Reaction Dynamics of Halorhodopsin Studied by Time-Resolved Diffusion

Reaction dynamics of a chloride ion pump protein, halorhodopsin (HR), from Natronomonas pharaonis (N. pharaonis) (NpHR) was studied by the pulsed-laser-induced transient grating (TG) method. A detailed investigation of the TG signal revealed that there is a spectrally silent diffusion process besides the absorption-observable reaction dynamics. We interpreted these dynamics in terms of release, diffusion, and uptake of the Cl⁻ ion. From a quantitative global analysis of the signals at various grating wavenumbers, it was concluded that the release of the Cl⁻ ion is associated with the L2 → (L2 (or N) ⇆ O) process, and uptake of Cl⁻ occurs with the (L2 (or N) ⇆ O) → NpHR′ process. The diffusion coefficient of NpHR solubilized in a detergent did not change during the cyclic reaction. This result contrasts the behavior of many photosensor proteins and implies that the change in the H-bond network from intra- to intermolecular is not significant for the activity of this protein pump.