The primary event in mammalian vision is a light-initiated cisto-trans isomerization of the retinal chromophore bound, via a protonated Schiff base, to a lysine residue in the opsin apoprotein ( Figure 1A). 1 This isomerization activates the protein which triggers a series of events resulting in a signal to the brain. This has been a basic tenet of our understanding of vision. Rod-rhodopsin, the opsin-retinal complex responsible for night vision, is a G-proteincoupled receptor, activated by light with an absorbance maximum at 500 nm. As the absorption from rhodopsin is minute above 600 nm, the pigment is not believed to be involved in vision at longer wavelengths.It has been proposed that the visual signal transduction pathway in a species of deep-sea fish involves the use of photosensitizers. 2 Molecules derived from chlorophyll 650 are thought to absorb longer-wavelength light and transfer the gained energy to shorterwavelength visual pigments ( Figure 1B), thus adding an extra step to their transduction pathway. This is based on the observations that: (i) the fish only possess visual pigments with λ max e 545 nm, (ii) bleaching of its 545 nm pigment with 671 nm light is faster than bleaching with 654 nm light, and (iii) chlorophyll derivatives which have strong absorbances centered at 665 nm have been isolated along with the 545 nm pigment. A triplet-triplet energytransfer mechanism from the chlorophyll 650 derivatives to the 545 nm pigment has been speculated. 2 Enhanced visual sensitivity is reported as a common side effect in patients exposed to porphyrins during photodynamic therapy. 3 In related photosynthetic systems found in plants, carotenoids are believed to act as quenchers of chlorophylls and singlet oxygen, in addition to their primary roles as light-harvesting complexes. 4 The quenching involves triplet-state energy transfer from chlorophyll to carotenoid. 4b To expand rhodopsin sensitivity into the near-IR we have investigated the bleaching of bovine rhodopsin upon exposure to λ max ) 675 nm light in the presence of various chromophores which are potential photosensitizers with strong absorptions around 665 nm. We report rate enhancements on the order of up to 3 times compared to that for the bleaching of rhodopsin alone with λ max ) 675 nm light.In all experiments the bleaching rates of bovine rhodopsin were measured using UV-vis spectroscopy by monitoring the absorbance at 500 nm corresponding to that for the Schiff base. A 0.009 mmol solution of bovine rhodopsin (90% in ROS suspension) solublized in 5% dodecyl--D-maltoside 5 in phosphate-buffered saline was used for all bleaching experiments. The curve marked rhodopsin in Figure 2 depicts a bleaching rate set to 1.0, for the initial 30 min of bleaching of bovine rhodopsin, with λ max ) 675 nm light at 25°C.