Infrared spectra of outer and cytoplasmic membranes of Escherichia coli

Abstract

Outer and cytoplasmic membranes of Escherichia coli were prepared by a method based on isopyenic centrifugation on a sucrose gradient. The infrared spectra of solid films of these membranes were studied. The cytoplasmic membrane had an amide I band at 1657 cm −1 and an amide II band at 1548 cm −1. The outer membrane had a broad amide I band at 1631–1657 cm −1 and an amid II band at 1548 cm −1 with a shoulder at 1520–1530 cm −1. Upon deuteration, the amide I band of the cytoplasmic membrane shifted to 1648 cm −1, whereas the band at 1631 cm −1 of the outer membrane remained unchanged. After extraction of lipids with chloroform and methanol, the infrared spectra in the amide I and amide II regions of both membranes remained unchanged. Although the outer membrane specifically contained lipopolysaccharide, this could not account for the difference in the infrared spectra of outer and cytoplasmic membranes. It is concluded that a large portion of proteins in the outer membrane is a β-structured polypeptide, while this conformation is found less, if at all in the cytoplasmic membrane.