Structure, Spectroscopy, and Spectral Tuning of the Gas-Phase Retinal Chromophore:  The β-Ionone “Handle” and Alkyl Group Effect

Abstract

The low-lying singlet states (i.e. S0, S1, and S2) of the chromophore of rhodopsin, the protonated Schiff base of 11-cis-retinal (PSB11), and of its all-trans photoproduct have been studied in isolated conditions by using ab initio multiconfigurational second-order perturbation theory. The computed spectroscopic features include the vertical excitation, the band origin, and the fluorescence maximum of both isomers. On the basis of the S0-->S1 vertical excitation, the gas-phase absorption maximum of PSB11 is predicted to be 545 nm (2.28 eV). Thus, the predicted absorption maximum appears to be closer to that of the rhodopsin pigment (2.48 eV) and considerably red-shifted with respect to that measured in solution (2.82 eV in methanol). In addition, the absorption maxima associated with the blue, green, and red cone visual pigments are tentatively rationalized in terms of the spectral changes computed for PSB11 structures featuring differently twisted beta-ionone rings. More specifically, a blue-shifted absorption maximum is explained in terms of a large twisting of the beta-ionone ring (with respect to the main conjugated chain) in the visual S-cone (blue) pigment chromophore. In contrast, the chromophore of the visual L-cone (red) pigment is expected to have a nearly coplanar beta-ionone ring yielding a six double bond fully conjugated framework. Finally, the M-cone (green) chromophore is expected to feature a twisting angle between 10 and 60 degrees. The spectroscopic effects of the alkyl substituents on the PSB11 spectroscopic properties have also been investigated. It is found that they have a not negligible stabilizing effect on the S1-S0 energy gap (and, thus, cause a red shift of the absorption maximum) only when the double bond of the beta-ionone ring conjugates significantly with the rest of the conjugated chain.

The color of rhodopsins at theab initiomulticonfigurational perturbation theory resolution

“…It is thus apparent that the opsins induce a blue-shift of the chromophore max a . Notice that, for the case of Rh, the difference in excitation energy of the relaxed gas-phase chromophore (the N-methylated form of the 6-s-cis,11-cis retinal protonated Schiff base) (29) and of the isolated Rh chromophore (this work) is 0.2 kcal⅐mol Ϫ1 . This demonstrates that the max a change induced by the protein environment is due to effects other than the limited changes in chromophore structure (the main differences are a Ϸ10°in-crease in the C 6 OC 7 single bond torsion and a planarization of the -C 12 AC 11 OC 10 AC 9 OC 8 AC 7 -moiety in the gas-phase structure).…”

“…2B) displays, with respect to the Rh chromophore, a Ͻ0.3 kcal⅐mol Ϫ1 change in excitation energy. In a recent report (29) we have shown that, in the gas-phase, the excitation energy of the retinal chromophore is sensitive to the deformation of the conjugated -framework. Here, the equilibrium structures of iso-Rh and G121L (see Fig.…”

Polarizable Embedding Density Matrix Renormalization Group

“…This system has become a benchmark system for advanced electronic structure methods in recent years (see, for instance, Refs. [103][104][105][106][107][108][109][110][111][112][113][114][115][116][117]. For the channelrhodopsin protein, we employ the M2P2 potential constructed in Ref.…”

A aplicação da fotoquímica inorgânica nas diversas áreas da ciência

“…Este tipo de reação é bem conhecido dos organismos vivos, onde encontramos moléculas fotorreceptoras naturais como a rodopsina (responsável pela visão) 24 e a clorofila (fotossíntese) 25 . A visão, por exemplo, inicia quando a luz é focalizada na retina, camada de células que recobre o interior do globo ocular.…”

“…Essa estratégia tem sido amplamente usada para controlar as geometrias e funções de biomoléculas, nos processos de visão, de materiais orgânicos e de complexos supramoleculares 24,[52][53][54] .…”