Photochemical reaction of 7,8-dihydrorhodopsin at low temperatures

7,8-Dihydro Retinals Outperform the Native Retinals in Conferring Photosensitivity to Visual Opsin

“…In contrast, the 7,8-dihydro spectrum displays hardly any shift of this peak, suggestive of at most only a small red-shift in the transition to 7,8-dihydro Batho. This agrees with cryospectral 27 as well as with time-resolved studies at room temperature 39 of 7,8-dihydro isorhodopsin that indicate strong spectral overlap between the ground state and the first photoproduct. It could be argued that the differences in Figure 3A between the native and the 7,8-dihydro pigments are due to lower stability of the Batho and BSI-like intermediates of the 7,8-dihydro pigment, because of the flexibility in their C 7 -C 9 segment, resulting in generation of a Lumi-like intermediate already at 80 K, instead of at about 130 K as in the native state.…”

“…This also can be explained by the absence of the 7,8 double bond, since a straightly extending polyene configuration usually exhibits a higher oscillator strength in the lowest electronic transition . The tendency of a straight polyene to display spectral fine structure is not only observed in the spectrum of 9- Z 7,8-dihydro retinal at room temperature (Figure ) but also in that of 7,8-dihydro isorhodopsin at 80 K 2 Spectral properties of native and 7,8-dihydro pigments compared in this study.…”

“…This also can be explained by the absence of the 7,8 double bond, since a straightly extending polyene configuration usually exhibits a higher oscillator strength in the lowest electronic transition. 31 The tendency of a straight polyene to display spectral fine structure is not only observed in the spectrum of 9-Z 7,8-dihydro retinal at room temperature (Figure 1) but also in that of 7,8-dihydro isorhodopsin at 80 K. 27 The most striking observation is the highly efficient photoisomerization of both analogue pigments (Table 2). The 11-Z 7,8-dihydro pigment is the first to equal native rhodopsin with respect to quantum yield Φ (0.68 versus 0.67), while the 9-Z 7,8-dihydro pigment is significantly more efficient (Φ ) 0.39) than native isorhodopsin (0.27).…”

“…Both show a large blue-shift relative to the native pigments, which is expected because of the shortening of the polyene and agrees with earlier reports. 12,26,27 However, the analogue pigments have about the same absorbance maxima (426 ( 2 nm for the 11-Z pigment and 428 ( 2 nm for the 9-Z pigment), in strong contrast to rhodopsin (498 nm) and isorhodopsin (485 nm) (Figure 2). This suggests that the two isomers now sense a similar protein environment and that the structural distortion of 9-Z retinal, because of its "misfit" in the opsin binding site, 32-34 can be relieved by allowing structural flexibility in the 7-9 segment of the retinal polyene chain.…”

“…The positive charge delocalization over the terminal part of the polyene chain creates a conjugation defect, a polaron that upon photoexcitation moves to the opposite end of the polyene. ,, We reasoned that shortening the polyene as in 5,6-dihydro- or 7,8-dihydro analogues would stabilize the polaron in the isomerization region, enhance the inversion of the π-distribution, and thereby might facilitate photoisomerization of a 9- Z and an 11- Z bond. Since ring modifications as in 5,6-dihydro may affect ligand−receptor interaction and scattered reports indicated that 7,8-dihydro retinals (Chart ) can generate rhodopsin analogues that appear to be functional, ,, we opted for the 7,8-dihydro modification.…”