Rhodopsin Chromophore in Proteins and Clay Interlayers: Mechanism of Color Tuning and Photoisomerization

抄録

Protonated 11-cis retinal Schiff base (RSB) is a chromophore molecule in our visual rhodopsins. Protonated RSB of the all-trans form is a chromophore of bacterial rhodopsins thatact for bacterial ion-pump or light-sensor. It is well known that rhodopsins are excellent light-sensor molecules in terms of (i) color tuning, (ii) photoisomerization, and (iii) functional expression. Protein environment of rhodopsins regulates visible absorption of a unique chromophore, so that we can distinguish so many colors. Photoisomerization of the rhodopsin chromophore is highly selective and efficient, which causes the molecular basis of high sensitivity of our vision. In fact, our ultrafast spectroscopy revealed that photoisomerization is an event occurring in femtoseconds. Photoisomerization in protein also takes place even at 77 K, where the environment of the chromophore is freezed. Relaxation of the photoisomerized chromophore leads tc various functional expressions of rhodopsins, such as visual excitation, bacterial phototaxis, and ion-pumping to create biological energy. Thus, rhodopsins are the goal for artificial light-sensors.
Although it has been difficult to mimic the properties of rhodopsins in other materials, it was found that clay acts as a novel model matrix to mimic visible absorption spectra of protonated RSB in rhodopsins. Montmorillonite (MONT) interlayers were modified by dimethyloctadecylamine (DOA) in benzene solution, and the appearance of a large spectral red shift (λ_mas at 530 nm) indicated that RSB was successfully intercalated into these layers. Absorption spectra of RSB at> 500 nm have been achieved only in the system other than in proteins. A proton is probably supplied from DOA to RSB, forming a protonated RSB in clay. We also found that illumination of RSB in clay at 77 K results in formation of a bathochromic product, which is reverted to the original state by light in a manner similar to what is observed in rhodopsins. Infrared spectral changes suggested that RSB is protonated in clay, and that trans-cis isomerization occurs at 77 K The clay interlayer thus works to not only regulate color, but also to mimic the primary photochemical reaction of RSB in rhodopsins.