).
Frequently asked
How do I know if a peptide is acidic or basic?
Count charged groups at pH 7: −1 for each Asp, Glu and a free C-terminus, +1 for each Lys, Arg and a free N-terminus. A negative total means acidic, positive means basic. His counts +1 only below about pH 6.
Why is a peptide least soluble at its isoelectric point?
At the pI the molecule carries no net charge, so there is little electrostatic repulsion between molecules and they associate and aggregate more easily. Moving the pH away from the pI increases the net charge.
Can I use DMSO for a peptide that contains cysteine or methionine?
It is generally avoided. DMSO can oxidize free cysteine thiols (it is used on purpose to form disulfides) and methionine, so DMF or acetonitrile is the usual alternative for in-vitro work.
What counts as a hydrophobic peptide?
Supplier guidelines use roughly 50% or more hydrophobic residues (W, L, I, F, M, V, Y, P, A) as the point where aqueous solvents usually fail and an organic co-solvent is needed first.
References
- Sigma-Aldrich. Solubility guidelines for peptides (technical article)Charge-counting rules, 10% and 25% charged-share thresholds, ammonium bicarbonate and acetic acid, DMSO caution for Cys and Met.
- GenScript. Peptide solubility guidelines (PDF)Stepwise water, acid or base, then DMSO approach; no NH4OH with Cys.
- SB-PEPTIDE. Peptide solubility guidelinesHydrophobic-residue threshold of 50% and organic co-solvents.
- Kozlowski LP. IPC – Isoelectric Point Calculator. Biol Direct. 2016;11:55. PMID 27769290Benchmark of pKa sets; average pI error about 0.25 pH units for peptides with the optimized sets.
- EMBOSS iep: isoelectric point and charge (documentation)Default pKa set used by the calculator; note on low solubility at the pI.
- Kyte J, Doolittle RF. A simple method for displaying the hydropathic character of a protein. J Mol Biol. 1982;157(1):105-132. PMID 7108955Hydropathy scale.
- Trevino SR, Scholtz JM, Pace CN. Amino acid contribution to protein solubility. J Mol Biol. 2007;366(2):449-460. PMID 17174328Asp, Glu and Ser contribute most favorably among hydrophilic residues.
- Tam JP, Wu CR, Liu W, Zhang JW. Disulfide bond formation in peptides by dimethyl sulfoxide. J Am Chem Soc. 1991;113(17):6657-6662DMSO as an oxidant for cysteine thiols.
