Further Characterization of Protein Secondary Structures in Purple Membrane by Circular Dichroism and Polarized Infrared Spectroscopies

The conformation and the orientation of the protein secondary structures in purple membrane was analyzed by infrared absorption and linear dichroism of oriented membranes as well as by UV circular dichroism of bacteriorhodopsin in intact purple membrane and in lipid vesicles. A large amount (74 ± 5%) of transmembrane α-helices is detected with no significant contribution of β-sheet strands running perpendicular to the membrane plane. Thus, these data do not support the recent structural model proposed by Jap et al. (Biophys. J. 1983, 43:81-89).