Phototransduction by Vertebrate Ultraviolet Visual Pigments: Protonation of the Retinylidene Schiff Base following Photobleaching†

The photochemical and subsequent thermal reactions of the mouse short-wavelength visual pigment (MUV) were studied by using cryogenic UV-visible and FTIR difference spectroscopy. Upon illumination at 75 K, MUV forms a batho intermediate (Imax 380 nm). The batho intermediate thermally decays to the lumi intermediate (Imax 440 nm) via a slightly blue-shifted intermediate not observed in other photobleaching pathways, BL (Imax 375 nm), at temperatures greater than 180 K. The lumi intermediate has a significantly red-shifted absorption maximum at 440 nm, suggesting that the retinylidene Schiff base in this intermediate is protonated. The lumi intermediate decays to an even more red-shifted meta I intermediate (Imax 480 nm) which in turn decays to meta II (Imax 380 nm) at 248 K and above. Differential FTIR analysis of the 1100-1500 cm -1 region reveals an integral absorptivity that is more than 3 times smaller than observed in rhodopsin and VCOP. These results are consistent with an unprotonated Schiff base chromophore. We conclude that the MUV-visual pigment possesses an unprotonated retinylidene Schiff base in the dark state, and undergoes a protonation event during the photobleaching cascade.