The initial reaction dynamics of the light-driven chloride pump halorhodopsin

The light-induced dynamics of halorhodopsin has been studied by transient absorption spectroscopy with a time resolution better than 100 fs. Similar to the process known from bacteriorhodopsin a fast initial process with a time constant of 170 ± 70 fs occurs indicating motions of the retinal chromophore on the excited-state potential energy surface. The decay of the excited electronic state is biexponential with time constants of 1.5 ± 0.7 and 8.5 ± 1.5 ps. The formation of the first red-shifted ground-state photoproduct occurs on the same timescale. A qualitative model for the primary reactions is proposed, which explains the lower quantum yield Φ of about 0.3 in halorhodopsin compared to 0.6 in bacteriorhodopsin.