Femtosecond polarized pump-probe and stimulated emission spectroscopy of the isomerization reaction of rhodopsin

The isomerization reaction of retinal in bovine rhodopsin was studied by femtosecond optical techniques. Probing at near-IR wavelengths allowed detection of stimulated emission, the rise and decay of which was too fast to be resolved with our 70 fs pulses. The fast dynamics of the emission suggest a multidimensional potential energy surface for isomerization, possibly involving carbon double-bond stretching in addition to bond torsion, as previously invoked for model systems by quantum mechanical calculations. A polarized pump-probe measurement of the appearance of the first isomerization product, bathorhodopsin, at 580 nm shows that the initial direction of the retinal electronic transition dipole moment in this product could be as much as 30 degrees away from the original direction, as deduced from an anisotropy of 0.25, If this is the case, there: would need to be significant electronic changes and charge translocations accompanying the reaction. The anisotropy settler, with an exponential fit time constant of 6.0 +/- 0.6 x 10(12) s(-1), to a value of 0.34 +/- 0.01, corresponding to an angle of 16.5 degrees. Coherent vibrational oscillations in the product well are shown to involve reorientations of the transition dipole direction of less than 4 degrees. A new transient absorption at ca. 700 nm was discovered and attributed to unreactive rhodopsin. The results presented here provide new benchmarks for theoretical evaluation of the isomerization mechanism and dynamics in rhodopsin.