Charge displacements during the photocycle of halorhodopsin

Halorhodopsin (hR)-containing, tween-washed membrane fragments were oriented by an electric field and immobilized in a polyacrylamide gel. Using this preparation, the photoelectric response of hR under various conditions was measured with a time resolution of 0.5 μs. At NACl or Na 2SO 4 concentrations below 25 mM a strong reduction of the signal was observed accompanied by bleaching of the sample. At 50 mM Cl −, four exponential components were distinguished with lifetimes of 0.5μs, 2 μs, 0.6 ms and 4 ms. The faster three components did not show a [Cl −] dependence. However, the slowest component only appeared in the presence of Cl −. We tentatively assign the fastest, non-resolved part of the signal (0.5 μs) to the primary charge separation in hR, and the two subsequent components (2 μs, 0.6 ms) to the relaxation of the dipole created in the first process. The additional component (4 ms), which only appears in the presence of Cl −, probably reflects the transport of Cl − ions. For comparison, an optical investigation was performed on the same membranes as used for the electric measurements. An approximate correlation between electric and optical signals was established.