Wavelength dependent cis-trans isomerization in vision

The primary event in vision is the light-driven cis-trans isomerization of the 11-cis-retinal chromophore in the G-protein coupled receptor rhodopsin. Early measurements showed that this photoisomerization has a reaction quan tum yield phi of similar to0.67 [Dartnall (1936) Proc. R. Soc. A 156, 158-1 70; Dartnall (1968) Vision Res. 8, 339-358] and suggested that the quantum yield was wavelength independent [Schneider (1939) Proc. Natl. Acad. Sci. U .S.A. 170, 102-112]. Here we more accurately determine phi (500) = 0.65 +/- 0.01 and reveal that phi surprisingly depends on the wavelength of the inc ident light. Although there is no difference in the quantum yield between 4 50 and 480 nm, the quantum yield falls significantly as the photon energy i s reduced below 20 000 cm(-1) (500 nm). At the reddest wavelength measured (570 nm), the quantum yield is reduced by 5 +/- 1% relative to the 500 nm v alue. These experiments correct the long-held presumption that the quantum yield in vision is wavelength independent, and support the hypothesis that the 200 fs photoisomerization reaction that initiates vision is dictated by nonstationary excited-state vibrational wave packet dynamics.