Reconstitution concepts: what happens when a dry peptide meets a solvent

What is in the vial

A lyophilized peptide is what remains after a frozen solution has had its ice removed under vacuum. The solid is left with pores where the ice was, so it is typically a light, porous cake or a thin film on the glass. The process is described in why peptides are lyophilized.

Review How a cake looks does not settle its quality. A commentary by formulation scientists from companies and universities notes that cake appearance may or may not be critical to quality, and that a non-ideal appearance sometimes has no impact on quality and is simply inherent to the formulation, the container and the drying process.⁷

The cake is usually not pure peptide either. Counter-ions and residual water can make up part of its mass, which is why the mass on a label and the mass of peptide in the vial can be different numbers.

What dissolving involves

Chemistry study Dissolving a cake is a sequence of events. A study of freeze-dried protein formulations at high concentration followed them: the liquid wets the solid, enters its pores and breaks it into pieces, and the pieces dissolve.¹ Cakes that were partly crystalline, with larger pores, wetted well and broke into small pieces that dissolved quickly.¹ Amorphous cakes wetted poorly.¹ In some of them the liquid did not enter the cake at all, and an intact lump floated among foam or bubbles and dissolved slowly by erosion from its surface.¹ The time taken tracked the viscosity of the concentrated layer at that surface.¹

Those were concentrated protein formulations, not peptide vials, so the study shows the steps and not the timing.

How sequence shapes solubility

Whether a peptide dissolves, and in what, depends largely on two properties of its sequence: its charge and its share of hydrophobic residues.

Charge

A peptide carries groups that gain or lose a proton as the pH changes, among them the amino group at one end of the chain, the acid group at the other, and the side chains of lysine, arginine, histidine, aspartate and glutamate. The pH at which the positive and negative charges cancel is the isoelectric point, written pI.

Chemistry study The pH at which a molecule is least soluble follows that point. In a study of the enzyme ribonuclease Sa and two engineered variants, with isoelectric points of 3.5, 6.4 and 10.2, the pH of minimum solubility moved with the pI.² The same study warns that a pI estimated from standard values for the ionizable groups can be wrong by more than one pH unit.²

Chemistry study A second study fused short tags of a single amino acid to a small model protein to measure what each residue contributes.⁸ Positively charged residues raised solubility significantly at both pH values tested, 4.7 and 7.7.⁸ Tags of aspartate or glutamate barely changed it at pH 4.7 and raised it eight- to tenfold at pH 7.7, where their side chains are ionized.⁸

Chemistry study Glucagon is one documented case. It is poorly soluble in aqueous buffers at or near physiological pH, and reaches a milligram or more per millilitre only at low or high pH.³ Its commercial preparation is supplied as a lyophilized solid with an acidic diluent.³ When chemists lowered the isoelectric point by replacing one asparagine with aspartate, solubility at physiological pH rose significantly.³

Hydrophobicity

Chemistry study Hydrophobic residues pull the other way. In the tag study, a run of isoleucines markedly reduced the solubility of the host protein.⁸

Review Solubility and aggregation are related questions. A review of peptide medicines lists sequence, concentration, pH and net charge among the factors that govern the association of peptide molecules with one another.⁹

Choosing a solvent

The solvent is chosen to suit the charge of the peptide and the experiment that follows.

Water

The United States Pharmacopeia defines its sterile water article as containing no antimicrobial agent or other added substance.¹⁰ That makes water the plainest choice: nothing enters the solution except the peptide and whatever came with it in the cake.

Chemistry study Whether water alone is enough depends on the charge the peptide carries at the pH that results: in the ribonuclease study, the pH of lowest solubility followed the isoelectric point.²

Dilute acid

Chemistry study A weak acid such as acetic acid lowers the pH. For a peptide rich in basic residues, or one whose isoelectric point lies near neutral pH, the charge studies suggest why that can raise solubility: the solution moves away from the pH at which the molecule is least soluble.² The acidic diluent supplied with glucagon is an instance of it.³ The mirror case holds for acidic residues, which added to solubility only at the higher pH where they were ionized.⁸

Chemistry study Acid has a cost. Held in dilute acid for extended periods or at raised temperature, glucagon formed degradation products.³

Buffers

A buffer holds the pH where the experiment needs it.

Review A review of peptide formulation ranks the choice of pH and of buffer as the most practical ways to keep a peptide stable in water.¹¹ A buffer is not always where a peptide dissolves, as the glucagon case above shows.

Organic co-solvents

A hydrophobic peptide may dissolve in none of the above. In general laboratory practice an organic solvent such as dimethyl sulfoxide (DMSO) is then a common resort: the material is dissolved in a small volume of it, and that solution is diluted into water or buffer.

Cell study DMSO can dissolve a wide variety of otherwise poorly soluble polar and nonpolar molecules, which has made its use ubiquitous, but it is not inert in a cell experiment.⁴ In a retinal neuronal cell line it was toxic at concentrations above 1% by volume.⁴ The authors recommended other ways of dissolving a substance where they exist and, where none does, computing the final DMSO concentration in an assay and including an untreated control as well as a solvent-only control.⁴

Preserved water

Bacteriostatic water is water that carries a preservative. The labelling of one licensed product describes a sterile preparation with 0.9% or 1.1% benzyl alcohol.¹²

Chemistry study The preservative is an ingredient like any other. When a freeze-dried protein was dissolved in water containing 0.9% benzyl alcohol, more of it aggregated than when plain water was used, although benzyl alcohol did not speed aggregation during later storage at room temperature.¹³

Bacteriostatic water explained covers the preservative in detail. The catalogue lists bacteriostatic water among its laboratory supplies.

Concentration: the arithmetic of a stock

A stock solution is a concentrated solution from which working solutions are made by dilution. Its concentration is the mass dissolved divided by the final volume: 2 mg brought to 1.0 mL in a tube is 2 mg/mL.

Dilution keeps the amount and changes the volume, so concentration times volume does not change: C₁ × V₁ = C₂ × V₂.

In the figure, 1 mL at 10 mg/mL made up to 10 mL gives 1 mg/mL. The same factor applies to everything in the stock, so a stock made in pure DMSO and diluted 1 in 200 leaves 0.5% DMSO by volume.

Two cautions apply to every such number. The mass on a label may be the mass of powder, not of peptide, and some of the peptide may not stay in solution, as the section on surfaces explains. Molar concentrations, net peptide content and serial dilutions are worked through in peptide units and concentration.

Agitation and the air surface

Review Agitation is one of the physical conditions that a review of peptide medicines lists among the influences on aggregation, together with surfaces and interfaces.⁹

Chemistry study An experiment with amyloid-beta, a peptide that readily forms fibrils, shows one mechanism. At the lower concentrations tested, a solution left still with no air surface formed no fibrils, and one with an air surface did.⁵ Vortexing or rotating a solution that had an air surface considerably accelerated fibril formation, and the first deposit appeared as a film at that surface.⁵ A polypropylene disc rotated in a solution without an air surface had a similar effect, with the first aggregates at the edge of the disc.⁵

Chemistry study Shaking a solution renews its air surface and can fold air into the liquid as foam. Foam or bubbles also surrounded the poorly wetted protein cakes that floated in the dissolution study.¹

Surfaces: glass and plastic

Chemistry study A dissolved peptide can leave the solution for the wall of its container. A Danish group measured three cationic, membrane-active peptides in commonly used glass and plastic sample containers and found that, at typical experimental concentrations, 90% or more could be lost through rapid adsorption.¹⁴

Chemistry study The loss differs from peptide to peptide. Eight radiolabelled peptide hormones, among them insulin, ghrelin and glucagon-like peptide-1, were left for 48 hours in glass and plastic tubes, plain or coated with a siliconizing fluid.⁶ Recovery differed from surface to surface; siliconizing lowered it, and adding the carrier protein bovine serum albumin raised it.⁶ The authors call the binding of peptides to surfaces unpredictable and conclude that the container has to be chosen for each peptide.⁶

A concentration worked out from mass and volume is therefore a nominal figure, not a measurement of what is actually in solution.

Aliquots and freeze-thaw

An aliquot is one of several small portions into which a solution is divided, so that each portion is thawed once and used once. The reason is the freeze-thaw cycle.

Chemistry study A 2025 study put glucagon solutions through repeated freeze-thaw cycles and followed aggregation by calorimetry.¹⁵ Of the two sugars tested as stabilizers, lactose stabilized the peptide better than trehalose: aggregation began later and then proceeded more slowly.¹⁵ A high ratio of sugar to peptide reduced the tendency to aggregate with either sugar.¹⁵

What freezing does to a solution is set out in peptide stability and degradation. Storage temperatures for dry and dissolved material are in the peptide storage guide.

Solution versus dry

Review Dissolving a peptide exposes it to the reactions that drying held back. A review of peptide formulation states that peptides are often unstable in aqueous solution.¹¹

Chemistry study The solvent chosen for dissolving also sets the chemistry afterwards. Oxytocin in solution degraded at a rate that depended on pH and temperature, slowest at pH 4.5 and fastest at pH 9 among the four values tested.¹⁶

Human evidence

None of the findings reported on this page comes from people or from animals. It is bench chemistry on peptides and proteins, with one finding from cultured cells. This guide describes how a material behaves in a container. It does not describe the preparation of anything for use in a person or an animal.