THE INITIAL REACTION DYNAMICS OF THE LIGHT-DRIVEN CHLORIDE PUMP HALORHODOPSIN

The light-induced dynamics of halorhodopsin has been studied by transi ent absorption spectroscopy with a time resolution better than 100 fs. Similar to the process known from bacteriorhodopsin a fast initial pr ocess with a time constant of 170 +/- 70 fs occurs indicating motions of the retinal chromophore on the excited-state potential energy surfa ce. The decay of the excited electronic state is biexponential with ti me constants of 1.5 +/- 0.7 and 8.5 +/- 1.5 ps. The formation of the f irst red-shifted ground-state photoproduct occurs on the same timescal e. A qualitative model for the primary reactions is proposed, which ex plains the lower quantum yield phi of about 0.3 in halorhodopsin compa red to 0.6 in bacteriorhodopsin.