Stability and Degradation
What actually destroys a peptide: heat, light, freeze-thaw, oxidation and time.
A peptide is not a stable small molecule. It is a chain of amino acids held together by bonds that water, heat, oxygen and light all attack, by different routes and at different rates depending on the sequence. A peptide rich in asparagine degrades differently from one rich in methionine, which is why there is no single shelf-life figure that applies to the category.
The dominant variable is whether water is present. Freeze-dried powder is stable for years; the same peptide in solution has a life measured in weeks. Everything else - refrigeration, light protection, avoiding freeze-thaw - modifies that basic fact rather than replacing it.
This section covers the specific chemistry of how peptides break down, what a lyophilised cake tells you about how it has been handled, and why a temperature excursion in transit spends part of a product's stability budget even when it arrives looking perfect.