Probing for the threshold energy for visual transduction: red-shifted visual pigment analogs from 3-methoxy-3-dehydroretinal and related compounds.

While azulenic retinal analogs failed to yield a red-shifted visual pigment analog, the 9-cis isomers of the push-pull polyenals 3-methoxy-3-dehydroretinal and 14F-3-methoxy-3-dehydroretinal yielded iodopsin pigment analogs with absorption maxima at, respectively, 663 and 720 nm. The former gave a relatively stable batho product (700 nm) and was able to activate transducin. A lower activity was observed for the latter. One possible explanation for the combined results is that the excitation energies of these red-shifted pigments are approaching the threshold energy for visual transduction (although at this time we cannot rigorously exclude a role of the added F-atom in reducing the transducin activity).

Analyzing the red-shift characteristics of azulenic, naphthyl, other ring-fused and retinyl pigment analogs of bacteriorhodopsin.

Prompted by the near infrared-absorbing properties of some of the azulenic bacteriorhodopsin (bR) analogs, we have analyzed their absorption characteristics along with 11 new related ring-fused analogs and the corresponding Schiff bases (SB) and protonated Schiff bases (PSB). The following three factors are believed to contribute to the total red shift of each of the pigment analogs (sigma RS): perturbation of the basic chromophore (SB shift, delta SB), protonation of the SB (PSB shift, PSBS) and protein perturbation (the opsin shift, OS). For each factor, effects of structural modifications were examined. For the red-shifted pigments, percent OS has been suggested as an alternate way of measuring protein perturbation. Computer-simulated chromophores provided evidence against any explanation involving altered shapes of the binding pocket as a major cause for absorption differences. Implications of the current bR results on preparation of further red-shifted bR and possible application to visual pigment analogs are discussed.

Modeling rhodopsin, a member of G-protein coupled receptors, by computer graphics. Interpretation of chemical shifts of fluorinated rhodopsins.

An attempt has been made to construct a 3-D model of rhodopsin, a member of G-protein coupled receptors. Sequence homology of rhodopsin with the latter was a factor considered in the modeling procedure. The constructed model has been used to compare currently available specific protein/substrate interaction information, the shape of the binding cavity derived from shape of binding retinal isomers and analogs and challenged to explain recently available results from a series of fluorinated rhodopsins.

9,11-Dicis-12-fluororhodopsin. Chromophore properties from 19F-NMR studies.

From a 19F-NMR study of 9,11-dicis-12-fluororhodopsin and its photobleached product, we concluded that the initially formed chromophore retained its configuration and the photoproduct corresponded to the two-bond isomerized all-trans. Upon standing, it slowly isomerized to the 9-cis isomer. The method represents a direct, non-destructive procedure for determining configuration purity of the pigment formed. Its unique fluorine opsin shift value is consistent with the expected different orientation of the fluoro-substituent in a dicis pigment.

NMR studies of fluorinated visual pigment analogs.

The 19F-nmr chemical shift data of isomeric pigments (11-cis and 9-cis) of four vinyl fluororhodopsins and two trifluororhodopsins have been recorded. When compared with model protonated Schiff bases, a set of F-nmr opsin shift parameter (FOS) was obtained. The data revealed regiospecific protein perturbations on the F-resonances. They can be interpreted in terms of specific protein interactions such as the postulated second point charge and other polar interactions as well as the common hydrophobic protein perturbation.