Synthesis and use of stable isotope enriched retinals in the field of vitamin A

The role of vitamin A and its metabolites in the life processes starting with the historical background and its up to date information is discussed in the introduction. Also the role of 11Z-retinal in vision and retinoic acid in the biological processes is elucidated. The essential role of isotopically enriched systems in the progress of vision research, nutrition research etc. is discussed. In part B industrial commercial syntheses of vitamin A by the two leading companies Hoffmann-La Roche (now DSM) and BASF are discussed. The knowledge obtained via these pioneering syntheses has been essential for the further synthetic efforts in vitamin A field by other scientific groups. The rest of the paper is devoted to the synthetic efforts of the Leiden group that gives an access to the preparation of site directed high level isotope enrichment in retinals. First the synthesis of the retinals with deuterium incorporation in the conjugated side chain is reviewed. Then, 13C-labeled retinals are discussed. This is followed by the discussion of a convergent synthetic scheme that allows a rational access to prepare any isotopomer of retinals. The schemes that provide access to prepare any possible isotope enriched chemically modified systems are discussed. Finally, nor-retinals and bridged retinals that give access to a whole (as yet incomplete) library of possible isotopomers are reviewed.

Stereoselective synthesis of annular 9-cis-retinoids and binding characterization to the retinoid X receptor

Analogues of 9-cis-retinoic acid incorporating an alicyclic ring between the C19 and C10 positions have been synthesized and evaluated as ligands for the RXRalpha nuclear receptor. The stereocontrolled synthesis of these configurationally constrained retinoids combines a Stille cross-coupling and the Wittig reaction as key bond-forming steps. The palladium-catalyzed cross-coupling reaction of the beta-bromo-alpha,beta-unsaturated aldehydes 5 to dienylstannane 6 is very fast at room temperature, and takes place with preservation of the dienylstannane geometry. A highly stereoselective Wittig reaction afforded the C7-C8 bond connecting the hydrophobic ring to the retinoid side chain. The binding affinities of these compounds for the receptor were determined, and the structural and energetic rationale behind the affinity profile of the cyclic 9-cis-retinoic acid derivatives for the RXRalpha nuclear receptor was characterized by using Molecular Mechanics protocols.

Role of the C9 methyl group in rhodopsin activation: characterization of mutant opsins with the artificial chromophore 11-cis-9-demethylretinal

Activation of the visual pigment rhodopsin involves both steric and electrostatic interactions between the chromophore and opsin within the retinal-binding site. Removal of the C9 methyl group of 11-cis-retinal inhibits light-dependent activation of the G protein, transducin, suggesting a direct steric contact. More recently, we have shown that steric interactions lead to receptor activation when Gly121 in the middle of transmembrane helix 3 is replaced by larger hydrophobic residues. In order to understand in more detail the role of the C9 methyl group of retinal in the structure and function of rhodopsin, we first studied the properties of recombinant 9-dm-Rho (opsin reconstituted with 11-cis-9-demethylretinal). The 9-dm-Rho pigment displayed a blue-shifted lambdamax, increased hydroxylamine reactivity, and decreased ability to activate transducin. These properties are consistent with the hypothesis that the C9 methyl group is a crucial structural anchor for the correct docking of the chromophore in its binding site. Next, we investigated the possible interaction between Gly121 of opsin and the C9 methyl group of retinal by characterizing recombinant pigments produced by combining mutant opsins (G121A, -V, -I, -L, and -W) with 11-cis-9-demethylretinal. Mutant opsins G121I, -L, and -W failed to bind the chromophore. However, the double mutant G121L/F261A bound 11-cis-9-demethylretinal to form a stable pigment with a lambdamax of 451 nm. When activity was assayed in membranes, the reduction in transducin activation by 9-dm-Rho caused by the lack of a C9 methyl group on the chromophore could be partially restored by replacing Gly121 with a bulky residue (leucine, isoleucine, or tryptophan). These results support a model of receptor activation that involves steric interaction between the C9 methyl group of the chromophore and the opsin in the vicinity of Gly121 on transmembrane helix 3.

Microsecond atomic force sensing of protein conformational dynamics: implications for the primary light-induced events in bacteriorhodopsin

In this paper a new atomic force sensing technique is presented for dynamically probing conformational changes in proteins. The method is applied to the light-induced changes in the membrane-bound proton pump bacteriorhodopsin (bR). The microsecond time-resolution of the method, as presently implemented, covers many of the intermediates of the bR photocycle which is well characterized by spectroscopical methods. In addition to the native pigment, we have studied bR proteins substituted with chemically modified retinal chromophores. These synthetic chromophores were designed to restrict their ability to isomerize, while maintaining the basic characteristic of a large light-induced charge redistribution in the vertically excited Franck-Condon state. An analysis of the atomic force sensing signals lead us to conclude that protein conformational changes in bR can be initiated as a result of a light-triggered redistribution of electronic charge in the retinal chromophore, even when isomerization cannot take place. Although the coupling mechanism of such changes to the light-induced proton pump is still not established, our data question the current working hypothesis which attributes all primary events in retinal proteins to an initial trans<==>cis isomerization.

Retinal analog study of the role of steric interactions in the excited state isomerization dynamics of rhodopsin

The role of intramolecular steric interactions in the isomerization of the 11-cis-retinal chromophore in the photoreceptor protein rhodopsin is examined with resonance Raman and CD spectroscopy combined with quantum yield experiments. The resonance Raman spectra and CD spectra of 13-demethylrhodopsin indicate that its chromophore, an analog in which the nonbonded interaction between the 10-H and the 13-CH3 groups is removed, is less distorted in the C10...C13 region than the native chromophore. The reduced torsional and hydrogen-out-of-plane resonance Raman intensities further indicate that the excited state potential energy surface has a much shallower slope along the isomerization coordinate. This is consistent with the decrease in quantum yield from 0.67 in rhodopsin to 0.47 in 13-demethylrhodopsin. The resonance Raman intensities show that the steric twist is reintroduced by addition of a methyl group at the C10 position. However, the quantum yield of 10-methyl-13-demethylrhodopsin is found to be only 0.35. This is attributed to nonisomorphous protein-analog interactions. The nonbonded interaction between the 10-hydrogen and the 13-methyl group in 11-cis-retinal makes this isomer particularly effective as the light-sensing chromophore in all visual pigments.