Fluoro Derivatives of Retinal Illuminate the Decisive Role of the C₁₂-H Element in Photoisomerization and Rhodopsin Activation

Abstract

Rhodopsin, the visual pigment of the vertebrate rod cell, is among the best investigated members of the G-protein-coupled receptor family. Within this family a unique characteristic of visual pigments is their covalently bound chromophore, 11-cis retinal, which acts as an inverse agonist. Upon illumination it can be transformed into the all-trans isomer that acts as a full agonist. This photoisomerization process is extremely efficient: 2 out of 3 photons are effective, full stereoselectivity is achieved, and stereoinversion occurs within 200 fs. The mechanism behind this process is still not really understood, although the available evidence points at the twisted C(9)-C(13) segment of the 11-cis ligand as the quintessence. To further dissect the role of this segment, we have generated the 10-fluoro, 12-fluoro, and 14-fluoro analogues of rhodopsin. A fluoro substituent brings in only little more volume than hydrogen, but considerably more mass and polarizability. The analogue pigments were compared to rhodopsin with respect to their photosensitivity (quantum yield), light-induced structural transitions (UV-vis and FT-IR spectroscopy), and signaling activity (G protein activation rate). We find that 14-F substitution is quite neutral, while 10-F and 12-F substitutions exert significant but distinct effects. The 10-F pigment exhibits a quantum yield similar to that of rhodopsin (0.65) but strongly perturbed thermodynamics of the structural transitions following photoactivation and only 20% of the native signaling activity. The 12-F pigment exhibits a significantly decreased quantum yield (0.47) and signaling activity (30%) but mixed effects on the structural transitions. These properties are compared to those of the corresponding methyl derivatives. We conclude that rotation of the C(12)-H bond of the rhodopsin chromophore is a major rate-limiting factor in the photoisomerization process, while the C(10)-H moiety plays a dominant role in ligand relaxation and further rearrangements following photoactivation.

The crystal structure of mycobacterial epoxide hydrolase A

“…Detecting this interaction suggests that it is in fact specific, and that it would likely be much stronger in solution. Moreover, such a small hydrophobic ligand would be unlikely to detect at all if not buried inside the molecule 30 .…”

Impacts of retinal polyene (de)methylation on the photoisomerization mechanism and photon energy storage of rhodopsin

“…[8][9][10][11][12][13][14][15][16][17][18][19][20][21] Therefore, it comes as no surprise that much effort has been devoted to experimental [8][9][10][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] and theoretical [31][32][33][34][35] investigations of synthetic pigments. However, there is a tight coupling between the chromophore and the receptor to achieve optimum stability of inactive dark and active conformations of the visual photopigment.…”

In Situ Raman Monitoring of Silver(I)‐Aided Laser‐Driven Cleavage Reaction of Cyclobutane

“…[3][4][5][6][7] It has been reported that pyridyl-substituted cyclobutane coordinated with metallic ions can undergo isomerization/cleavage reactions upon thermalt reatment, [8][9][10][11] and recently,m etal-organic organopolymeric hybrid frameworks have been produced by using [2+ +2] cycloaddition reactions. [17][18][19] Moreover,R aman spectroscopyi su ltrasensitive to bond vibration, [19][20][21] as opposed to in situ X-ray diffraction, which is only sensitivet oc rystalline structure, so Ramans pectroscopy can even be applied to amorphousm aterials during the reactionp rocess. [14,15] Typically,e xsitu techniques such as nuclear magnetic resonance (NMR) spectroscopy and mass spectrometrya re used to study the yield of the cyclobutane cleavage reactions, which fail to monitor the whole reaction process.…”

“…[12,13] Besidesc onventional heat resources, laser irradiation can also be applied to induce cleavage reaction. [17][18][19] Moreover,R aman spectroscopyi su ltrasensitive to bond vibration, [19][20][21] as opposed to in situ X-ray diffraction, which is only sensitivet oc rystalline structure, so Ramans pectroscopy can even be applied to amorphousm aterials during the reactionp rocess. [9,16] In situ time-resolved Raman spectroscopy is an advancedt echnique that can monitor the reaction processes and structure changes of molecules.…”

“…[9,16] In situ time-resolved Raman spectroscopy is an advancedt echnique that can monitor the reaction processes and structure changes of molecules. [17][18][19] Moreover,R aman spectroscopyi su ltrasensitive to bond vibration, [19][20][21] as opposed to in situ X-ray diffraction, which is only sensitivet oc rystalline structure, so Ramans pectroscopy can even be applied to amorphousm aterials during the reactionp rocess. [22][23][24] In our previousw ork, [25] we managed to monitor the dimerization reaction process of pyridyl-substituted olefinsi nto cyclobutanes through the in situ Ramans pectra,w here the laser is the heat source and silver(I)i onsa ct as at emplate to control the pyridyl-substituted paralleled olefins' distance to within 4.2 .…”