Ultrafast Pump-Probe Study of Halide Dependence in Primary Reaction Dynamics of Halorhodopsin

Halorhodopsin is a retinal protein in \textit{Haloarchaeal} cell membrane. The light-induced all-trans to 13-cis isomerization of the retinal chromophore triggers unidirectional chloride-ion pump in millisecond timescale. Here, we present pump-probe study of the primary ultrafast dynamics of \textit{Natronobacterium pharaonis} halorhodopsin that contains Cl$^{-}$, Br$^{-}$ or I$^{-}$. All the temporal behaviors of the S$_{1}$ absorption, ground-state bleaching, and stimulated emission consisted of three components, and their time constants showed halide-ion dependency. The $\sim $50-fs component corresponds to the spectral shift of the S$_{1}$ absorption and stimulated emission bands, which is due to the wavepacket motion from the Franck-Condon region, forming the reactive and nonreactive S$_{1}$ states. Referring to previous reports, the $\sim $2-ps component is assignable to the isomerization process from the reactive S$_{1}$ state to the ground-state 13-\textit{cis} form via the conical intersection, while the $\sim $5-ps component to the internal conversion of the nonreactive S$_{1}$ state. Quantitative analysis indicated that the isomerization quantum yield increased in order of Cl$^{-}$, Br$^{-}$ and I$^{-}$. On the basis of the halide-ion dependence observed, we discuss the relation between the initial halide-ion pump process and the isomerization mechanism.