Conversion of a photon to an electrical signal by sudden polarisation in the N-retinylidene visual chromophore

Ultrafast protein response in the Pfr state of Cph1 phytochrome

Photoisomerization is a fundamental process in several classes of photoreceptors. Phytochromes sense red and far-red light in their Pr and Pfr states, respectively. Upon light absorption, these states react via individual photoreactions to the other state. Cph1 phytochrome shows a photoisomerization of its phycocyanobilin (PCB) chromophore in the Pfr state with a time constant of 0.7 ps. The dynamics of the PCB chromophore has been described, but whether or not the apoprotein exhibits an ultrafast response too, is not known. Here, we compare the photoreaction of ¹³C/¹⁵N labeled apoprotein with unlabeled apoprotein to unravel ultrafast apoprotein dynamics in Cph1. In the spectral range from 1750 to 1620 cm^-1 we assigned several signals due to ultrafast apoprotein dynamics. A bleaching signal at 1724 cm^-1 is tentatively assigned to deprotonation of a carboxylic acid, probably Asp207, and signals around 1670 cm^-1 are assigned to amide I vibrations of the capping helix close to the chromophore. These signals remain after photoisomerization. The apoprotein dynamics appear upon photoexcitation or concomitant with chromophore isomerization. Thus, apoprotein dynamics occur prior to and after photoisomerization on an ultrafast time-scale. We discuss the origin of the ultrafast apoprotein response with the 'Coulomb hammer' mechanism, i.e. an impulsive change of electric field and Coulombic force around the chromophore upon excitation.

Conical Intersections in Solution with Polarizable Embedding: Integral-Exact Direct Reaction Field

A common strategy to exploring the properties and reactivity of complex systems is to use quantum mechanics/molecular mechanics (QM/MM) embedding, wherein a QM region is defined and treated with electronic structure theory, and the remainder of the system is treated with a force field. Important to the description of electronic excited states, especially those of charge-transfer character, is the treatment of the coupling between the QM and MM subsystems. The state of the art is to use a polarizable force field for the MM region and mutually couple the QM wavefunction and MM induced dipoles, in addition to the usual electrostatic embedding, yielding a polarizable embedding (QM/MM-Pol) approach. However, we showed previously that current popular QM/MM-Pol approaches exhibit issues of root flipping and/or incorrect descriptions of electronic crossings in multistate calculations [J. Chem. Theory Comput. 14, 2137 (2018)]. Here, we demonstrate a solution to these problems with an integral-exact reformulation of the direct reaction field approach of Thole and Van Duijnen (QM/MM-IEDRF). The resulting embedding potential includes one- and two-electron operators and many-body dipole-induced dipole interactions and thus includes a natural description of the screening of electron-electron interactions by the MM induced dipoles. Pauli repulsion from the environment is mimicked by effective core potentials on the MM atoms. Inherent to the DRF approach is the assumption that MM dipoles respond instantaneously to the positions of the QM electrons; therefore, dispersion interactions are captured approximately. All electronic states are eigenfunctions of the same Hamiltonian, while the polarization induced in the environment and the associated energetic stabilization are unique to each state. This allows for a consistent definition of transition properties and state crossings. We demonstrate QM/MM-IEDRF by exploring the influence of a (polarizable) inert xenon matrix environment on the conical intersection underlying the photoisomerization of ethylene.

Description of Sudden Polarization in the Excited Electronic States with an Ensemble Density Functional Theory Method

Sudden polarization (SP) is one of the manifestations of electron transfer in the electronically excited states of molecules. Proposed initially to explain the unusual reactivity of photoexcited olefins, SP often occurs in the excited states of molecules possessing strongly correlated diradical ground state. Theoretical description of SP involves mixing between the singly excited and the doubly excited zwitterionic states, which makes it inaccessible with the use of the popular linear-response time-dependent density functional theory methods. In this work, an extended variant of the state-interaction state-averaged spin-restricted ensemble-referenced Kohn-Sham (SI-SA-REKS, or SSR) method is applied to study SP in a number of organic diradical systems. To this end, the analytical derivative formalism is derived and implemented for the SSR(3,2) method (see the main text for explanation of the acronym), which enables the automatic geometry optimization and obtains the relaxed density matrices as well as the electron binding energies and respective Dyson's orbitals. Application of the new method to SP in the lowest singlet excited state of ethylene agrees with the results obtained previously with the use of multireference methods of wavefunction theory. A number of interesting manifestations of SP are observed, such as the charge transfer in photoexcited tetramethyleneethene (TME) diradical mediated by the vibrational motion and conductivity switching in the excited state of a donor-acceptor dyad placed in an external electric field.

Sudden polarization and zwitterion formation as a pseudo-Jahn–Teller effect: a new insight into the photochemistry of alkenes

We show that the intermediates of photochemical reactions—sudden polarization and zwitterion formations—are consequences of the pseudo-Jahn–Teller effect (PJTE), which facilitates a better understanding, rationalization, prediction, and manipulation of the corresponding chemical and biological processes.

Schiff Base Proton Acceptor Assists Photoisomerization of Retinal Chromophores in Bacteriorhodopsin

In this study, we investigated the ultrafast dynamics of bacteriorhodopsins (BRs) from Haloquadratum walsbyi (HwBR) and Haloarcula marismortui (HmBRI and HmBRII). First, the ultrafast dynamics were studied for three HwBR samples: wild-type, D93N mutation, and D104N mutation. The residues of the D93 and D104 mutants correspond to the control by the Schiff base proton acceptor and donor of the proton translocation subchannels. Measurements indicated that the negative charge from the Schiff base proton acceptor residue D93 interacts with the ultrafast and substantial change of the electrostatic potential associated with chromophore isomerization. By contrast, the Schiff base proton donor assists the restructuring of the chromophore cavity hydrogen-bond network during the thermalization of the vibrational hot state. Second, the ultrafast dynamics of the wild-types of HwBR, HmBRI, and HmBRII were compared. Measurements demonstrated that the hydrogen-bond network in the extracellular region in HwBR and HmBRII slows the photoisomerization of retinal chromophores, and the negatively charged helices on the cytoplasmic side of HwBR and HmBRII accelerate the thermalization of the vibrational hot state of retinal chromophores. The similarity of the correlation spectra of the wild-type HmBRI and D104N mutant of HwBR indicates that inactivation of the Schiff base proton donor induces a positive charge on the helices of the cytoplasmic side.

Nonadiabatic photodynamics and UV absorption spectrum of all-trans-octatetraene

The short-time molecular quantum dynamics of all-trans-octatetraene after electronic excitation to the first bright valence state is theoretically investigated.

Space and Time Evolution of the Electrostatic Potential During the Activation of a Visual Pigment

Animal and microbial retinal proteins employ the Schiff base of retinal as their chromophore. Here, the possible consequences of the charge translocation associated with the light-induced dynamics of the chromophore of a visual opsin are investigated along a representative semiclassical trajectory. We show that the evolution of the electrostatic potential projected by the chromophore onto the surrounding protein displays intense but topographically localized sudden variations in proximity of the decay region. pKa calculations carried out on selected snapshots used as probes, indicate that the only residue which may be sensitive to the electrostatic potential shift is Glu181. Accordingly, our results suggest that the frail Tyr191/268-Glu181-Wat2-Ser186 hydrogen bond network may be perturbed by the transient variations of the electrostatic potential.

The role of retinal light induced dipole in halorhodopsin structural alteration

The present work studies the mechanism of light induced protein conformational changes in the over-expressed mutant of halorhodopsin (phR) from Natronomonas pharaonis. The catalytic effect of light is reflected in accelerating hydroxyl amine reaction rate of light adapted phR. Light catalysis was detected in native phR but also in artificial pigments derived from tailored retinal analogs locked at the crucial C13=C14 double bond. It is proposed that the photoexcited retinal chromophore induces protein concerted motion that decreases the energy gap between reactants ground and transition states. This energy gap is overcome by coupling to specific protein vibrations. Surprisingly, the rate constants show unusual decreasing trend following temperature increase both for native and artificial pigments.

Evidence from Chlamydomonas on the photoactivation of rhodopsins without isomerization of their chromophore

Attachment of retinal to opsin forms the chromophore N-retinylidene, which isomerizes during photoactivation of rhodopsins. To test whether isomerization is crucial, custom-tailored chromophores lacking the β-ionone ring and any isomerizable bonds were incorporated in vivo into the opsin of a blind mutant of the eukaryote Chlamydomonas reinhardtii. The analogs restored phototaxis with the anticipated action spectra, ruling out the need for isomerization in photoactivation. To further elucidate photoactivation, responses to chromophores formed from naphthalene aldehydes were studied. The resulting action spectral shifts suggest that charge separation within the excited chromophore leads to electric field-induced polarization of nearby amino acid residues and altered hydrogen bonding. This redistribution of charge facilitates the reported multiple bond rotations and protein rearrangements of rhodopsin activation. These results provide insight into the activation of rhodopsins and related GPCRs.

Vibrational motions associated with primary processes in bacteriorhodopsin studied by coherent infrared emission spectroscopy

The primary energetic processes driving the functional proton pump of bacteriorhodopsin take place in the form of complex molecular dynamic events after excitation of the retinal chromophore into the Franck-Condon state. These early events include a strong electronic polarization, skeletal stretching, and all-trans-to-13-cis isomerization upon formation of the J intermediate. The effectiveness of the photoreaction is ensured by a conical intersection between the electronic excited and ground states, providing highly nonadiabatic coupling to nuclear motions. Here, we study real-time vibrational coherences associated with these motions by analyzing light-induced infrared emission from oriented purple membranes in the 750-1400 cm(-)(1) region. The experimental technique applied is based on second-order femtosecond difference frequency generation on macroscopically ordered samples that also yield information on phase and direction of the underlying motions. Concerted use of several analysis methods resulted in the isolation and characterization of seven different vibrational modes, assigned as C-C stretches, out-of-plane methyl rocks, and hydrogen out-of-plane wags, whereas no in-plane H rock was found. Based on their lifetimes and several other criteria, we deduce that the majority of the observed modes take place on the potential energy surface of the excited electronic state. In particular, the direction sensitivity provides experimental evidence for large intermediate distortions of the retinal plane during the excited-state isomerization process.

Experimental evidence for secondary protein-chromophore interactions at the Schiff base linkage in bacteriorhodopsin: Molecular mechanism for proton pumping

Resonance Raman spectroscopy of the retinylidene chromophore in various isotopically labeled membrane environments together with spectra of isotopically labeled model compounds demonstrates that a secondary protein interaction is present at the protonated Schiff base linkage in bacteriorhodopsin. The data indicate that although the interaction is present in all protonated bacteriorhodopsin species it is absent in unprotonated intermediates. Furthermore, kinetic resonance Raman spectroscopy has been used to monitor the dynamics of Schiff base deprotonation as a function of pH. All our results are consistent with lysine as the interacting group. A structure for the interaction is proposed in which the interacting protein group in an unprotonated configuration is complexed through the Schiff base proton to the Schiff base nitrogen. These data suggest a molecular mechanism for proton pumping and ion gate molecular regulation. In this mechanism, light causes electron redistribution in the retinylidene chromophore, which results in the deprotonation of an amino acid side chain with pK >10.2 +/- 0.3 (e.g., arginine). This induces subsequent retinal and protein conformational transitions which eventually lower the pK of the Schiff base complex from >12 before light absorption to 10.2 +/- 0.3 in microseconds after photon absorption. Finally, in this low pK state the complex can reprotonate the proton-deficient high pK group generated by light, and the complex is then reprotonated from the opposite side of the membrane.

All-trans retinal constitutes the functional chromophore in Chlamydomonas rhodopsin

Orientation of the green alga Chlamydomonas in light (phototaxis and stop responses) is controlled by a visual system with a rhodopsin as the functional photoreceptor. Here, we present evidence that in Chlamydomonas wild-type cells all-trans retinal is the predominant isomer and that it is present in amounts similar to that of the rhodopsin itself.The ability of different retinal isomers and analog compounds to restore photosensitivity in blind Chlamydomonas cells (strain CC2359) was tested by means of flash-induced light scattering transients or by measuring phototaxis in a taxigraph. All-trans retinal reconstitutes behavioral light responses within one minute, whereas cis-isomers require at least 50 x longer incubation times, suggesting that the retinal binding site is specific for all-trans retinal. Experiments with 13-demethyl(dm)-retinal and short-chained analogs reveal that only chromophores with a beta-methyl group and at least three double bonds in conjugation with the aldehyde mediate function. Because neither 13-dm-retinal, nor 9,12-phenylretinal restores a functional rhodopsin, a trans/13-cis isomerisation seems to take place in the course of the activation mechanism. We conclude that with respect to its chromophore, Chlamydomonas rhodopsin bears a closer resemblence to bacterial rhodopsins than to visual rhodopsins of higher animals.

Primary photoinduced protein response in bacteriorhodopsin and sensory rhodopsin II

Essential for the biological function of the light-driven proton pump, bacteriorhodopsin (BR), and the light sensor, sensory rhodopsin II (SRII), is the coupling of the activated retinal chromophore to the hosting protein moiety. In order to explore the dynamics of this process we have performed ultrafast transient mid-infrared spectroscopy on isotopically labeled BR and SRII samples. These include SRII in D(2)O buffer, BR in H(2)(18)O medium, SRII with (15)N-labeled protein, and BR with (13)C(14)(13)C(15)-labeled retinal chromophore. Via observed shifts of infrared difference bands after photoexcitation and their kinetics we provide evidence for nonchromophore bands in the amide I and the amide II region of BR and SRII. A band around 1550 cm(-1) is very likely due to an amide II vibration. In the amide I region, contributions of modes involving exchangeable protons and modes not involving exchangeable protons can be discerned. Observed bands in the amide I region of BR are not due to bending vibrations of protein-bound water molecules. The observed protein bands appear in the amide I region within the system response of ca. 0.3 ps and in the amide II region within 3 ps, and decay partially in both regions on a slower time scale of 9-18 ps. Similar observations have been presented earlier for BR5.12, containing a nonisomerizable chromophore (R. Gross et al. J. Phys. Chem. B 2009, 113, 7851-7860). Thus, the results suggest a common mechanism for ultrafast protein response in the artificial and the native system besides isomerization, which could be induced by initial chromophore polarization.

Insights into excited-state and isomerization dynamics of bacteriorhodopsin from ultrafast transient UV absorption

A visible-pump/UV-probe transient absorption is used to characterize the ultrafast dynamics of bacteriorhodopsin with 80-fs time resolution. We identify three spectral components in the 265- to 310-nm region, related to the all-trans retinal, tryptophan (Trp)-86 and the isomerized photoproduct, allowing us to map the dynamics from reactants to products, along with the response of Trp amino acids. The signal of the photoproduct appears with a time delay of approximately 250 fs and is characterized by a steep rise ( approximately 150 fs), followed by additional rise and decay components, with time scales characteristic of the J intermediate. The delayed onset and the steep rise point to an impulsive formation of a transition state on the way to isomerization. We argue that this impulsive formation results from a splitting of a wave packet of torsional modes on the potential surface at the branching between the all-trans and the cis forms. Parallel to these dynamics, the signal caused by Trp response rises in approximately 200 fs, because of the translocation of charge along the conjugate chain, and possible mechanisms are presented, which trigger isomerization.

Probing the ultrafast charge translocation of photoexcited retinal in bacteriorhodopsin

The ultrafast evolution of the electric field within bacteriorhodopsin was measured by monitoring the absorption changes of a tryptophan residue after excitation of retinal. The Trp absorption decreases within the first 200 femtoseconds and then recovers on time scales typical for retinal isomerization and vibrational relaxation. A model of excitonic coupling between retinal and tryptophans shows that the signal reflects a gradual rise of the retinal difference dipole moment, which precedes and probably drives isomerization. The results suggest an intimate connection between the progressive dipole moment change and the retinal skeletal changes reported over the same time scale.

Absorption of Schiff-Base Retinal Chromophores in Vacuo

The absorption spectrum of the all-trans retinal chromophore in the protonated Schiff-base form, that is, the biologically relevant form, has been measured in vacuo, and a maximum is found at 610 nm. The absorption of retinal proteins has hitherto been compared to that of protonated retinal in methanol, where the absorption maximum is at 440 nm. In contrast, the new gas-phase absorption data constitute a well-defined reference for spectral tuning in rhodopsins in an environment devoid of charges and dipoles. They replace the misleading comparison with absorption properties in solvents and lay the basis for reconsidering the molecular mechanisms of color tuning in the large family of retinal proteins. Indeed, our measurement directly shows that protein environments in rhodopsins are blue- rather than red shifting the absorption. The absorption of a retinal model chromophore with a neutral Schiff base is also studied. The data explain the significant blue shift that occurs when metharhodopsin I becomes deprotonated as well as the purple-to-blue transition of bacteriorhodopsin upon acidification.

The hydroxylamine reaction of sensory rhodopsin II: light-induced conformational alterations with C13=C14 nonisomerizable pigment

Sensory rhodopsin II, a repellent phototaxis receptor from Natronomonas (Natronobacterium) pharaonis (NpSRII), forms a complex with its cognate transducer (NpHtrII). In micelles the two proteins form a 1:1 heterodimer, whereas in membranes they assemble to a 2:2 complex. Similarly to other retinal proteins, sensory rhodopsin II undergoes a bleaching reaction with hydroxylamine in the dark which is markedly catalyzed by light. The reaction involves cleavage of the protonated Schiff base bond which covalently connects the retinal chromophore to the protein. The light acceleration reflects protein conformation alterations, at least in the retinal binding site, and thus allows for detection of these changes in various conditions. In this work we have followed the hydroxylamine reaction at different temperatures with and without the cognate transducer. We have found that light irradiation reduces the activation energy of the hydroxylamine reaction as well as the frequency factor. A similar effect was found previously for bacteriorhodopsin. The interaction with the transducer altered the light effect both in detergent and membranes. The transducer interaction decreased the apparent light effect on the energy of activation and the frequency factor in detergent but increased it in membranes. In addition, we have employed an artificial pigment derived from a retinal analog in which the critical C13=C14 double bond is locked by a rigid ring structure preventing its isomerization. We have observed light enhancement of the reaction rate and reduction of the energy of activation as well as the frequency factor, despite the fact that this pigment does not experience C13=C14 double bond isomerization. It is suggested that retinal excited state polarization caused by light absorption of the "locked" pigment polarizes the protein and triggers relatively long-lived protein conformational alterations.

Hopping of a Single Hole in hexakis[4-(1,1,2-Triphenyl-ethenyl)phenyl]benzene Cation Radical through the Hexaphenylbenzene Propeller

A versatile synthesis of a dendritic structure (5) is described in which six tetraphenylethylene moieties are connected to a central benzene ring in such a way that one of the phenyl rings of each tetraphenylethylene is also part of the propeller of the hexaphenylbenzene core. Observation of multiple oxidation waves in its cyclovoltammogram as well as an intense charge-resonance transition in the near-IR region in its cation radical spectrum suggests that a single hole delocalizes via electron transfer over six identical redox-active centers. [structure--see text]

Nonisomerizable non-retinal chromophores initiate light-induced conformational alterations in bacterioopsin

The photoactivation of retinal proteins is usually interpreted in terms of C=C photoisomerization of the retinal moiety, which triggers appropriate conformational changes in the protein. In this work several dye molecules, characterized by a completely rigid structure in which no double-bond isomerization is possible, were incorporated into the binding site of bacteriorhodopsin (bR). Using a light-induced chemical reaction of a labeled EPR probe, it was observed that specific conformational alterations in the protein are induced following light absorption by the dye molecules occupying the binding site. The exact nature of these changes and their relationship to those occurring in the bR photocycle are still unclear. Nevertheless, their occurrence proves that C=C or C=NH(+) isomerization is not a prerequisite for protein conformational changes in a retinal protein. More generally, we show that conformational changes, leading to changes in reactivity, may be induced in proteins by optical excitation of simple nonisomerizable dyes located in the macromolecular matrix.

Bacteriorhodpsin experiences light-induced conformational alterations in nonisomerizable C(13)=C(14) pigments. A study with EPR

The mechanism by which bacteriorhodopsin is activated following light absorption is not completely clear. We have detected protein conformational alterations following light absorption by retinal-based chromophores in the bacteriorhodopsin binding site by monitoring the rate of reduction-oxidation reactions of covalently attached spin labels, using EPR spectroscopy. It was found that the reduction reaction with hydroxylamine is light-catalyzed in the A103C-labeled pigment but not in E74C or M163C. The reaction is light-catalyzed even when isomerization of the C(13)=C(14) bond of the retinal chromophore is prevented. The reverse oxidation reaction with molecular oxygen is effective only in apomembrane derived from the mutant A103C. This reaction is light-accelerated following light absorption of the retinal oxime, which occupies the binding site. The light-induced acceleration is evident also in "locked" bacteriorhodopsin in which isomerization around the C(13)=C(14) bond is prevented. It is evident that the chromophore-protein covalent bond is not a prerequisite for protein response. In contrast to the case of the retinal oxime, a reduced C=N bond A103C-labeled pigment did not exhibit acceleration of the oxidation reaction following light absorption. Acceleration was observed, however, following substitution of the polyene by groups that modify the excited state charge delocalization. It is suggested that protein conformational alterations are induced by charge redistribution along the retinal polyene following light absorption.

Photochemistry in dried polymer films incorporating the deionized blue membrane form of bacteriorhodopsin

The preparation and photochemical properties of dried deionized blue membrane (dIbR600; lambdamax approximately 600 nm, epsilon approximately 54, 760 cm-1 M-1, f approximately 1.1) in polyvinyl alcohol films are studied. Reversible photoconversion from dIbR600 to the pink membrane (dIbR485; lambdamax approximately 485 nm) is shown to occur in these films under conditions of strong 647-nm laser irradiation. The pink membrane analog, dIbR485, has a molar extinction coefficient of approximately 39,000 cm-1 M-1 (f approximately 1.2). The ratio of pink --> blue and blue --> pink quantum efficiencies is 33 +/- 5. We observe an additional blue-shifted species (dIbR455, lambdamax approximately 455 nm) with a very low oscillator strength (f approximately 0.6, epsilon approximately 26,000 cm-1 M-1). This species is the product of fast thermal decay of dIbR485. Molecular modeling indicates that charge/charge and charge/dipole interactions introduced by the protonation of ASP85 are responsible for lowering the excited-state all-trans --> 9-cis barrier to approximately 6 kcal mol-1 while increasing the corresponding all-trans --> 13-cis barrier to approximately 4 kcal mol-1. Photochemical formation of both 9-cis and 13-cis photoproducts are now competitive, as is observed experimentally. We suggest that dIbR455 may be a 9-cis, 10-s-distorted species that partially divides the chromophore into two localized conjugated segments with a concomitant blue shift and decreased oscillator strength of the lambdamax absorption band.

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.

Unbleachable rhodopsin with an 11-cis-locked eight-membered ring retinal: the visual transduction process

Visual transduction occurs through photorhodopsin, the primary photoproduct of rhodopsin, which relaxes to bathorhodopsin and a series of other intermediates until it reaches the metarhodopsin II stage, upon which the enzymatic cascade leading to vision is activated. Despite advances in areas related to visual transduction, the triggering process itself, a key problem in the chemistry of rhodopsin, has remained unsolved. In order to clarify the extent of involvement of the chromophoric excited state versus the 11-cis to trans isomerization, and as an extension of past studies with 11-cis-locked seven-membered ring rhodopsin (Rh7), 11-cis eight- and nine-membered ring retinal analogs, ret8 and ret9, respectively, have been synthesized. The bulkiness of the tetramethylene bridge in ret8 led to numerous unexpected obstacles in attempts to reconstitute a ret8-containing rhodopsin (Rh8) embedded in lipid bilayer membranes. These obstacles were solved by using methylated rhodopsin which gave MeRh8 containing 11-cis-ret8 as its chromophore. MeRh8 exhibited UV-vis and CD spectra very similar to those of native rhodopsin (Rh); furthermore, the quantum efficiency of photorhodopsin formation was comparable to that of Rh.(ABSTRACT TRUNCATED AT 250 WORDS)

Spectral sensitivity, structure and activation of eukaryotic rhodopsins: activation spectroscopy of rhodopsin analogs in Chlamydomonas

Retinal normally binds opsin forming the chromophore of the visual pigment, rhodopsin. In this investigation synthetic analogs were bound by the opsin of living cells of the alga Chlamydomonas reinhardtii; the effect was assayed by phototaxis to give an activation spectrum for each rhodopsin analog. The results show the influence of different chromophores and the protein on the absorption of light. The maxima of the phototaxis action spectra shifted systematically with the number of double bonds conjugated with the imine (C = N+H) bond of the chromophore. Chromophores lacking a beta-ionone ring, methyl groups and all C = C double bonds photoactivated the rhodopsin of Chlamydomonas with normal efficiency. On the basis of a simple model involving one-electron transitions between occupied and virtual molecular orbitals, we estimate the charge distribution along the chromophore in the binding site. With this restraint we define a unique structural model for eukaryotic rhodopsins and explain the spectral clustering of pigments, the spectral differences between red and green rhodopsins and the molecular basis of color blindness. Our results are consistent with the triggering of the activation of rhodopsin by the light-mediated change in electric dipole moment rather than the steric cis-trans isomerization of the chromophore.