N-retinylidene-1-amino-2-propanol: a Schiff base analog for rhodopsin

Multistate Multiconfiguration Quantum Chemical Computation of the Two-Photon Absorption Spectra of Bovine Rhodopsin

Recently, progress in IR sources has led to the discovery that humans can detect infrared (IR) light. This is hypothesized to be due to the two-photon absorption (TPA) events promoting the retina dim-light rod photoreceptor rhodopsin to the same excited state populated via one-photon absorption (OPA). Here, we combine quantum mechanics/molecular mechanics and extended multiconfiguration quasi-degenerate perturbation theory calculations to simulate the TPA spectrum of bovine rhodopsin (Rh) as a model for the human photoreceptor. The results show that the TPA spectrum of Rh has an intense S₀ → S₁ band but shows also S₀ → S₂ and S₀ → S₃ transitions whose intensities, relative to the S₀ → S₁ band, are significantly increased when compared to the corresponding bands of the OPA spectrum. In conclusion, we show that IR light in the 950 nm region can be perceived by rod photoreceptors, thus supporting the two-photon origin of the IR perception. We also found that the same photoreceptor can perceive red (i.e., close to 680 nm) light provided that TPA induces population of S₂.

Substituents at the c(13) position of retinal and their influence on the function of bacteriorhodopsin

Retinal analogues in which the 13-methyl group is replaced by H, C(2)H(5), CF(3), and OCH(3) residues are studied by means of quantumchemical modified neglect of diatomic overlap-correlated version (MNDOC) calculations. The analogues are suitable to test the stereochemical mechanism of proton pumping in bacteriorhodopsin. The results explain the proton-pumping activities of bacterio-opsin reconstituted with these analogues and elucidate the decisive role of retinal's ground-state intramolecular properties in the pump cycle of bacteriorhodopsin.

The effect of protonation and electrical interactions on the stereochemistry of retinal schiff bases

Based on quantumchemical MNDOC calculations it is shown that the ground-state properties of a retinal Schiff base depend sensitively on its protonation state and charge environment. This is exemplified for the equilibrium geometry, for the distribution of partial charges and, in particular, for the thermal isomerization barriers around the pi-bonds. It is demonstrated that a protein, by protonating the retinal Schiff base and by providing one or two negative ions in its environment, can reduce double-bond isomerization barriers from 50 kcal/mol for the unprotonated compound to approximately 5 kcal/mol and can increase single bond barriers from 5 kcal/mol to approximately 20 kcal/mol. Thereby, the specific location of the ions relative to the polyene chain of the protonated retinal Schiff base determines the barrier heights. The results explain the ground-state isomerization reactions of retinal observed in bacteriorhodopsin and in squid retinochrome.

Electrostatic potential at the retinal of three archaeal rhodopsins: implications for their different absorption spectra

The color tuning mechanism of the rhodopsin protein family has been in the focus of research for decades. However, the structural basis of the tuning mechanism in general and of the absorption shift between rhodopsins in particular remains under discussion. It is clear that a major determinant for spectral shifts between different rhodopsins are electrostatic interactions between the chromophore retinal and the protein. Based on the Poisson-Boltzmann equation, we computed and compared the electrostatic potential at the retinal of three archaeal rhodopsins: bacteriorhodopsin (BR), halorhodopsin (HR), and sensory rhodopsin II (SRII) for which high-resolution structures are available. These proteins are an excellent test case for understanding the spectral tuning of retinal. The absorption maxima of BR and HR are very similar, whereas the spectrum of SRII is considerably blue shifted--despite the structural similarity between these three proteins. In agreement with their absorption maxima, we find that the electrostatic potential is similar in BR and HR, whereas significant differences are seen for SRII. The decomposition of the electrostatic potential into contributions of individual residues, allowed us to identify seven residues that are responsible for the differences in electrostatic potential between the proteins. Three of these residues are located in the retinal binding pocket and have in fact been shown to account for part of the absorption shift between BR and SRII by mutational studies. One residue is located close to the beta-ionone ring of retinal and the remaining three residues are more than 8 A away from the retinal. These residues have not been discussed before, because they are, partly because of their location, no obvious candidates for the spectral shift among BR, HR, and SRII. However, their contribution to the differences in electrostatic potential is evident. The counterion of the Schiff base, which is frequently discussed to be involved in the spectral tuning, does not contribute to the dissimilarities between the electrostatic potentials.

Absorbance changes by aromatic amino acid side chains in early rhodopsin photointermediates

Absorbance changes were monitored from 250 to 650 nm during the first microsecond after photolysis of detergent suspensions of bovine rhodopsin at 20 degrees C. Global analysis of the resulting data produced difference spectra for bathorhodopsin, BSI and lumirhodopsin which give the change in absorbance of the aromatic amino acid side chains in these photointermediates relative to rhodopsin. These spectra show that the significant bleaching of absorbance near 280 nm, which has been seen previously for the lumirhodopsin, metarhodopsin I and metarhodopsin II intermediates, extends to times as early as bathorhodopsin. Because no corresponding absorbance increase is observed in the 250-275 nm region, the earliest bleaching of the 280 nm absorbance in rhodopsin is attributed to disruption of a hyperchromic interaction affecting Trp265. Partial decay of this 280 nm bleaching as bathorhodopsin converts to BSI takes place maximally near 290 nm, where Trp265 has been shown to absorb, and could be due to the ring of the retinylidene chromophore resuming a position at the BSI stage that reestablishes the hyperchromic interaction with Trp265. A subsequent change in the 250-300 nm region, which has no counterpart in the visible chromophore bands, indicates the possible presence of a protein-localized process as lumirhodopsin is formed.

Spectroscopic observation of solvent interaction with selected retinal Schiff bases

Iodide salts of several N-retinylidenedialkylamines were prepared and their UV-VIS spectra recorded. Their lambda max values increased as the number of hydrogen atoms on the carbons alpha to nitrogen increased. In separate experiments, iodide salts of N-retinylidene-n-butylammonium (1) and N-retinylidene-n-butylmethylammonium (2) were prepared, and their excitation energies (delta E) were measured in selected solvents of varying dielectric constant (epsilon). Data from each compound gave a straight line which converged at epsilon = 0. On the other hand, when delta E values of the iodide and bromide of 2 were plotted vs solvent epsilon, parallel rather than convergent lines were obtained. When lambda max values of 1 and 2 were measured in a greater number of solvents, the solvents fell into four main groups. The first group, regular solvents, are rigid, have fixed dipoles, neither donate nor accept H-bonds, and the delta E of 1 and 2 decreases linearly as solvent epsilon increases. This line for 2 is taken as a standard against which other solvents are judged. A second class of solvents that are good H-bond donors like CHCl3, whose dipole moment coincides with the C-H bond axis, is located in an area below the standard line. A third group, nucleophilic solvents like tetrahydrofuran, whose dipole moment is coincident with a strongly nucleophilic oxygen atom are good H-bond acceptors and are found above the standard line. Solvents with unpredictable spectroscopic behavior are classed as anomalous.

A rhodopsin is the functional photoreceptor for phototaxis in the unicellular eukaryote Chlamydomonas

Rhodopsin is a visual pigment ubiquitous in multicellular animals. If visual pigments have a common ancient origin, as is believed, then some unicellular organisms might also use a rhodopsin photoreceptor. We show here that the unicellular alga Chlamydomonas does indeed use a rhodopsin photoreceptor. We incorporated analogues of its retinal chromophore into a blind mutant; normal photobehaviour was restored and the colour of maximum sensitivity was shifted in a manner consistent with the nature of the retinal analogue added. The data suggest that 11-cis-retinal is the natural chromophore and that the protein environment of this retinal is similar to that found in bovine rhodopsin, suggesting homology with the rhodopsins of higher organisms. This is the first demonstration of a rhodopsin photoreceptor in an alga or eukaryotic protist and also the first report of behavioural spectral shifts caused by exogenous synthetic retinals in a eukaryote. A survey of the morphology and action spectra of other protists suggests that rhodopsins may be common photoreceptors of chlorophycean, prasinophycean and dinophycean algae. Thus, Chlamydomonas represents a useful new model for studying photoreceptor cells.

Wavelength regulation in rhodopsin: effects of dipoles and amino acid side chains

The effects of dipoles and aromatic amino acid side-chain models on the absorption and optical activity of the rhodopsin chromophore were calculated by using perturbation theory, and the results were compared with those of a Pariser-Parr-Pople calculation for the unperturbed system. The interaction was assumed to result from purely electrostatic interactions. It was concluded that the side chains of phenylalanine and tryptophan should have no important effects. However, the charge separation in tyrosine is sufficient to cause substantial electrostatic perturbation; in fact, the effect of tyrosine is large enough to approximately many of the spectral properties of rhodopsin quantitatively. This is encouraging because the use of aromatic amino acid side-chain analogs probably provides a better physical model than the use of isolated full charges, except in the case of the counterion to the protonated Schiff base.

Ultraviolet chromophore transitions in the rhodopsin spectrum

Difference spectra measured at -105 degrees show two decreases in the ultraviolet absorption spectrum of rhodopsin upon bleaching that cannot be attributed to changes in protein conformation. These absorbancy decreases in rhodopsin are consistent with a cis-trans isomerization of the chromophore.