Peptide Storage: A Laboratory Reference

This reference covers one thing: keeping lyophilized (freeze-dried) peptide material in good condition in its sealed vial, as supplied. It does not cover solutions or preparation of any kind. A certificate of analysis describes material as it stood on the report date (see reading a certificate of analysis); storage conditions influence how that material changes afterwards.

At a glance

Why a dry solid still changes

Freeze-drying removes most of the water that drives peptide degradation, which is why peptides are kept and shipped as a lyophilized solid. It slows chemical change; it does not stop it. Lai and Topp (1999) reviewed the reactions that continue in solid peptides and proteins (deamidation, peptide-bond cleavage, oxidation, the Maillard reaction, beta-elimination and dimerization/aggregation) and discussed the factors that influence them, temperature and moisture content among them.

The sequence matters too. Bachem's handling guidelines single out peptides that contain asparagine, glutamine, methionine, cysteine or tryptophan as having limited shelf lives. The peptide-related impurities catalogued by D'Hondt et al. (2014) include oxidized side chains and the products of degradation routes such as beta-elimination and diketopiperazine, pyroglutamate and succinimide formation.

Temperature

• Long-term: −20 °C or colder. Bachem's guidelines put long-term holding of the lyophilizate, in a tightly closed container, below −15 °C, with lower temperatures preferred.
• Short-term: 2–8 °C, a standard laboratory refrigerator. The same guidelines describe refrigerator storage at 4 °C as acceptable for the short term. For anything longer, hold at −20 °C or colder.
• Room temperature: acceptable in transit (the same guidelines state that lyophilized peptides may be shipped at room temperature), but not a holding condition.

Light

Kerwin and Remmele (2007) reviewed photodegradation of proteins and identified tryptophan, tyrosine, phenylalanine and cysteine/cystine as the residues that undergo primary photooxidation. The same residues occur in many peptide sequences. Keep vials in a closed box, drawer or freezer, out of direct light, whether the glass is clear or amber.

Moisture and desiccation

Lyophilized peptides tend to be hygroscopic: the solid takes up water from the air. Bachem's guidelines note that absorbed moisture reduces the overall peptide content and may also decrease stability, and Lai and Topp (1999) list moisture content among the factors that influence solid-state degradation. The vial's intact seal is the primary barrier, so vials stay sealed in storage. For longer holding, a closed secondary container (a box or a sealable bag) with a desiccant pack adds a second barrier against humidity.

Freeze-thaw and the sealed vial

Moving a sealed vial between cold storage and room temperature is the dry solid's version of a freeze-thaw cycle. Each move is a temperature excursion, and a cold vial collects condensation on its surface as it warms. Two habits limit both:

• Move vials between cold storage and room temperature as rarely as possible, and keep them at one stable temperature in between.
• Leave a cold vial sealed until it has reached room temperature. Bachem's guidelines recommend that a peptide container reach ambient temperature in a desiccator before it is opened, because the solid takes up moisture from the air.

Shipping and receiving

PepsMartUSA orders ship from the USA at ambient temperature, sealed, without cold packs. That is consistent with the guidance above: room temperature is acceptable in transit, and moisture is the risk the packaging is built around (see shipping & guarantee). On arrival:

1. Check that each cap and seal is intact and the contents look dry. If a seal is broken, a vial is damaged or the contents look wet, keep the vial and contact us with the order number and the lot number.
2. Note the lot number on the vial label. Entering it in the lot lookup opens the certificate published for that lot.
3. Move the vials into cold storage, sealed and in the dark: −20 °C for long-term holding, 2–8 °C for a short hold.

Storage in short form, and the storage line on a certificate

In short. −20 °C long term, 2–8 °C short term, Protect from light: the freezer for long-term holding, a refrigerator for a short hold, and dark in both cases.

The certificate. The AxisPharm certificates in the COA library have a "Sample Received" block with a receipt date, a receiving condition and a storage condition. That storage condition varies between certificates (room temperature on some, −20 °C on others, another temperature written in on a few). It sits with the receipt details, so read it as a record of how the laboratory held that sample, not as a storage recommendation.

Most certificates also print MFD and BBD (manufacture and best-before) dates in the sample description, next to the lot number. The certificates report no stability study, so a BBD is the date given for that lot, not a shelf life measured under a stated storage condition. What a certificate does report (identity by LC-MS, UV purity, the measured amount of material and, for most lots, endotoxin) describes the sample on its report date.