How long do reconstituted peptides last?

A sealed vial of lyophilized peptide can keep for a long time. The moment liquid is added, that changes. This is one of the most common questions about research peptides, and the honest answer is that there is no single number that applies to every compound. There are, however, clear principles and a widely used reference point.

Why dissolving shortens the life of a peptide

In a dry powder, the reactions that break peptides down happen extremely slowly, because there is almost no water for them to happen in. In solution, water allows several kinds of degradation to proceed:

• Hydrolysis — water splits the bonds between amino acids.
• Oxidation — some amino acids, such as methionine, cysteine and tryptophan, react with oxygen. Light and air speed this up.
• Deamidation — certain amino acids gradually change their chemical structure.
• Aggregation — peptide molecules stick together into clumps, which can make a solution cloudy.

Each compound is vulnerable to these to a different degree, depending on its sequence. That is why stability varies from one peptide to another.

The factors you control

Temperature. Refrigeration at 2–8 °C slows every one of these reactions. A solution kept at room temperature degrades far faster than one kept in the fridge.

Light. Keeping the vial in its box or wrapped in foil limits light-driven oxidation, which matters most for light-sensitive compounds such as Semax.

The liquid used. A solution made with bacteriostatic water contains a preservative that limits bacterial growth each time the stopper is punctured. A solution made with sterile water has no such protection and should be treated as single-use or kept for a much shorter time. See bacteriostatic water vs sterile water.

How it is handled. Every puncture of the stopper is a chance for contamination. Clean technique and a fresh swab each time matter.

Freezing and thawing. Repeatedly freezing and thawing a solution damages peptides. Reconstituted vials are normally kept refrigerated, not frozen.

The 28-day reference point

Pharmacy standards generally set a maximum of 28 days for a multiple-dose container once it has been opened, unless the manufacturer specifies a different period. That figure is about limiting the risk of contamination in a vial used repeatedly, not a guarantee of chemical stability. In practice it is widely used as the outer limit for a refrigerated vial reconstituted with bacteriostatic water.

Some compounds degrade noticeably sooner than that. Others are more robust. Where we have compound-specific information, it appears on that compound's product page.

Signs a solution may have degraded

• It has turned cloudy or hazy
• Particles or strands are visible
• The colour has changed in a way you would not expect for that compound
• It has been left out of the fridge for an extended period, or frozen and thawed

Any of these is a reason not to rely on the vial. A solution can also lose potency without any visible change, which is why the date it was mixed matters.

Good practice

• Write the date of reconstitution and the volume added on the vial.
• Reconstitute only what you expect to use within the vial's useful life.
• Keep sealed vials sealed until they are needed.