Peptide Reconstitution Guide: How Lyophilised Peptides Are Reconstituted in the Laboratory
How to reconstitute a lyophilised peptide for in-vitro research: what the powder is, which solvent to use and why, the aseptic bench technique, the concentration arithmetic with worked examples, dilution for assays, storage of the stock, and the mistakes that ruin an experiment.
Reconstitution is the step that turns the dry powder in a research peptide vial back into a solution of known concentration, and it is where most experimental error is introduced. Get the volume wrong and every concentration downstream is wrong. Shake instead of swirl and the peptide aggregates. Use the wrong solvent and it never dissolves, or it degrades in a week. This guide describes how a laboratory reconstitutes a lyophilised peptide for in-vitro work: what the powder is, which solvent and why, the bench technique, the arithmetic with worked examples, dilution for assays, and storage of the resulting stock. It is written for research use. Nothing in it is guidance for preparing anything for administration to a person or animal; research peptides are not for that, and this page does not go there.
What is in the vial before you start
Research peptides are supplied lyophilised, meaning freeze-dried. The synthesised peptide was dissolved, frozen, and the ice removed under vacuum by sublimation, leaving a dry cake or powder at the bottom of the vial. The mass stated on the label, for example 5 mg or 10 mg, is the nominal mass of peptide in that solid. Lyophilised peptide is stable for a long time when kept cold, dry and dark, which is why it ships this way; in solution the same molecule may last days. Our storage guide covers how long each state lasts and how to hold it.
Two things about the powder matter for the calculation that follows. First, the peptide is usually present as a salt, most often a trifluoroacetate or acetate salt left over from purification, and the label mass may be gross peptide mass rather than pure peptide content; a certificate of analysis states net peptide content where it has been measured, and for quantitative work that figure is the one to use. Second, the cake can look like almost nothing, a thin film or a small pellet; that is normal for a few milligrams of peptide and does not mean the vial is short.
Choosing the solvent
Most research peptides dissolve readily in water. For a stock drawn from over several sessions, sterile water with single-use aliquots frozen at -20°C is the usual laboratory choice. Bacteriostatic water, sterile water with 0.9% benzyl alcohol as a preservative, is an alternative for a refrigerated stock that will be re-entered, where the assay tolerates benzyl alcohol; a vehicle control is then essential. Our bacteriostatic water guide explains the difference between it, plain sterile water and saline.
Some peptides do not dissolve well in water, and forcing them produces a cloudy suspension rather than a solution. The behaviour is predictable from the sequence. Strongly basic peptides, rich in arginine and lysine, usually dissolve in water or in a small volume of dilute acetic acid; strongly acidic peptides, rich in aspartate and glutamate, prefer a small volume of dilute ammonium bicarbonate or a mildly basic buffer; very hydrophobic sequences may need a small amount of an organic solvent such as DMSO or acetonitrile before dilution into aqueous buffer. The peptide solubility calculator estimates net charge and hydropathy from a sequence and suggests where to start, with the limitations stated. Whatever the solvent, the same rule applies: dissolve in the minimum volume of the strong solvent, then dilute into the buffer the assay needs, and include a vehicle control so the solvent's own effect is measured.
Aseptic bench technique
Reconstitution is done at the bench with aseptic technique, because anything introduced into the vial will be in the stock for its whole life. The sequence below is standard laboratory practice for a stoppered vial.
Let the vial reach room temperature before opening it, about 15 to 20 minutes out of the freezer. Opening a cold vial draws moisture from the air onto the cake. Remove the flip-off cap and clean the rubber stopper with an alcohol wipe; let it dry. Measure the calculated volume of solvent with a calibrated pipette, or a sterile syringe if the vial is to stay stoppered. Accuracy here is accuracy everywhere afterwards; use an instrument sized to the volume so the graduations are readable. Add the solvent slowly down the inside wall of the vial rather than onto the cake. A jet onto the powder foams and can denature the peptide. Swirl, do not shake. Roll the vial gently between the fingers until the solid has dissolved. Shaking creates air-liquid interfaces and shear that promote aggregation. Inspect against a light. The solution should be clear; a faint tint is normal for some peptides. Persistent particles or cloudiness mean incomplete dissolution or the wrong solvent, not something to proceed with. Label the vial with the compound, concentration, solvent and date, and move it to the storage it needs, usually refrigerated for a preserved aqueous stock. The concentration arithmetic
The concentration of the stock is the mass of peptide divided by the volume of solvent added. Everything else follows from that one line.
concentration (mg/mL) = peptide mass (mg) / solvent volume (mL)
Worked examples for a laboratory stock:
Stock concentration from mass and solvent volume Peptide in vial Solvent added Stock concentration Same value in µg/µL 5 mg 1 mL 5 mg/mL 5 µg/µL 5 mg 2 mL 2.5 mg/mL 2.5 µg/µL 10 mg 2 mL 5 mg/mL 5 µg/µL 10 mg 5 mL 2 mg/mL 2 µg/µL 2 mg 1 mL 2 mg/mL 2 µg/µL
Milligrams per millilitre and micrograms per microlitre are the same number, which is convenient when pipetting microlitre volumes into assay plates. To convert a mass concentration to a molar one, which most cell and binding assays are specified in, divide by the molecular weight: a 5 mg/mL stock of a peptide with a molecular weight of 1,000 g/mol is 5 mmol/L, or 5 mM. The molarity calculator does this conversion in either direction, and the molecular weight calculator gives the molecular weight from a sequence if the certificate does not.
Two refinements matter for quantitative work. If the certificate of analysis gives a net peptide content, for example 82%, the effective peptide mass in a 10 mg vial is 8.2 mg and the concentration should be calculated from that. And the volume of the cake itself is negligible at these masses, so the solvent volume can be taken as the final volume without correction.
Diluting the stock for an assay
A stock at milligrams per millilitre is far too concentrated for most in-vitro work, where working concentrations are commonly in the nanomolar to low micromolar range. Dilution uses the standard relation C1 × V1 = C2 × V2: the stock concentration times the volume taken equals the final concentration times the final volume. To make 10 mL of a 10 µM working solution from a 5 mM stock, V1 = (10 µM × 10 mL) / 5 mM = 0.02 mL, or 20 µL of stock into 9.98 mL of buffer.
Serial dilution, in steps of ten or so, is more accurate than one large dilution when the ratio is very high, because pipetting 2 µL is far less precise than pipetting 20 µL twice. Dilute into the buffer the assay actually uses, not into more water, so that pH and ionic strength are those of the experiment. And carry the vehicle control through the same dilutions, so that any effect of benzyl alcohol, DMSO or acetic acid carried over from the stock is measured rather than attributed to the peptide.
Storing the reconstituted stock
A peptide in aqueous solution degrades faster than the same peptide as a dry powder, by hydrolysis, oxidation and, for some sequences, deamidation, and the rate roughly doubles for every 10°C rise in temperature. A preserved stock in bacteriostatic water is normally kept refrigerated at 2 to 8°C and used within the period the preservative is relied on, which by convention is 28 days; the peptide's own stability is often shorter and is compound-specific. For longer holding, the stock is aliquoted into single-use volumes and frozen at -20°C or below once, because each freeze-thaw cycle concentrates solutes as ice forms and stresses the peptide. Never refreeze a thawed aliquot. The storage guide sets out the reasoning and a checklist.
The mistakes that ruin experiments
Wrong volume. A 2 mL addition recorded as 1 mL doubles every concentration downstream. Write the volume on the vial before any solvent goes in. Shaking. Aggregated peptide gives a lower effective concentration and can behave differently in the assay. Wrong solvent. A hydrophobic or strongly charged peptide forced into plain water gives a suspension, and the assay is run on whatever fraction dissolved. Ignoring net peptide content. Treating gross mass as peptide mass overstates the concentration by the counter-ion and water fraction, often 10 to 30%. Repeated freeze-thaw of the bulk stock. Aliquot first. No vehicle control. The solvent has effects too, and without a control they become the peptide's. Cross-contamination from re-entering the vial without cleaning the stopper or with a used tip or syringe. The preservative slows bacteria; it does not undo a dirty entry. Why this page stops where it does
Searches for "how to reconstitute peptides" are often followed by questions about use on a person. This page does not answer those, and no page on this site will, because research peptides are supplied for laboratory work and not for human use; presenting them otherwise would make them unauthorised medicines under UK law, as our guide to whether peptides are legal in the UK explains. The technique and arithmetic above are the same in any laboratory. What is done with the solution is defined by the experiment, and the experiment is in vitro.
Vivera Labs supplies lyophilised research peptides and bacteriostatic water from UK stock, lists the specification and quantity on every product page, and will publish original batch reports against the matching product as they are received. The reconstitution calculator, molarity calculator and solubility calculator on this site are free and do the arithmetic described here.
Frequently Asked Questions How do you reconstitute a lyophilised peptide? Let the vial reach room temperature, clean the stopper, add the calculated volume of solvent slowly down the inside wall, swirl gently until dissolved, inspect for clarity, label and store. For a stock used over several sessions, sterile water with single-use aliquots frozen at -20°C is the usual laboratory choice; bacteriostatic water is an alternative where the assay tolerates benzyl alcohol. This describes laboratory technique for in-vitro research only.
How do you calculate peptide concentration after reconstitution? Divide the peptide mass by the solvent volume: 10 mg in 2 mL gives 5 mg/mL, which is the same as 5 µg/µL. For molar concentration, divide by the molecular weight: 5 mg/mL of a 1,000 g/mol peptide is 5 mM. If the certificate gives net peptide content, use that fraction of the label mass.
What solvent should be used to reconstitute peptides? Sterile water, with single-use aliquots frozen at -20°C for a stock used over several sessions; bacteriostatic water is an alternative for a refrigerated, re-entered stock where the assay tolerates benzyl alcohol. Poorly soluble peptides may need a small volume of dilute acetic acid, dilute ammonium bicarbonate, or an organic solvent such as DMSO before dilution into buffer, depending on the sequence's charge and hydrophobicity.
How much bacteriostatic water do you add to a peptide vial? The volume that gives the stock concentration your experiment needs: volume equals mass divided by target concentration. A 5 mg vial reconstituted with 1 mL gives 5 mg/mL; with 2 mL, 2.5 mg/mL. The choice is set by the assay and the vial size, and the reconstitution calculator does the arithmetic. There is no correct volume independent of the experiment.
Why should peptides be swirled and not shaken? Shaking creates air-liquid interfaces and shear forces that cause peptides to aggregate and, for some sequences, to unfold. Aggregated peptide gives a lower effective concentration and can behave differently in an assay. Gentle rolling of the vial dissolves the cake without that stress.
How long does a reconstituted peptide last? In bacteriostatic water, refrigerated at 2 to 8°C, the preservative is relied on for 28 days, but the peptide itself may degrade sooner depending on its sequence. For longer storage, aliquot into single-use volumes and freeze once at -20°C or below; never refreeze a thawed aliquot.
Does the peptide powder weigh less than the label says? The label mass is usually gross peptide mass, which includes the counter-ion salt and residual water. Net peptide content, where stated on the certificate of analysis, is commonly 70 to 90% of gross mass. For quantitative work, calculate concentration from net content.
Can this guide be used to prepare peptides for personal use? No. It describes laboratory technique for in-vitro research. Research peptides are not supplied for human use, and presenting them for it would make them unauthorised medicines under UK law. Questions about any medicine belong with a prescriber or pharmacist.
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For in-vitro laboratory research use only. Not for human or veterinary use, consumption, or therapeutic application. No medical claims are made.
