Secondary structure of halorhodopsin.

Ultraviolet circular dichroism (CD) spectroscopy in the interval from 190 to 240 nm has been used to estimate the secondary structural content of halorhodopsin (hR), a light-driven chloride pump isolated from the membranes of Halobacterium halobium. Least-squares curve fitting of the CD spectrum for hR solubilized with octyl glucoside yields an alpha-helical content of approximately 50% and a beta-structure content of approximately 30%. The CD spectrum of hR is unaffected by the absence or presence of chloride ions or by the ionic strength of the medium. The CD spectrum of halorhodopsin is very similar to that of bacteriorhodopsin, indicating that these light-driven pumps possess nearly identical fractions of alpha- and beta-secondary structures.