Reconstitution is where a vial of well-characterised peptide most often loses purity — through poor solvent choice, foaming, adsorption to plasticware, or imprecise volumes. This guide describes a defensible workflow.

Step 1 — Equilibrate the vial

Remove the lyophilised vial from cold storage and let it sit, sealed, at room temperature for 20–30 minutes before opening. Cold glass condenses atmospheric moisture; the cake will absorb it within seconds and your reported water content is suddenly wrong.

Step 2 — Choose the diluent

Diluent	When to use	Notes
Bacteriostatic water (0.9% benzyl alcohol)	Stocks intended for repeated draws over 2–4 weeks at 4 °C	Most common for research peptide reconstitution
Sterile water for injection	Single-use stocks, cell-assay aliquots	No preservative; freeze leftover aliquots immediately
0.1% acetic acid in water	Basic peptides that resist dissolution in neutral water	Particularly useful for Lys/Arg-rich sequences
1× PBS pH 7.4	Direct addition to assays where pH must be neutral	Some peptides precipitate in PBS — test on a small aliquot first
DMSO (≤5% final in assays)	Hydrophobic peptides	Stock solutions in DMSO are stable; cell-assay DMSO ≤0.1–1%

Step 3 — Calculate the volume

Use the net peptide content from the COA, not the label fill mass.

target volume (mL) = (net peptide mass in vial, mg) / (target concentration, mg/mL)

Example: a vial labelled 5 mg with 88% net peptide contains 4.4 mg. To reach 1 mg/mL:

volume = 4.4 / 1 = 4.4 mL

If you reach the wrong concentration because you used the label mass, every downstream IC50 is wrong by the same multiplicative factor.

Step 4 — Add diluent slowly

Direct the diluent stream onto the inner glass wall of the vial, never directly onto the cake. Foam destroys peptide. The cake will dissolve from the bottom up over 1–10 minutes; do not rush.

If the cake refuses to dissolve, add 1–2 µL of glacial acetic acid or warm the vial gently in your closed palm (never above 30 °C). Do not vortex.

Step 5 — Confirm dissolution

The solution should be clear and free of particulates. Cloudiness indicates one of:

pH-driven aggregation (try 0.1% acetic acid instead of water).
Hydrophobic aggregation (try a small DMSO co-solvent).
A genuinely insoluble peptide for the chosen diluent (consult the COA).

A faint blue colour is normal for copper-containing peptides such as GHK-Cu.

Step 6 — Aliquot

Repeated freeze–thaw cycles are the single biggest avoidable cause of stock-solution decay. Immediately after dissolution:

Mix gently by inversion.
Pipette single-use volumes into low-binding tubes (Protein LoBind or equivalent).
Label each aliquot with peptide name, lot, concentration, diluent and date.
Freeze upright at −80 °C for long-term, −20 °C for medium-term.

Each freeze–thaw cycle reduces effective concentration by 5–15% for sensitive peptides.

Step 7 — Verify (optional but recommended)

For high-stakes experiments, verify the reconstituted concentration with a quick UV scan:

Trp-containing peptides: A280 with the calculated extinction coefficient (ProtParam value).
BCA or Bradford assay using a peptide standard for short sequences.

A measured concentration within 10% of the calculated value is usually good enough; a discrepancy of 20%+ suggests poor dissolution or adsorption to the tube.

Common pitfalls

Polypropylene adsorption. Short hydrophobic peptides (≤15 residues) lose 10–40% of nominal concentration to standard polypropylene tubes within minutes. Use Protein LoBind tubes, or pre-coat plasticware with 0.1% BSA where the assay tolerates it.
Pipette accuracy at low volume. Below 5 µL, a P10 pipette can read 10–15% high or low. Use larger volumes and serial dilutions.
Concentration drift after freezing. Some peptides precipitate on thawing; spin briefly and re-dissolve before drawing.
Label confusion. Always label aliquots with concentration and diluent. A 1 mg/mL stock in DMSO behaves differently from 1 mg/mL in PBS in every assay.

Cross-references

How to read a peptide COA — understanding the net peptide content used in step 3.
Lyophilised vs liquid peptides — why the lyophilised form is the stable starting point.
Storage protocols: see the blog Best Practices for Peptide Storage and Handling.