A peptide stock solution is a reconstituted liquid form of a lyophilised peptide, prepared at a concentration higher than your final assay needs so you can dilute accurately into working volumes. You make one whenever an experiment requires a defined, reproducible peptide concentration rather than weighing powder each time. Three rules apply from the start:
- Target a practical stock concentration that keeps assay volumes small and limits solvent carryover.
- Never add a complex assay buffer as your first solvent. Start simple, escalate only when needed.
- Aliquot immediately and limit freeze–thaw cycles to three or fewer per aliquot.
Record solvent identity, concentration calculation, COA lot number, and preparation date before the vial leaves the bench. That metadata is what makes the solution reproducible months later.
Pro Tip: Write the COA lot number directly on each aliquot label at prep time. Tracking it down after the fact is where reproducibility breaks down.
Table of Contents
- How does a peptide stock solution differ from lyophilised peptide?
- How do you prepare a peptide stock solution step by step?
- Which solvents should you use, and what concentration is right?
- How do you calculate the right volume for your target concentration?
- What aseptic technique and filtration do you need?
- How should you store peptide stock solutions?
- What do you do when the peptide will not dissolve?
- What records does a Canadian lab SOP need?
- Key takeaways
- A supplier’s perspective on COAs and why they change how you prep
- Peptilab: Canadian-sourced peptides with the documentation your lab needs
- Useful sources
How does a peptide stock solution differ from lyophilised peptide?
Lyophilised peptides are shipped as freeze-dried powder because the dry state is far more stable than solution. Reconstitution restores the peptide to a usable liquid form, but the moment water or solvent contacts the powder, the chemistry changes: oxidation becomes possible, hydrolysis begins at susceptible bonds, and any aggregation-prone sequences can start to cluster.
Which sequences cause the most trouble? Hydrophobic residues (Leu, Ile, Val, Phe, Trp in high density) reduce aqueous solubility and drive aggregation. Net charge matters too: a peptide with several Arg or Lys residues dissolves readily in water, while a neutral or negatively charged sequence may need pH adjustment. Cysteine residues introduce a separate problem: free thiols oxidise to disulfide bonds in air, scrambling the intended structure.
Short peptides under five residues often dissolve easily in water regardless of sequence. Very hydrophobic sequences, cysteine-containing peptides, and those with glutamine or asparagine at the N-terminus all warrant extra planning before you open the vial.
How do you prepare a peptide stock solution step by step?
Follow this sequence every time. Deviating from the order is where most solubility failures originate.
- Test a small amount first. Weigh out roughly 0.1–0.2 mg into a clean low-bind microcentrifuge tube. This protects the bulk material if your first solvent choice fails.
- Try sterile distilled or bacteriostatic water first. Add water in small increments, invert gently, and observe. A clear solution means water works; proceed to dissolve the full amount.
- If water fails, escalate to dilute acid or base. For basic peptides (net positive charge), try 0.1% acetic acid or dilute HCl. For acidic peptides, try dilute NH₄OH (less than 50 µL). Do not use NH₄OH if the sequence contains cysteine.
- If acid or base fails, introduce a small DMSO or DMF volume. Add 50–100 µL DMSO per mg of peptide, sonicate briefly (30–60 seconds in a bath sonicator), then slowly titrate aqueous buffer into the organic phase while gently mixing.
- Confirm true dissolution. A clear solution is dissolved. A haze, gel, or surface scum indicates suspension or aggregation, not dissolution. Do not proceed with a suspended preparation if your assay requires a defined concentration.
- Filter if preparing a multi-use stock. Pass through a 0.22 µm membrane (see Section 6 for filter compatibility).
- Aliquot, label, and freeze immediately if the stock will not be used within 24–48 hours.
Peptide reconstitution best practices covers edge cases like gel-forming sequences and very short peptides in more detail.
Pro Tip: Sonication accelerates dissolution but generates heat. Keep sonication pulses short and cool the tube between cycles to avoid thermal degradation.
Which solvents should you use, and what concentration is right?
Solvent choice depends on the peptide’s net charge and hydrophobicity. The table below summarises the most common options.
| Solvent | Best for | Cautions |
|---|---|---|
| Sterile distilled water | Hydrophilic, charged peptides | May not dissolve hydrophobic sequences |
| Bacteriostatic water | Multi-use stocks requiring sterility | Contains benzyl alcohol; check assay compatibility |
| 0.1% acetic acid | Basic peptides (Arg, Lys-rich) | Avoid for acid-sensitive sequences |
| Dilute NH₄OH | Acidic peptides (Asp, Glu-rich) | Do not use with Cys-containing peptides |
| DMSO (neat, small volume) | Hydrophobic peptides; difficult sequences | Limit to ≤1% v/v in final assay; check filter compatibility |
| DMF | Alternative to DMSO for very hydrophobic sequences | Higher toxicity; handle in fume hood |
| Chaotropes (urea, guanidinium) | Aggregation-prone or beta-sheet-forming sequences | May interfere with downstream assays |
For most research applications, a moderate stock concentration is practical. Aim for a stock concentration of 1–2 mg/mL for most peptides so the initial solvent volume is small and solvent carryover does not affect cell viability or enzyme activity. When you must use DMSO, prepare a concentrated stock and dilute heavily into aqueous buffer to keep the final DMSO percentage below 1%.
pH matters as much as solvent identity. A peptide that dissolves at pH 4 may precipitate when you add it to a pH 7.4 assay buffer. Check the final assay pH against the peptide’s isoelectric point before committing to a solvent system. The peptide compatibility checklist on Peptilab’s site is a useful reference for solvent–assay pairing decisions.
How do you calculate the right volume for your target concentration?
Two formulas cover most lab scenarios.
mg/mL calculation:
Volume (mL) = Mass (mg) ÷ Target concentration (mg/mL)
Example: You have a 2 mg vial and want a 1 mg/mL stock. Add 2.0 mL of solvent.
mM calculation (requires molecular weight):
Volume (mL) = [Mass (mg) ÷ (MW (g/mol) × Target concentration (mM))] × 1,000,000
Example: You have a 1 kDa peptide (MW = 1,000 g/mol), 2 mg in the vial, and want a 10 mM stock.
Volume = [2 ÷ (1,000 × 10)] × 1,000,000 = 200 µL
| Scenario | Mass | MW | Target | Solvent volume to add |
|---|---|---|---|---|
| 1 mg/mL aqueous stock | 2 mg | N/A | 1 mg/mL | 2.0 mL |
| 2 mg/mL DMSO stock | 2 mg | N/A | 2 mg/mL | — |
| 10 mM stock | 2 mg | 1,000 g/mol | 10 mM | 200 µL |
| 5 mM stock | 2 mg | — | 5 mM | 200 µL |
Always have a second person verify the calculation before adding solvent to the vial. A tenfold pipetting error at this step propagates through every downstream dilution. When preparing a dilution series, prepare the stock first, confirm it is fully dissolved, then dilute stepwise rather than preparing each concentration independently from powder.
What aseptic technique and filtration do you need?
Contamination at the stock preparation stage is difficult to detect and impossible to undo once aliquots are frozen. Prepare stocks in a biosafety cabinet or clean bench whenever the peptide will contact cells or be used in sterility-sensitive assays.
- Use bacteriostatic water (contains 0.9% benzyl alcohol) for multi-use vials that will be accessed repeatedly with a syringe.
- For single-use aliquots, sterile distilled water is sufficient if the stock is frozen immediately.
- Sterile filtration through a 0.22 µm membrane removes microbial contaminants from multi-use stocks, but check filter membrane compatibility before use: cellulose acetate membranes are incompatible with high DMSO or DMF content. Use PVDF or nylon membranes for organic solvent-containing solutions.
- Use low-protein-binding tubes and pipette tips throughout. Standard polypropylene tubes can adsorb peptides, particularly short or hydrophobic sequences, reducing the effective concentration before you even run the assay.
- Avoid glass containers for peptides prone to adsorption; siliconised glass or polypropylene is preferable.
Pro Tip: Weigh the filter unit before and after filtration when working with small volumes. Peptide adsorption onto the membrane can be significant, and a mass balance check tells you whether your post-filtration concentration is still accurate.
How should you store peptide stock solutions?
Storage temperature is the single biggest lever on solution stability. The general guidance:
| Storage condition | Recommended use | Expected hold time |
|---|---|---|
| 4 °C (refrigerator) | Short-term use within 1–2 weeks | Up to 2 weeks for stable sequences |
| −20 °C (standard freezer) | Routine long-term storage | Several months for most sequences |
| −80 °C (ultra-low freezer) | Sensitive sequences; C, M, N, Q, W residues | Extended stability; preferred for unstable peptides |
Peptides containing Cys, Met, Asn, Gln, or Trp residues are less stable in solution and should go straight to −20 °C or colder. Tryptophan is light-sensitive; store those solutions in amber tubes or foil-wrapped vials. Cysteine-containing peptides are prone to oxidation and should be stored under inert gas (argon or nitrogen) when possible.
Avoid frost-free freezers. The repeated freeze–thaw cycles from the auto-defrost mechanism degrade solutions faster than a conventional freezer at the same nominal temperature.
Aliquot into single-use volumes before freezing. Three freeze–thaw cycles is a reasonable upper limit for most sequences; beyond that, activity and concentration both become unreliable. Label each aliquot with the freeze–thaw count and update it at every use. For detailed degradation kinetics by residue type, Peptilab’s guide on peptide stability testing covers the analytical methods used to measure solution-phase degradation.
What do you do when the peptide will not dissolve?
Work through this decision sequence before concluding the peptide is insoluble.
- Still a powder after water? Try 0.1% acetic acid (basic peptide) or dilute NH₄OH (acidic peptide, no Cys). Sonicate for 30–60 seconds between attempts.
- Gel or haze after adding solvent? The peptide may be forming a beta-sheet gel. Add DMSO (50–100 µL per mg), sonicate, then dilute slowly with aqueous buffer. Heating to 37–40 °C briefly can break some gels, but avoid temperatures above 50 °C.
- Precipitate on dilution into buffer? The pH shift is likely the cause. Check the buffer pH against the peptide’s predicted isoelectric point and adjust before adding the peptide stock.
- Cysteine-containing peptide clumping? Oxidation has probably formed disulfide bonds. Add a reducing agent (DTT or TCEP, typically 1–5 mM) to the solvent before reconstitution, and work quickly under inert gas.
- Aggregating hydrophobic sequence? Dissolve in neat DMSO first, confirm clarity, then titrate aqueous buffer slowly while vortexing. Reversing the order (adding DMSO to an aqueous suspension) causes irreversible precipitation.
- Nothing works? Re-lyophilise the solution by speed-vac or freeze-drying, then retry with a different solvent system from scratch.
Pro Tip: If you suspect aggregation rather than true insolubility, a brief centrifugation (10,000 × g, 5 minutes) followed by careful removal of the supernatant tells you whether the bulk of the peptide is in solution or pellet.
What records does a Canadian lab SOP need?
A prepared peptide solution is a distinct material from the parent lyophilised vial and requires its own documentation record. The minimum fields for a Canadian lab SOP:
- Peptide name, lot number, and COA reference
- Solvent identity and lot number
- Mass weighed (with balance ID and calibration date)
- Target concentration and calculated solvent volume
- Second-person verification of the calculation
- Container type (tube material, volume, low-bind status)
- Preparation date and time
- Storage condition and location
- Freeze–thaw limit and current freeze–thaw count
- Initials of preparer and verifier
An aliquot label should carry at minimum: peptide name, concentration, solvent, preparation date, freeze–thaw count, and COA lot. A label reading “BPC-157 | 1 mg/mL in 0.9% NaCl | 2025-11-14 | F/T: 1 | Lot: PL-2411” gives any lab member enough information to use or discard the aliquot correctly.
Retain at least one reference aliquot from each preparation batch, frozen at −80 °C, for re-analysis if experimental results are anomalous. The complete lab protocol guide on Peptilab’s site includes SOP templates formatted for Canadian research environments.
Key takeaways
A peptide stock solution requires the right solvent, a verified concentration calculation, proper aliquoting, and full preparation metadata to be reproducible across experiments.
| Point | Details |
|---|---|
| Definition | A stock solution is a reconstituted, higher-concentration liquid prepared from lyophilised peptide for accurate assay dilution. |
| Target concentration | Aim for 1–2 mg/mL for most peptides; use a concentrated DMSO stock when the sequence is hydrophobic. |
| Solvent escalation | Start with water, escalate to dilute acid or base, then DMSO or DMF only when simpler solvents fail. |
| Storage and aliquoting | Store at −20 °C or −80 °C; limit freeze–thaw cycles to three; keep sensitive residues (C, M, N, Q, W) at −80 °C. |
| Peptilab resources | Peptilab supplies Canadian researchers with COA-verified peptides and protocol resources for SOP development. |
A supplier’s perspective on COAs and why they change how you prep
The COA is not a formality. It is the document that tells you whether the purity figure on the vial label reflects the actual batch you received, and it is the first thing to consult before you choose a solvent or set a target concentration. A peptide at 85% purity requires a different concentration calculation than one at >99%, because the impurity mass contributes to the total weight but not to the active peptide content.
What most researchers underestimate is how much the COA’s solubility note, when one is provided, compresses the trial-and-error phase of reconstitution. Suppliers who conduct solubility testing at synthesis report the effective solvent and approximate concentration achieved, which means you can skip the first two rungs of the escalation ladder entirely. At Peptilab, third-party testing and COA documentation are standard for every batch, and researchers ordering custom or catalogue peptides can request solubility test data at the time of order. Include that data in your lot record alongside the COA reference. It becomes part of the preparation metadata and makes the next researcher who uses that peptide significantly faster.
The broader point: treat the COA as a preparation input, not an archive document. Pull it before you open the vial.
Peptilab: Canadian-sourced peptides with the documentation your lab needs
Sourcing research-grade peptides domestically means your COA arrives with the shipment, not weeks later through customs. Peptilab supplies Canadian researchers with >99% purity peptides, third-party verified and accompanied by full certificates of analysis, shipped from within Canada with no import delays.
Beyond the peptides themselves, Peptilab’s protocol library covers reconstitution workflows, storage SOPs, and solubility guidance formatted for Canadian lab environments. Whether you are setting up a new peptide assay or standardising an existing workflow, the research peptides Canada page is the practical starting point: browse the catalogue, review COA documentation, and request solubility test data for your specific sequence before your order ships.
Useful sources
The following references informed this guide and are worth consulting directly for additional protocol depth.
- Peptide storage guidance | NIBSC — Public-sector UK reference covering solvent escalation, sequence-dependent stability, and storage temperature recommendations. Authoritative for general peptide handling principles.
- Peptide reconstitution guide | Peptide Mind — Supplier protocol resource with stepwise reconstitution instructions, DMSO volume guidance, and stock concentration rationale.
- Research peptide storage SOP for Canadian labs | Northern Compound — Canadian-context SOP guidance covering preparation metadata, sterile filtration, and freeze–thaw tracking.
- Peptide solubility guidelines | GenScript — Detailed solubility decision flowchart with solvent options by peptide charge class; includes DMSO volume recommendations.
- Synthetic peptide handling and storage protocol | Merck Millipore — Working stock preparation guidance including acetic acid reconstitution and re-lyophilisation recovery strategy.
- Peptide lab protocol: a complete guide for researchers | Peptilab — Canadian supplier protocol resource with SOP templates and COA use guidance; internal Peptilab reference.