This study represents the first physicochemical analysis of the recently cloned methionine repressor protein (Met aporepressor) from Escherichia coli. Infrared spectrometry was used to investigate the secondary structure and the hydrogen-deuterium exchange behavior of the E. coli Met aporepressor. The secondary structure of the native bacterial protein was derived by analysis of the amide I mode. The amide I band contour was found to consist of five major component bands (at 1625, 1639, 1653, 1665, and 1676 cm⁻¹) which reflect the presence of various substructures. The relative areas of these component bands are consistent with a high α-helical content of the peptide chain secondary structure in solution (43%) and a small amount of β-sheet structure (7%). The remaining substructure is assigned to turns (10%) and to unordered (or less ordered) structures (40%). The temperature dependence of the infrared spectra of native Met aporepressor in D₂0 medium over the temperature interval 20-80ºC indicates that there are two discrete thermal events: the first thermal event, centered at 42ºC, is associated with the hydrogen-deuterium exchange of the hard-to-exchange a-helical peptide bonds accompanied by a partial denaturation of the protein, while the second event, centered around 50ºC, represents the irreversible thermal denaturation of the protein.