Halide dependence of the halorhodopsin photocycle as measured by time-resolved infrared spectra

Time-resolved Fourier transform infrared (FTIR) difference spectra of the h alorhodopsin (hR) photocycle have been collected from 3 mus to 100 ms in sa turating concentrations of KCl or KBr, Kinetic analysis of these data revea led two decay processes, with time constants of tau (1) similar or equal to 150 mus and tau (2) similar or equal to 16 ms in the presence of either ha lide, with tau (2) describing the return to the starting (hR) state. Compar ison to previous low-temperature FTIR spectra of hR intermediates confirms that characteristic hK and hL spectral features are both present before the tau (1) decay, in a state previously defined as hK, (Dioumaev, A., and M. Braiman. 1997. Photochem. Photobiol. 66:755-763). However, the relative siz es of these features depend on which halide is present. In Br-, the hL feat ures are clearly more dominant than in Cl-. Therefore, the state present be fore tau (1) is probably best described as an hK(L)/hL(1) equilibrium, inst ead of a single hK(L) state. Different halides affect the relative amounts of hK(L) and hL(1) present, i.e., Cl- produces a much more significant back -reaction from hL(1) to hK(L) than does Br-. The halide dependence of this back-reaction could therefore explain the halide selectivity of the halorho dopsin anion pump.