FEMTOSECOND DYNAMICS OF CIS-TRANS ISOMERIZATION IN A VISUAL PIGMENT ANALOG - ISORHODOPSIN

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 change s in absorption are measured from 450 to 570 nm using a 10-fs probe pu lse centered at 500 nm and from 570 to 670 nm using a 10-fs probe puls e centered at 620 nm. Immediately following the pump pulse we observe a strong excited-state absorption with a maximum at approximate to 530 nm and a stimulated emission feature extending from 590 to 670 nm. Bo th of these features persist for 100-150 fs, indicating that the excit ed-state torsional dynamics of 9-cis isorhodopsin are significantly sl ower than those of the 11-cis chromophore in rhodopsin. An absorption feature between 525 and 670 nm fully develops in approximate to 600 fs , indicating the formation of photoproduct on this time scale. The iso merization time in isorhodopsin is significantly slower than that of r hodopsin (200 fs). The slower isomerization time and the slower excite d-state dynamics in isorhodopsin are consistent with the idea that the photochemical quantum yield in these pigments (Phi(iso) = 0.22, Phi(r ho) = 0.67) depends on the dynamics of the excited-state torsional wav e packet, according to a Landau-Zener potential-surface crossing mecha nism.