Optical properties of an outer membrane lipoprotein from Escherichia coli

The infrared spectrum of a structural lipoprotein from the Escherichia coli outer membrane indicated the lipoprotein had an alpha-helical conformation but no sign for the existence of beta-structures. From circular dichroism spectra of the lipoprotein, the alpha-helical content of the protein was found to be as high as 88% in 0.01-0.03% sodium dodecyl sulfate in the presence of 10(-5) M Mg2+ at pH 7.1 and 23 degrees C. When sodium dodecyl sulfate concentration increased higher than 0.1%, the alpha-helical content of the lipoprotein decreased to about 57%. Divalent cations, such as Mg2+ and Mn2+, were found to increase the helical content of the lipoprotein. The high alpha-helical content of the lipoprotein was observed in a wide range of temperatures (23 to 55 degrees C). The significance of the high alpha-helical content of the lipoprotein is discussed in light of the three-dimensional molecular models of the lipoprotein proposed previously.