Excipients are the non-peptide ingredients in a freeze-dried vial. They bulk out a tiny amount of peptide into a stable cake, protect it while it freezes and dries, and keep it intact in storage. The most common are sugars such as sucrose and trehalose, polyols such as mannitol, amino acids such as glycine, buffers and surfactants. If you work with lyophilised peptides in a UK laboratory, you need to know which of them are in your vial, because they change your working concentrations, your assay results and how the material ages.
TL;DR
- Excipients fall into a few jobs: bulking, cryoprotection, lyoprotection, buffering and surfactant action. Many are listed under more than one role.
- Mannitol is a popular crystalline bulking agent, but sucrose generally protects peptides better in the solid state, so a balanced combination is common.
- An undeclared excipient is a quality problem: mannitol has been found at 20% and 43% w/w in two commercial custom peptides that HPLC alone did not flag.
- Ask for a Certificate of Analysis that declares the full composition, and weigh your material accordingly before calculating molarity.
Table of Contents
- Why peptides are freeze-dried in the first place
- The main excipient classes and what each one does
- Mannitol in peptides: useful, but not neutral
- Why the ratio of peptide to excipient matters
- Can excipients change the result of an experiment?
- A worked example: why net peptide content matters
- How to check what is actually in your vial
- What the choice of excipient means for stability and shelf life
- Our perspective on excipients and research peptides
- Buying lyophilised peptides for the lab
- Frequently asked questions
- Sources
Why peptides are freeze-dried in the first place
Most peptides degrade in water. They deamidate, oxidise, hydrolyse and aggregate, and the rate climbs with temperature. Removing the water by freezing the solution and subliming the ice under vacuum gives a dry solid that can sit in a freezer for far longer than a solution would. Freeze-drying is widely treated as the method of choice for sensitive biomolecules that are not stable enough in solution, a point made in a 2025 mini review of short peptides as excipients.
The process itself is hard on the molecule, though. Freezing creates ice crystals and concentrates solutes. Drying strips away the water shell that holds structure in place. Excipients exist to soften both insults and to give the finished solid a physical form you can handle.
Some peptides are self-buffering and need little or nothing besides the peptide and a volatile counterion, so a lyophilised vial is not automatically a formulated one. That is why it matters to read what a supplier declares rather than assume.
The main excipient classes and what each one does
The classification below follows the mechanisms described in a review of regulatory guidelines for therapeutic peptide and protein analysis. That review groups common choices into sugars (sucrose, trehalose), polyols (mannitol, sorbitol), surfactants (polysorbate 20 and 80) and amino acids (glycine, arginine).
| Role | Typical examples | What it does |
|---|---|---|
| Cryoprotectant | Sucrose, trehalose | Limits damage from ice crystals and concentration effects during freezing |
| Lyoprotectant | Sugars, polyols | Forms an amorphous glassy matrix that restricts molecular mobility while drying and in storage |
| Bulking agent | Mannitol, glycine | Gives mechanical strength and an elegant, intact cake |
| Buffer | Phosphate, acetate, citrate | Holds pH steady in the solution before drying and, to a degree, afterwards |
| Surfactant | Polysorbate 20 or 80 | Reduces surface-induced aggregation and adsorption |
The categories overlap. Mannitol is a polyol that can act as a lyoprotectant in some conditions, yet it is mostly used as a bulking agent because it crystallises. Sucrose stays amorphous, which is exactly why it protects well but gives a weaker cake on its own.
Bulking agents
A research vial may hold only a few milligrams of peptide. On its own that can leave a thin film or a fluffy wisp stuck to the glass. A bulking agent supplies the mass that forms a proper cake. Mannitol and glycine are the usual choices, and a crystalline bulking agent makes the structure robust enough to survive aggressive drying.
There is a catch. According to the same mini review, crystalline bulking agents risk collapse if crystallisation is incomplete, so authors recommend a balanced strategy that pairs an amorphous lyoprotectant such as sucrose with a crystalline bulking agent. That gives both stability and process efficiency.
Cryoprotectants and lyoprotectants
Sugars replace the hydrogen bonds that water would normally form with the peptide, and they set into a glass. A glass is a rigid, disordered solid in which molecules barely move, so degradation reactions slow dramatically. Non-reducing disaccharides, sucrose and trehalose, are the standard choices because they do not undergo Maillard reactions with free amines the way reducing sugars can.
Buffers and surfactants
Buffers fix the pH of the pre-lyophilisation solution, which determines which degradation pathways are active. Surfactants prevent peptide from sticking to vial walls and stop aggregation at interfaces. Both are used at low levels, but low does not mean irrelevant: a few percent of mass still counts when you weigh out a milligram.
Mannitol in peptides: useful, but not neutral
Mannitol is probably the most common reason people search this topic. It is cheap, chemically inert in most respects, and it makes a good-looking cake. Two points deserve attention in a research setting.
It can be less protective than sucrose. A 2005 study in the Journal of Pharmaceutical Sciences looked at asparagine deamidation in model peptides. In the solid state, the peptides deamidated 5 to 30 times faster in mannitol-based formulations than in sucrose-stabilised ones. Crystalline mannitol provides structure but little of the glassy protection that sucrose offers, so a peptide with a deamidation-prone Asn-Gly or similar sequence may fare worse in a mannitol-only cake.
It can be present without being declared. The ScienceDirect paper on NMR analysis of custom synthetic peptides reported mannitol at 20% and 43% w/w in two commercially sourced peptides, with no declaration. Mannitol has no chromophore, so a standard UV-detected HPLC purity run does not see it. A vial can show a clean chromatogram and still be nearly half sugar alcohol by mass.
The practical lesson is not that mannitol is bad. It is that you need to know whether it is there and how much.
Why the ratio of peptide to excipient matters
Formulation work shows that composition drives physical stability. In the amorphous excipients work presented via Pharma Excipients, amorphous disaccharides such as sucrose are prone to collapse when primary drying is pushed at a higher shelf temperature. The reason is the collapse temperature: if the product warms above it during primary drying, the cake loses structure.
The consequences are practical for a bench scientist:
- A collapsed cake looks poor and often takes longer to reconstitute.
- Slow dissolution can mask aggregation or degradation, because you may not notice a problem that is hidden behind a cloudy, slowly clearing solution.
- Storage temperature should stay well below the glass transition temperature (Tg) of the dry product, otherwise the glass softens and degradation speeds up.
The mini review makes these points about Tg and collapse temperature, and a related insulin-sucrose-polymer study examines how added polymers can stabilise amorphous peptide glasses against heat and humidity. For you, the direct takeaway is simple: keep lyophilised material cold and dry, and read our guide to lyophilised peptide storage for the detail.
Can excipients change the result of an experiment?
Yes, and in both directions. In a 2018 study, researchers lyophilised peptide-polymer nano-polyplexes of a MAPKAP kinase 2 inhibitory peptide. With 300 mM lactosucrose present, cellular uptake rose 5.23-fold and bioactivity 2.39-fold compared with samples freeze-dried without excipients. That is a specialised nanoparticle system, so do not expect the same numbers for a simple peptide. But it shows that lyophilisation without protection can wreck function, and that the right sugar can preserve it.
The reverse also holds. If your vial contains a large fraction of an excipient you did not account for, your stock concentration is wrong from the start. Cells, enzymes or receptor-binding assays then run at a lower peptide dose than you intended, and a sugar alcohol at high concentration can add osmotic load or interfere with certain assays.
A worked example: why net peptide content matters
Suppose a vial is labelled 5 mg. If that mass includes 20% non-peptide material (a figure the ScienceDirect paper observed for mannitol in one product), the peptide itself is about 4 mg. You reconstitute in 1 mL expecting 5 mg/mL, but the real concentration is about 4 mg/mL. That is a 20% error before any pipetting variation, and it propagates through every dilution.
Counterions and residual water add to the gap, which is why serious users work from net peptide content or quantify by amino acid analysis or a validated method, not from gross weight alone. Our guide to peptide reconstitution covers calculating concentrations in practice.
How to check what is actually in your vial
HPLC tells you how pure the peptide is relative to other UV-absorbing species. It does not tell you what else is in the vial. A thorough check combines several approaches.
- Read the full composition on the product page and the CoA. A legitimate supplier should name every component, not use a vague phrase like "stabilisers". If excipient concentrations are not given, you cannot calculate working concentrations or judge chemical compatibility.
- Look for orthogonal methods. Mass spectrometry confirms identity. Karl Fischer titration gives water content. NMR can detect non-chromophoric additives such as mannitol, as the custom-peptide study showed.
- Ask for raw data. A chromatogram image is a summary. The underlying file lets someone else check integration and baseline.
- Match the lot. The CoA should carry the batch number printed on your vial.
- Check the cake. An intact, uniform white cake is reassuring. A shrunken, melted-looking or collapsed plug suggests a drying or storage problem, and you should record it and ask the supplier.
For a line-by-line walkthrough of the document itself, see our guide to reading a peptide Certificate of Analysis.
What the choice of excipient means for stability and shelf life
The excipient system shapes how long a peptide lasts. A sucrose or trehalose glass protects against deamidation and aggregation. A fully crystalline mannitol matrix gives a sturdy cake but less molecular-level protection, which is why combinations are common. Amorphous polymers with a high glass transition temperature, such as PVP or PVPVA, are being explored to stabilise more complex systems against moisture and temperature swings, according to the insulin-sucrose-polymer work and the amorphous excipients paper cited earlier.
One trend from this literature is that raising the peptide-to-excipient ratio tends to lower the glass transition temperature and reduce stability. In plain terms, the more peptide you pack in relative to protectant, the less protective the glass becomes. That is a trade-off between dose per vial and robustness, and it is why formulators do not simply use as little excipient as possible.
What you can control as a user is handling. Do not open cold vials in humid air, since moisture lowers Tg and invites degradation. Return them to the freezer promptly. Our overview of how long research peptides last sets out what affects shelf life after purchase.
Our perspective on excipients and research peptides
We supply research peptides, and our focus is on what a laboratory can verify: 99%+ HPLC purity and a Certificate of Analysis you can check against your batch. We have no interest in glossing over the limits of that number. Purity by HPLC describes the peptide relative to its related impurities. It was never a statement about the whole vial.
So our advice to researchers is the advice we would follow ourselves. Treat composition as a first-class specification. Ask what is in the vial, in what amount, and by which method that was determined. If an answer is vague, treat it as a flag. Account for non-peptide mass when you calculate concentrations, and where an assay is sensitive to sugars, salts or buffers, tell your supplier before you order so you can choose the right presentation.
Our peptides are supplied strictly for laboratory and scientific research. They are not for human or veterinary consumption, and the formulation questions in this article are about analytical rigour at the bench, not about preparing anything for administration.
— Neil
Buying lyophilised peptides for the lab
If you are sourcing material, work through a short checklist: composition declared in full, batch-matched CoA, identity confirmed by mass spectrometry, water content reported, and clear storage instructions. Our range of research peptides is supplied with 99%+ HPLC purity and supporting documentation, with a secure checkout. You can see what the research-use-only designation means for your order on our Research Use Only page, and our guide to research use only peptides in the UK explains how the label is meant to be used.
Frequently asked questions
Is mannitol always present in lyophilised peptides?
No. Some peptides are self-buffering and need no excipient, and others use sucrose, trehalose or glycine instead. Mannitol is common as a bulking agent, so check the declared composition. If the product page and CoA do not state it, ask the supplier directly.
Does mannitol show up on an HPLC purity report?
Usually not. Mannitol lacks a chromophore, so a UV-based HPLC method does not detect it. In the NMR study, undeclared mannitol at up to 43% w/w was found in commercial peptides. Techniques such as NMR or a dedicated quantitation method are needed to see it.
Is sucrose better than mannitol for peptide stability?
In the solid state, often yes. A 2005 study found model peptides deamidated 5 to 30 times faster in mannitol formulations than in sucrose ones. Even so, formulators frequently combine the two, using sucrose for protection and mannitol for a strong cake. The best choice depends on the peptide sequence.
Why does a lyophilised cake sometimes collapse?
Collapse happens when the product warms above its collapse temperature during primary drying, or when a crystalline bulking agent has not fully crystallised. It can also develop in storage if the temperature rises towards the glass transition. A collapsed cake often reconstitutes slowly, which can hide aggregation or degradation.
Sources
- NMR reveals an undeclared constituent in custom synthetic peptides, ScienceDirect
- Short Peptides as Excipients in Parenteral Protein Formulations: A Mini Review, PMC
- Regulatory Guidelines for the Analysis of Therapeutic Peptides and Proteins, PMC
- Excipients for the Lyoprotection of MAPKAP Kinase 2 Inhibitory Peptide Nano-Polyplexes, PMC
- Effects of sucrose and mannitol on asparagine deamidation rates of model peptides, J Pharm Sci
- Be Aggressive! Amorphous Excipients Enabling Single-Step Freeze-Drying, Pharma Excipients
- Design of Co-lyophilised Ternary Insulin-Sucrose-Polymer Systems, Pharma Excipients