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.