Most peptide degradation in a well-run lab does not happen in the freezer. It happens in the minutes a stock spends freezing and thawing, and in the repeated trips a single vial makes between the bench and the -20 C shelf. Understanding freeze-thaw cycles, and aliquoting peptide stocks so that each portion is thawed only once, is one of the simplest ways to keep in-vitro data consistent. This article explains what physically happens when a peptide solution freezes, which variables make it worse, and a practical aliquoting workflow.

What happens as a solution freezes

Water crystallizes first, as pure ice. Everything dissolved in it, including the peptide, buffer salts, counterions and any preservative, is pushed into the shrinking volume of liquid between the crystals. This freeze-concentration can raise local solute concentrations many-fold before the remaining liquid finally solidifies. Reviews of freezing and lyophilization describe the resulting stresses: high local concentration, altered pH and ionic strength, and extensive ice-liquid interfaces where molecules can adsorb and unfold [4].

For peptides, which lack the folded structure of larger proteins, the main consequences are faster chemical reactions in the concentrated liquid phase and a higher chance of self-association. Thawing replays the process in reverse, and slow thawing prolongs the time spent in the concentrated state.

Buffer choice: the phosphate pH trap

Some buffer salts crystallize during freezing, which removes one component of the buffer pair from solution and shifts the pH of what remains. A study of sodium phosphate buffers found that crystallization of disodium phosphate produced abrupt pH drops shortly after ice formation began [1]. Solutions at 50 and 100 mM with a starting pH of 7.4 reached about pH 4.2 at -10 C, while an 8 mM solution reached about pH 5.2. Lower buffer concentration and lower starting pH both reduced the shift.

A peptide that is stable at pH 7.4 may therefore spend part of every freeze at pH 4 or 5, which can favour different degradation routes (see deamidation and other peptide degradation pathways). Practical responses include using lower buffer concentrations for frozen stocks, choosing buffers with smaller freezing pH shifts, or freezing in water and adding buffer after thawing.

Containers and interfaces

The container matters more than is often assumed. A freeze-thaw study of a monoclonal antibody varied pH, salt, protein concentration, cooling and warming rates, and container type [2]. Aggregation increased at lower pH, but the effect of container material was especially pronounced: samples in ordinary plastic or glass formed little aggregate, whereas samples in Teflon and certain commercial freezing containers formed significantly more [2]. The study was on a protein, but the underlying mechanism, adsorption and unfolding at surfaces during freezing, applies to many peptides.

Surface adsorption is also a problem at low concentrations independent of freezing. In a study of eight radiolabelled endocrine peptides, the fraction retained in solution varied widely between glass and plastic tubes, siliconizing the surfaces made retention worse, and 1% bovine serum albumin improved it; lyophilizing with BSA in the best-suited tube type kept more than 89% of each peptide [3]. Freezing dilute stocks in tubes with high binding capacity compounds both losses.

The role of the solvent and preservative

Bacteriostatic water contains 0.9% benzyl alcohol as a preservative, which suits a vial that is punctured repeatedly at the bench. For frozen single-use aliquots, that preservative function is less relevant, and benzyl alcohol has been reported to promote aggregation of some proteins. In one study, reconstituting a lyophilized interleukin-1 receptor antagonist with 0.9% benzyl alcohol caused more aggregation than reconstituting it with water, and the effect was larger when the protein's structure had been perturbed during drying [5]. Whether that matters for a given short peptide is sequence-dependent, but it is a reason to choose the solvent for frozen stocks deliberately. Our overview of reconstitution solvents for peptide research compares the options.

An aliquoting workflow

  1. Plan the aliquot size. Work out the volume one experiment uses, add a small margin, and make that the aliquot volume.
  2. Dissolve once, at a practical concentration. More concentrated stocks lose a smaller fraction to surfaces; dilute to working concentration on the day of use.
  3. Use low-binding tubes of a material suited to the peptide, with tight caps to prevent sublimation and evaporation.
  4. Dispense quickly and keep cold. Work on ice and cap tubes promptly.
  5. Freeze fast. Snap-freezing in dry ice or liquid nitrogen shortens the time in the freeze-concentrated state compared with slow freezing in a -20 C freezer.
  6. Store at -80 C where possible. Avoid frost-free freezers, whose automatic defrost periodically warms the contents.
  7. Label completely. Peptide, lot number, concentration, solvent, date and aliquot number.
  8. Thaw once, then discard the remainder. Thaw quickly, mix gently, spin briefly to collect the liquid, and do not refreeze.

Keeping track of aliquots

A box of identical tubes becomes a liability if nobody knows which were made from which lot, or which have already been thawed. A simple aliquot log, kept with the freezer map, should record the parent lot, the date the stock was prepared, the solvent and concentration, the number of aliquots made and each one's withdrawal date. When an unexpected assay value appears, the log lets you check whether the aliquot came from an older stock, a different lot or a tube that was accidentally thawed. The same principle underpins lot-level traceability in general, discussed in lot numbers and traceability in peptide research.

When refreezing cannot be avoided

Sometimes a stock must be refrozen. In that case, record the number of freezing events on the tube, keep the count low, and run a quick check such as an HPLC analysis against the original chromatogram before relying on a heavily reused stock. Comparing with the lot's original data, such as that on our lab reports page, shows whether new peaks or a smaller main peak have appeared. A simple validation, freezing and thawing a test aliquot three to five times and analysing it, tells you how tolerant a particular peptide and solvent combination is.

Key takeaways

  • Freezing concentrates solutes between ice crystals and creates large ice-liquid interfaces.
  • Sodium phosphate buffers can drop from pH 7.4 to about pH 4 during freezing as disodium phosphate crystallizes.
  • Container material and surface adsorption strongly influence losses and aggregation.
  • Choose the solvent for frozen stocks deliberately; a preservative designed for repeated vial access is not required for single-use aliquots.
  • Aliquot to single-experiment volumes, freeze fast, store at -80 C, thaw once and discard the remainder.

This article summarizes published research for informational purposes. All Ascent Sciences products are for laboratory research use only and are not for human or animal consumption.

References

  1. Gómez G, Pikal MJ, Rodríguez-Hornedo N. Effect of initial buffer composition on pH changes during far-from-equilibrium freezing of sodium phosphate buffer solutions. Pharm Res. 2001;18(1):90-97. PubMed
  2. Kueltzo LA, Wang W, Randolph TW, et al. Effects of solution conditions, processing parameters, and container materials on aggregation of a monoclonal antibody during freeze-thawing. J Pharm Sci. 2008;97(5):1801-1812. PubMed
  3. Goebel-Stengel M, Stengel A, Taché Y, et al. The importance of using the optimal plasticware and glassware in studies involving peptides. Anal Biochem. 2011;414(1):38-46. PubMed
  4. Wang W. Lyophilization and development of solid protein pharmaceuticals. Int J Pharm. 2000;203(1-2):1-60. PubMed
  5. Roy S, Jung R, Kerwin BA, et al. Effects of benzyl alcohol on aggregation of recombinant human interleukin-1-receptor antagonist in reconstituted lyophilized formulations. J Pharm Sci. 2005;94(2):382-396. PubMed

Frequently asked questions

Why is repeated freezing and thawing harmful to peptide solutions?

Freezing concentrates the peptide and buffer salts into shrinking pockets of liquid between ice crystals, can shift pH sharply when buffer components crystallize, and creates large ice-liquid interfaces. Each of these can promote aggregation and chemical degradation.

Can sodium phosphate buffer change pH when frozen?

Yes. In one study, 50 and 100 mM sodium phosphate solutions starting at pH 7.4 fell to about pH 4.2 at -10 C because disodium phosphate crystallized during freezing.

What volume should aliquots be?

Each aliquot should hold what one experiment consumes, so that a tube is thawed once and any remainder is discarded rather than refrozen.

Is bacteriostatic water needed for frozen aliquots?

Not usually. Its benzyl alcohol preservative is intended to limit microbial growth in a vial that is accessed repeatedly. Single-use frozen aliquots are often prepared in sterile water or buffer instead, depending on the assay.

All Ascent Sciences products are for laboratory research use only and are not for human or animal consumption. This article summarizes published research and is not medical advice. See our Research Use Agreement.