Fluorescence quantum yield of visual pigments: evidence for subpicosecond isomerization rates.

The fluorescence quantum yields (phi f) for bovine and squid rhodopsins are determined. Both pigments yield similar results, with an average value for phi f of 1.2 (+/- 0.5) X 10(-5). Since the estimated radiative lifetime of rhodopsin is 5 nsec, the rate constant of the process that competes with fluorescence must be on the order of 0.1 psec. Given the large quantum yield for isomerization of rhodopsin's retinal chromophore, this process is likely to correspond to the motion along retinal's C11-C12 torsional coordinate that leads to cis-trans isomerization. An empirical excited-state potential energy curve along this coordinate is derived. It is shown that subpicosecond torsional motion to highly twisted nonfluorescing regions of the potential is possible and, in fact, likely. Our results require the existence of a barrier-less excited-state potential energy curve and suggest that cis-trans isomerization occurs in less than 1 psec.