What happens
The sulfur in a methionine side chain reacts with oxygen to form methionine sulfoxide, adding 16 daltons to the molecular mass. That mass shift is detectable by mass spectrometry, which is how oxidation is usually identified.
Cysteine oxidises too, forming disulfide bonds either within a chain or between chains. Inter-chain bonds produce dimers and higher aggregates.
What drives it
Dissolved oxygen, trace metal ions that catalyse the reaction, light, and elevated pH all accelerate oxidation. Peroxide impurities in solvents are another route, which is one reason solvent quality matters.
What limits it
Keeping material dry and cold, protected from light, and limiting exposure to air. Each vial entry admits fresh oxygen, so repeated entry accelerates the process in a way continuous cold storage does not.
Consequence
An oxidised peptide is a different molecule. Whether that matters depends entirely on the experiment, but it is a real change to what is in the vial and not merely a cosmetic one.
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.