Spontaneous emission study of the femtosecond isomerization dynamics of rhodopsin

The spontaneous emission spectra and yields of the visual pigment rhodopsin and its 9-cis retinal analog isorhodopsin have been measured following excitation at 472.7, 514.5, and 568.2 nm. The fluorescence quantum yields are (0.9 +/- 0.3) x 10(-5) for rhodopsin and (1.8 +/- 0.7) x 10(-5) for isorhodopsin. By use of a Strickler-Berg analysis, these quantum yields correspond to excited electronic state lifetimes of 50 and 100 fs for rhodopsin and isorhodopsin, respectively. The fluorescence spectra also undergo a blue-shift of similar to 1800 cm(-1) upon shifting the excitation wavelength from 568.2 to 472.7 nm. This is consistent with the idea that the emission arises from unrelaxed, nonstationary vibrational states. For rhodopsins, there is a correlation among these rapid initial nuclear dynamics out of the Franck-Condon region, the rapid photoisomerization rates, and the high isomerization quantum yields. These results support a new mechanism for the isomerization reaction dynamics in visual pigments where the reaction efficiency is mechanistically linked to the reaction rate through a dynamic Landau-Zener tunneling process.