Crystal structures of the isomeric dipeptides L-glycyl-L-methionine and L-methionyl-L-glycine

The oxidation of methionyl peptides can contribute to increased biological (oxidative) stress and development of various inflammatory diseases. The conformation of peptides has an important role in the mechanism of oxidation and the intermediates formed in the reaction. Herein, the crystal structures of the isomeric dipeptides Gly-Met (Gly = glycine and Met = methionine) and Met-Gly, both C 7 H 14 N 2 O 3 S, are reported. Both molecules exist in the solid state as zwitterions with nominal proton transfer from the carboxylic acid to the primary amine group. The Gly-Met molecule has an extended backbone structure, while Met-Gly has two nearly planar regions kinked at the C atom bearing the NH 3 group. In the crystals, both structures form extensive three-dimensional hydrogen-bonding networks via N—H...O and bifurcated N—H...(O,O) hydrogen bonds having N...O distances in the range 2.6619 (13)–2.8513 (13) Å for Gly-Met and 2.6273 (8)–3.1465 (8) Å for Met-Gly.