Which residues
Tryptophan, tyrosine and phenylalanine absorb in the ultraviolet — the same property that makes them detectable during chromatography also makes them susceptible. Absorbed energy can drive photo-oxidation, particularly of tryptophan, and can promote free-radical reactions affecting neighbouring residues.
Cysteine is also implicated, since light can promote disulfide rearrangement.
What exposure does
Photo-degradation produces modified species that may be chromatographically similar to the parent compound and are not always obvious. Unlike thermal degradation, there is often no visible change to indicate it has happened.
Practical handling
Vials are supplied in outer boxes and are best kept there. Amber glass and opaque packaging exist for this reason rather than for appearance. Bench lighting over a working day is a smaller exposure than direct sunlight, but neither is neutral.
Research use only. This page is explanatory and does not recommend any use, quantity or procedure. REVIVE LAB supplies reference materials for laboratory research. These are not medicines and are not for human or veterinary consumption. Anyone considering the use of any compound in humans should do so only under qualified medical supervision.