Lyophilised peptides: reconstitution and solution handling

Synthetic peptides for research are usually supplied as a dry, freeze-dried solid, often called a lyophilised cake or powder. Before use in an experiment, the solid has to be dissolved. From that moment, the clock on its stability starts to run. This article explains why lyophilisation is used, what degrades peptides in solution, and the general laboratory principles behind accurate and stable stock solutions.

This article is written for laboratory work. It contains no dosing information and no instructions for administration to people or animals.

Why peptides are freeze-dried

Many peptides and proteins are chemically and physically unstable in water. To achieve an acceptable shelf life, they are often converted into solid form, and the most common method is lyophilisation, or freeze-drying^[1]. Water is removed in the solid state, which slows hydrolysis and other water-dependent reactions^[2].

The process has three broad stages^[3]:

1. Freezing. The solution is frozen, separating water as ice from a concentrated mixture of peptide and other solutes.
2. Primary drying. Ice is removed by sublimation under reduced pressure.
3. Secondary drying. Much of the remaining bound water is removed by desorption at a higher temperature.

Each stage has to be controlled. The product must stay below its collapse temperature during primary drying, or the cake loses its structure^[3]. Freezing and drying themselves impose stresses that can damage proteins, and a dried product may still have limited long-term stability^[1]. Lyophilisation reduces degradation; it does not abolish it.

What degrades peptides

Peptide instability is usually divided into chemical and physical pathways^[4][5].

Chemical instability

• Oxidation. Methionine, cysteine, histidine, tryptophan and tyrosine are the residues most susceptible to oxidation. It can be triggered by contaminating oxidants, catalysed by trace metal ions or induced by light, and it is influenced by pH, temperature and buffer composition^[6].
• Deamidation and isomerisation. Asparagine and aspartic acid residues can form a cyclic succinimide intermediate. This can lead to deamidation, isomerisation and racemisation. In one model peptide, the asparagine residue deamidated with a half-life of only 1.4 days at 37 °C and pH 7.4. The rate depended strongly on the neighbouring residue^[7].
• Backbone cleavage and other reactions, including proteolysis, β-elimination and racemisation^[4].

Physical instability

• Aggregation. Peptides can self-associate into amorphous aggregates or highly ordered fibrils. Relevant factors include sequence, concentration, pH, net charge, excipients, surfaces, temperature, agitation and lyophilisation^[8].
• Interfacial stress. Air–water and other interfaces are a major trigger of aggregation in solution^[9]. This is one reason vigorous shaking is avoided.
• Freeze–thaw damage. In studies of model proteins, the rates of freezing and thawing affected how much activity was lost, and recrystallisation during thawing added further damage^[10].
• Adsorption to container surfaces, discussed below.

Moisture and temperature

Residual moisture matters even in a dry cake. In a study of a lyophilised antibody formulation stored for up to a year, higher moisture reduced chemical stability. The effect on physical stability depended on whether storage was below the formulation's glass transition temperature^[11]. The glass transition temperature fell from 80 °C at 1% moisture to 25 °C at 8% moisture^[11], which shows how sensitive a dried product can be to water.

This is the reasoning behind a common laboratory practice: letting a refrigerated or frozen vial reach room temperature before opening it. Opening a cold vial draws in humid air, which can condense on the cold solid.

Lower temperatures generally slow degradation, but they introduce their own risks. Solutions that are frozen and thawed repeatedly are exposed to freeze–thaw stress each time^[10]. For this reason, stock solutions are often divided into single-use portions before freezing.

Choosing a solvent

No single solvent suits every peptide. The choice depends on the sequence, its net charge, its solubility and the intended experiment. Some general considerations apply:

• pH and buffer. Both affect oxidation and aggregation rates^[6][8], so the solvent's pH should be compatible with the peptide's stability, not only with its solubility.
• Preservatives. Phenol and benzyl alcohol are the antimicrobial preservatives most often used in licensed peptide and protein products^[12]. They are not inert. Reconstituting a lyophilised protein with 0.9% benzyl alcohol caused more aggregation than reconstitution with water. The extent depended on how well the protein's structure had been preserved during freeze-drying^[13]. A preservative suited to one molecule may be unsuitable for another.
• Sterility. Where a solution must remain free of microbial contamination, laboratories use sterile diluents, sterile consumables and aseptic technique, in line with local procedures.

Dissolving the solid

Reconstitution is not always instant. For concentrated lyophilised protein formulations, reconstitution time depended on the structure of the cake, its crystallinity and pore size, and the viscosity near the dissolving surface. Some amorphous cakes floated amid foam without being penetrated by the liquid^[14]. Gentle swirling or inversion is generally preferred to vigorous shaking or vortexing, because agitation and air–liquid interfaces promote aggregation^[9]. A solution that remains cloudy or contains visible particles has not simply "not dissolved yet". It may indicate aggregation or poor solubility in the chosen solvent.

Adsorption: losing peptide to the container

Peptides can bind to the walls of tubes and vials, sometimes to a striking degree. For three cationic membrane-active peptides, 90% or more could be lost from solution at typical experimental concentrations, through rapid adsorption to glass and plastic^[15]. A study of eight radiolabelled endocrine peptides found large differences in recovery between surfaces. Siliconisation reduced recovery, while adding a carrier protein improved it^[16]. The authors concluded that choosing the right container avoids unpredictable losses that lead to inaccurate measurements and false conclusions^[16]. Losses are greatest in dilute solutions, where a fixed amount of adsorbed peptide is a larger fraction of the total.

Laboratory concentration calculations

Accurate concentrations begin with understanding what the weighed solid contains. A lyophilised synthetic peptide is rarely pure peptide by mass. It can contain counter-ions such as trifluoroacetate left over from synthesis and purification, together with residual water and peptide-related impurities^[17]. Peptide specifications therefore distinguish identity, purity and assay, meaning the actual content of the substance^[18]. Where the net peptide content has been determined, calculations should use it rather than the gross weight of powder.

The arithmetic is general and applies to any solute:

Two further points avoid common errors. First, the final volume of a solution is not always exactly the volume of solvent added, particularly at high concentrations. Second, converting between units, such as mg/mL to µg/µL (which are equal), or mM to µM (a factor of 1,000), is a frequent source of tenfold and thousandfold mistakes. Recording every value with its units reduces this risk.

Stability in solution

Once dissolved, a peptide is exposed to all the water-dependent pathways described above. How long a solution remains usable depends on the sequence, solvent, pH, concentration, container and temperature. It can only be established by measurement, for example by chromatographic analysis of purity over time. The same analytical methods used to characterise a peptide on receipt can show whether a stored solution has changed. Labelling each stock with its identity, concentration, solvent, date and storage condition makes such checks possible.

Summary

Lyophilisation removes water and slows the chemical and physical reactions that degrade peptides, but it does not stop them. Once a peptide is dissolved, oxidation, deamidation, hydrolysis, aggregation and adsorption all become possible, influenced by pH, temperature, interfaces, container materials and additives such as preservatives. Accurate stock solutions depend on knowing the net peptide content, careful unit handling and suitable containers. Stability in solution varies from peptide to peptide and should be established by measurement rather than assumed.