Synthesis of 11-cis-locked bicyclo[5.1.0]octanyl retinal and an enantioselective binding to bovine opsin

Palladium-Catalyzed [3 + 2]-C-C/N-C Bond-Forming Annulation

The synthesis of bi- and tricyclic structures incorporating pyrrolidone rings is disclosed, starting from resonance-stabilized acetamides and cyclic α,β-unsaturated-γ-oxycarbonyl derivatives. This process involves an intermolecular Tsuji-Trost allylation/intramolecular nitrogen 1,4-addition sequence. Crucial for the success of this bis-nucleophile/bis-electrophile [3 + 2] annulation is its well-defined step chronology in combination with the total chemoselectivity of the former step. When the newly formed annulation product carries a properly located o-haloaryl moiety at the nitrogen substituent, a further intramolecular keto α-arylation can join the cascade, thereby forming two new cycles and three new bonds in the same synthetic operation.

Synthesis of the nakanishi ring-locked retinoid

An optimized synthetic route to prepare ring-locked retinoid 1a has been developed. We fully describe a purification protocol that provides isomerically pure 1a in support of on-going proof of concept studies for the development of therapeutic agents to treat human ADRP. Additionally, we have found that isomerically pure 1a can be stored in amber vials under argon at −20°C for use over time (up to six months) without degradation. Thus, enabling 1a to be an accessible and valuable biological tool.

Synthesis of an 11-cis-locked biotinylated retinoid for sequestering 11-cis-retinoid binding proteins

The synthesis of a seven-membered ring locked analogue of 11-cis-retinol, tethered to a cross-linking moiety on C-15 and a lysine-extended biotin on the C-3, was accomplished for its utilization as a probe to fish out retinol binding proteins that may be involved in the reisomerization of retinal from all-trans to 11-cis in the visual cycle.Key words: retinoids, biotin, 11-cis-locked, cross-linking, retinol binding proteins.

Primary events in dim light vision: a chemical and spectroscopic approach toward understanding protein/chromophore interactions in rhodopsin

The visual pigment rhodopsin (bovine) is a 40 kDa protein consisting of 348 amino acids, and is a prototypical member of the subfamily A of G protein-coupled receptors (GPCRs). This remarkably efficient light-activated protein (quantum yield = 0.67) binds the chromophore 11-cis-retinal covalently by attachment to Lys296 through a protonated Schiff base. The 11-cis geometry of the retinylidene chromophore keeps the partially active opsin protein locked in its inactive state (inverse agonist). Several retinal analogs with defined configurations and stereochemistry have been incorporated into the apoprotein to give rhodopsin analogs. These incorporation results along with the spectroscopic properties of the rhodopsin analogs clarify the mode of entry of the chromophore into the apoprotein and the biologically relevant conformation of the chromophore in the rhodopsin binding site. In addition, difference UV, CD, and photoaffinity labeling studies with a 3-diazo-4-oxo analog of 11-cis-retinal have been used to chart the movement of the retinylidene chromophore through the various intermediate stages of visual transduction.

Pharmacological chaperone-mediated in vivo folding and stabilization of the P23H-opsin mutant associated with autosomal dominant retinitis pigmentosa

Protein conformational disorders, which include certain types of retinitis pigmentosa, are a set of inherited human diseases in which mutant proteins are misfolded and often aggregated. Many opsin mutants associated with retinitis pigmentosa, the most common being P23H, are misfolded and retained within the cell. Here, we describe a pharmacological chaperone, 11-cis-7-ring retinal, that quantitatively induces the in vivo folding of P23H-opsin. The rescued protein forms pigment, acquires mature glycosylation, and is transported to the cell surface. Additionally, we determined the temperature stability of the rescued protein as well as the reactivity of the retinal-opsin Schiff base to hydroxylamine. Our study unveils novel properties of P23H-opsin and its interaction with the chromophore. These properties suggest that 11-cis-7-ring retinal may be a useful therapeutic agent for the rescue of P23H-opsin and the prevention of retinal degeneration.

Biochemical and physiological properties of rhodopsin regenerated with 11-cis-6-ring- and 7-ring-retinals

Phototransduction is initiated by the photoisomerization of rhodopsin (Rho) chromophore 11-cis-retinylidene to all-trans-retinylidene. Here, using Rho regenerated with retinal analogs with different ring sizes, which prevent isomerization around the C(11)=C(12) double bond, the activation mechanism of this G-protein-coupled receptor was investigated. We demonstrate that 11-cis-7-ring-Rho does not activate G-protein in vivo and in vitro, and that it does not isomerize along other double bonds, suggesting that it fits tightly into the binding site of opsin. In contrast, bleaching 11-cis-6-ring-Rho modestly activates phototransduction in vivo and at low pH in vitro. These results reveal that partial activation is caused by isomerization along other double bonds in more rigid 6-locked retinal isomers and protonation of key residues by lowering pH in 11-cis-6-ring-Rhos. Full activation is not achieved, because isomerization does not induce a complete set of conformational rearrangements of Rho. These results with 6- and 7-ring-constrained retinoids provide new insights into Rho activation and suggest a potential use of locked retinals, particularly 11-cis-7-ring-retinal, to inactivate opsin in some retinal degeneration diseases.

Solution and biologically relevant conformations of enantiomeric 11-cis-locked cyclopropyl retinals

To gain information on the conformation of the 11-cis-retinylidene chromophore bound to bovine opsin, the enantiomeric pair (2a and 2b) of 11-cis-locked bicyclo[5.1.0]octyl retinal (retCPr) 2 was prepared and its conformation was investigated by NMR, geometry optimization, and CD calculations. This compound is also of interest since it contains a unique moiety in which a chiral cyclopropyl group is flanked by triene and enal chromophores, and hence would clarify the little-known chiroptical contribution of a cyclopropyl ring linked to polyene systems. NMR revealed that the seven-membered ring of retCPr adopts a twist chair conformation. The NMR-derived structure constraints were then used for optimizing the geometry of 2 with molecular mechanics and ab initio methods. This revealed that enantiomer 2a with a 11 beta,12 beta-cyclopropyl group exists as two populations of diastereomers depending on the twist around the 6-s bond; however, the sense of twist around the 12-s is positive in both rotamers. The theoretical Boltzmann-weighted CD obtained with the pi-SCF-CI-DV MO method and experimental spectra were consistent, thus suggesting that the conjugative effect of the cyclopropyl moiety is minimal. It was found that only the beta-cyclopropyl enantiomer 2a, but not the alpha-enantiomer 2b, binds to opsin. This observation, together with earlier retinal analogues incorporation results, led to the conclusion that the chromophore sinks into the N-terminal of the opsin receptor from the side of the 4-methylene and 15-aldehyde, and that the binding cleft accommodates 11-cis-retinal with a slightly positive twist around C12/C13. A reinterpretation of the previously published negative CD couplet of 11,12-dihydrorhodopsin also leads to a chromophoric C12/C13 twist conformation with the 13-Me in front as in 1b. Such a conformation for the chromophore accounts for both the observed biostereoselectivity of retCPr 2a and the observed negative couplet of 11,12-dihydro-Rh7.