Abstract
The photochemical cis-trans isomerization of the visual pigment analog isorhodopsin is investigated using femtosecond pump-probe techniques. The 9-cis-retinal protonated Schiff base chromophore in isorhodopsin is excited with a 40-fs pump pulse at 500 nm, and the transient changes in absorption are measured from 450 to 570 nm using a 10-fs probe pulse centered at 500 nm and from 570 to 670 nm using a 10-fs probe pulse centered at 620 nm. Immediately following the pump pulse we observed a strong excited-state absorption with a maximum at approximately 530 and a stimulated emission feature extending from 590 to 670 nm. Both of these features persist for 100-150 fs, indicating that the excited-state torsional dynamics of 9-cis isorhodopsin are significantly slower than those of the 11-cis chromophore in rhodopsin. An absorption feature between 525 and 670 nm fully develops in approximately fs, indicating the formation of of photoproduct on this time scale. The isomerization time in isorhodopsin is significantly slower than that of rhodopsin (200 fs). 33 refs., 4 figs.
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