Time-resolved Fourier transform infrared (FTIR) difference spectra of the halorhodopsin (hR) photocycle have been collected from 3μs to 100 ms in saturating concentrations of KCI or KBr. Kinetic analysis of these data revealed two decay processes, with time constants of τ1≃150 μs and τ2≃16 ms in the presence of either halide, with τ2 describing the return to the starting (hR) state. Comparison to previous low-temperature FTIR spectra of hR intermediates confirms that characteristic hK and hL spectral features are both present before the τ1 decay, in a state previously defined as hKL (Dioumaev, A., and M. Braiman. 1997. Photochem. Photobiol. 66:755-763). However, the relative sizes of these features depend on which halide is present. In Br- the hL features are clearly more dominant than in Cl-. Therefore, the state present before τ1 is probably best described as an hKL/hL1 equilibrium, instead of a single hKL state. Different halides affect the relative amounts of hKL and hL1 present, i.e., Cl- produces a much more significant back-reaction from hL1 to hKL than does Br-. The halide dependence of this back-reaction could therefore explain the halide selectivity of the halorhodopsin anion pump.
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