Glycylmethionine

Glycyl-methionine or Gly-Met is a dipeptide consisting of the amino acids glycine and methionine. It plays a role as a metabolite. It is reverse of Methionylglycine or Met-Gly sequence.

Oxidation
The oxidation of Gly-Met and its reverse sequence, Met-Gly, has differences. Both photooxidation and collision-induced dissociation of Gly-Met happen through electron transfer from either the sulfur atom or the terminal amino group when it is in its uncharged state. This process leads to the formation of α-protons and sulfur-centered cation radicals in Gly-Met.

However, the photooxidation of Met-Gly behaves differently from Gly-Met. The peptide’s conformation plays a crucial role in understanding the oxidation mechanism. In Met-Gly, the process leads to the formation of an open-chain sulfur-centered cation radical. This radical then releases a proton from the N-terminal amino group, resulting in a five-membered cyclic radical structure with a three-electron bond between the sulfur and nitrogen atoms.

Peptides can adopt different conformations—cationic, zwitterionic, or anionic—depending on the solvent and pH. The zwitterionic form of Gly-Met is particularly important as it is responsible for the formation of sulfur-centered radicals. This same mechanism can cause similar damage in proteins.