Photoisomerization Of Rhodopsin Research Articles

ChemInform Abstract: A Quantum Chemical Study on the Mechanism of cis‐trans Isomerization in Retinal‐Like Protonated Schiff Bases

The dynamics of the photochemical cis-trans isomerization in retinal-like protonated Schiff bases is studied by means of MNDO/CI calculations. The aim of these calculations is a better understanding of the mechanism which accounts for the highly regioselective and efficient photoisomerization of rhodopsin and bacteriorhodopsin in the primary step after light absorption. Calculations on the model compound protonated l-imino-2,4-pentadiene show that the regioselectivity and efficiency of this reaction can be explained from the intrinsic properties of this molecule. Whereas it is found that the protonated Schiff bases have a lowest 'B,+-like excited state, the second 'A,:like excited state is particularly photochemical labile. This latter state serves in diminishing (or even removing) the barrier on the potential energy surface of the initially excited state, thus enhancing the rate for photoisomerization. The transition probability for a radiationless return of the excited molecule to its ground state was evaluated explicitly for the photoisomerization around the various double bonds in protonated 1 -imino- 2,4-pentadiene by means of semiclassical trajectory calculations. The transition probability depends on the energy gap between the ground and excited state and the nonadiabatic coupling between these states for the 90° twisted molecule. The extent of the energy gap is related to the distance from the twisted bond to the nitrogen atom. The role of this electron-deficient nitrogen atom is to stabilize the polarized resonance structure which describes the 90° twisted molecule in the excited state. When this stabilization is too strong, the polarized resonance structure drops below the diradicalar ground state which results in an increased energy gap and a reduced efficiency of photoisomerization. The possibility for a concerted bicycle pedal isomerization around two double bonds is investigated by a calculation of the two-dimensional energy surfaces and nonadiabatic couplings for a combined rotation around these two bonds. A strictly bicycle pedal motion is found to be unfavorable, but a mechanism which involves a complete rotation around one double bond assisted by a partial rotation of the second double bond might provide a route for the photoisomerization of the retinylidene chromophore in the confined environment of a protein. Calculations on a model compound of the protonated Schiff base of retinal show that the extent of the stability of the 90° twisted molecule in the excited state can be directed by locating external point-charges around the molecule. In nature, these point-charges are provided by the protein opsin, and their presence has been used to explain the opsin shift of the various intermediates in the photocycles of rhodopsin and bacteriorhodopsin. Our calculations show that these external point-charges also have an important impact on the energy gap between the ground and excited state and, therefore, on the regioselectivity and efficiency of photoisomerization in the retinylidene chromophore. The primary step in the photoisomerization in bac- teriorhodopsin can be best understood from an external point-charge model with a negative counterion near the protonated nitrogen atom and an ion pair near the cyclohexene ring.

A quantum chemical study on the mechanism of cis-trans isomerization in retinal-like protonated Schiff bases

The dynamics of the photochemical cis-trans isomerization in retinal-like protonated Schiff bases is studied by means of MNDO/CI calculations. The aim of these calculations is a better understanding of the mechanism which accounts for the highly regioselective and efficient photoisomerization of rhodopsin and bacteriorhodopsin in the primary step after light absorption. Calculations on the model compound protonated l-imino-2,4-pentadiene show that the regioselectivity and efficiency of this reaction can be explained from the intrinsic properties of this molecule. Whereas it is found that the protonated Schiff bases have a lowest 'B,+-like excited state, the second 'A,:like excited state is particularly photochemical labile. This latter state serves in diminishing (or even removing) the barrier on the potential energy surface of the initially excited state, thus enhancing the rate for photoisomerization. The transition probability for a radiationless return of the excited molecule to its ground state was evaluated explicitly for the photoisomerization around the various double bonds in protonated 1 -imino- 2,4-pentadiene by means of semiclassical trajectory calculations. The transition probability depends on the energy gap between the ground and excited state and the nonadiabatic coupling between these states for the 90° twisted molecule. The extent of the energy gap is related to the distance from the twisted bond to the nitrogen atom. The role of this electron-deficient nitrogen atom is to stabilize the polarized resonance structure which describes the 90° twisted molecule in the excited state. When this stabilization is too strong, the polarized resonance structure drops below the diradicalar ground state which results in an increased energy gap and a reduced efficiency of photoisomerization. The possibility for a concerted bicycle pedal isomerization around two double bonds is investigated by a calculation of the two-dimensional energy surfaces and nonadiabatic couplings for a combined rotation around these two bonds. A strictly bicycle pedal motion is found to be unfavorable, but a mechanism which involves a complete rotation around one double bond assisted by a partial rotation of the second double bond might provide a route for the photoisomerization of the retinylidene chromophore in the confined environment of a protein. Calculations on a model compound of the protonated Schiff base of retinal show that the extent of the stability of the 90° twisted molecule in the excited state can be directed by locating external point-charges around the molecule. In nature, these point-charges are provided by the protein opsin, and their presence has been used to explain the opsin shift of the various intermediates in the photocycles of rhodopsin and bacteriorhodopsin. Our calculations show that these external point-charges also have an important impact on the energy gap between the ground and excited state and, therefore, on the regioselectivity and efficiency of photoisomerization in the retinylidene chromophore. The primary step in the photoisomerization in bac- teriorhodopsin can be best understood from an external point-charge model with a negative counterion near the protonated nitrogen atom and an ion pair near the cyclohexene ring.

A model of the light dependent regulation of retinal rod phosphodiesterase, guanylate cyclase and the cation flux.

In the 1880’s Kuhn dissected dark adapted eyes and observed that the retina’s red/purple “visual pigment”, changed rapidly to a pale yellow upon illumination. Years later, George Wald and Ruth Hubbard chemically characterized the rod visual pigment as consisting of opsin and 11-cis retinal (Wald, 1968). They demonstrated that the photoisomerization of rhodopsin was driven by a photon induced change in the configuration of 11-cis retinal (to all-trans), which was accompanied by conformational changes in the protein opsin. Subsequently, Wald, Yoshizawa and others were able to identify a series of spectral intermediates that appeared in rapid succession following the illumination of rhodopsin. The early intermediates could only be captured by stabilization at low temperatures or ultrafast spectroscopy. The major opsin photoconformers are called batho, hypso, meta I, meta II and meta III rhodopsin (Wald 1968; Birge, 1981). The meta II conformation was subsequently found to be enzymatically active (see below).

Models for quick activation of transducin and spontaneous deactivation of activated rhodopsin in rod outer segments

Serial-parallel activation (S-P) and parallel-parallel activation (P-P) models had been proposed to account for the quick activation of transmitter molecules (transducin) in the visual transduction process in rod outer segments10. In these models, unbleached rhodopsin molecules are activated by bleached rhodopsin molecules, and both bleached and activated rhodopsin molecules activate transducin. This activation mechanism enables bleached rhodopsin molecules to transfer the information of photon detection quickly to transducin. In the previous models, however, the rhodopsin photoisomerization process and the deactivation process of activated rhodopsin molecules were excluded. In the present study, these two processes are incorporated into the models. The deactivation process of activated rhodopsin molecules is assumed to proceed spontaneously. It is presented that, in spite of the incorporation of these two processes. the present S-P and P-P models can account for the quick activation of transducin in rod outer segments.

Models for quick activation of transducin and spontaneous deactivation of activated rhodopsin in rod outer segments

Serial-parallel activation (S-P) and parallel-parallel activation (P-P) models had been proposed to account for the quick activation of transmitter molecules (transducin) in the visual transduction process in rod outer segments 10. In these models, unbleached rhodopsin molecules are activated by bleached rhodopsin molecules, and both bleached and activated rhodopsin molecules activate transducin. This activation mechanism enables bleached rhodopsin molecules to transfer the information of photon detection quickly to transducin. In the previous models, however, the rhodopsin photoisomerization process and the deactivation process of activated rhodopsin molecules were excluded. In the present study, these two processes are incorporated into the models. The deactivation process of activated rhodopsin molecules is assumed to proceed spontaneously. It is presented that, in spite of the incorporation of these two processes, the present S-P and P-P models can account for the quick activation of transducin in rod outer segments.

Light-induced increases in cGMP metabolic flux correspond with electrical responses of photoreceptors.

The metabolism of photoreceptor cGMP and the relationship of its light-sensitive regulation to rhodopsin photoisomerization and to the photoreceptor electrical response was examined in isolated, intact rabbit retinas. The dynamics of cGMP metabolism were assessed by measuring the rate of 18O incorporation from 18O-water into the alpha-phosphoryls of the guanine nucleotides. The photoreceptor electrical response was determined by measuring the aspartate-isolated mass receptor potential. Basal cGMP flux in dark-adapted retinas was 33 pmol cGMP X mg protein-1 X s-1 which translates into a metabolic rate in the rod outer segment (ROS) of 1.7 mM/min in ATP equivalents. Photic stimulation increased this flux as much as 4.5-fold. With continuous illumination, increasing intensity caused increments in cGMP metabolic flux to a maximum of 4.5-fold, with corresponding increases in the electrical response over the same 3-log unit intensity range. Tight coupling between activation of guanylate cyclase and phosphodiesterase was indicated by either no changes in cGMP steady state concentrations or relatively small fluctuations represented by increases of 50% at lower light intensities and a 12% decrease at one of the highest intensities. A stoichiometry of about 10,000 molecules of cGMP generated and hydrolyzed per photon absorbed was calculated for the lowest light intensity when the increment in cGMP metabolic flux per photon was maximal. Flashing light caused an increase in flux in proportion to frequency up to 1 Hz and a nearly proportional increase in the voltage time integral of the electrical response up to 0.5 Hz. This indicates that the temporal resolution, or "on"/"off" rate, of the cGMP metabolic response was as fast or faster than the temporal resolution of the electrical response. The concentration of cGMP remained relatively stable in spite of the marked acceleration of cGMP flux that occurred over the 32-fold range of frequencies tested. Taken together these results show that the light-accelerated rate of cGMP synthesis tightly coupled to hydrolysis becomes a primary energy-utilizing system in the photoreceptor and represents a response that fulfills certain of the fundamental criteria required of a metabolic event playing an essential role in phototransduction.

Colour dependence of the early receptor potential and late receptor potential in scallop distal photoreceptor.

1. Intracellular voltage and current responses to short (blue) and long (red) wave-length lights were measured in the distal hyperpolarizing photoreceptor (;off receptor') of the isolated and perfused scallop (Pecten irradians) retina.2. The early receptor potential (e.r.p.) was isolated by holding membrane potential at the reversal potential for the late receptor potential (l.r.p.) or by working at temperatures (< 5.0 degrees C) that abolished the l.r.p.3. The e.r.p., measured using intense flashes of white light, consisted of a positive phase followed by a negative phase, but was converted to a monophasic, negative-going wave following pre-adaptation with red light and to a monophasic, positive-going wave following pre-adaptation with blue light.4. The spectral sensitivity curve for the negative e.r.p. was maximum at 500 nm, whereas the spectral sensitivity curve for the positive e.r.p. was maximum at 575 nm.5. The positive or negative e.r.p.s approached their maximum amplitude exponentially when tested with red or blue flashes of increasing intensity. The results suggest that the positive (or negative) e.r.p. is proportional to the number of photopigment molecules photo-isomerized.6. The photosensitivity maximum of rhodopsin calculated at 500 nm, using the exponential constant and the spectral sensitivity data, was estimated to be 2.1 x 10(-16) cm(2) photon(-1), whereas the photosensitivity maximum of metarhodopsin calculated at 575 nm was estimated to be 2.6 x 10(-16) cm(2) photon(-1).7. In cells pre-adapted with white light, stimulation with blue light caused a hyperpolarizing l.r.p. which was followed by a prolonged hyperpolarizing after-potential (p.h.a.). Stimulation with red light under similar conditions caused an initial hyperpolarization which was followed by a small depolarization during the stimulus, but no after-potential.8. The duration of the p.h.a. was increased by pre-adaptation with a red light, which caused the maximum net transfer of metarhodopsin to rhodopsin; however, its decay was always complete in 5 min or less.9. The photo-isomerization of metarhodopsin by red light suppressed the p.h.a. and caused an after-depolarizing response that decayed in less than 1 min.10. The spectral sensitivity curve for the induction of the p.h.a. was maximum at 500 nm and corresponded to the spectral sensitivity for the negative e.r.p. and for the l.r.p. studied in the dark-adapted retina, whereas the spectral sensitivity curve for the suppression of the p.h.a. and for the induction of the after-depolarization was maximum at 575 nm and corresponded to the spectral sensitivity for the positive e.r.p.11. In photoreceptors clamped to the resting potential in normal ASW, the photo-isomerization of rhodopsin, in the absence of light absorption by metarhodopsin, activated a persistent outward current that had the same time course of decay as the p.h.a. The photo-isomerization of metarhodopsin suppressed the persistent outward current and activated an inward current whose decay took longer than the decay of the after-depolarizing response.12. In the absence of external Ca(2+) and Na(+) ions, the persistent outward current produced by light absorption by rhodopsin, and the inward current produced by light absorption by metarhodopsin, both reversed at the K(+) equilibrium potential. The results show that the induction of the prolonged hyperpolarizing after-potential and the after-depolarizing response involve only the movement of K(+) ions through the same light-dependent K(+) channels that determine the hyperpolarizing l.r.p. of the distal cells.

54] Photosensitivity and quantum efficiency of photoisomerization in rhodopsin and retinal

Publisher Summary This chapter discusses the photosensitivity and quantum efficiency of photoisomerization in rhodopsin and retinal. The photochemistry of inorganic and organic molecules is of wide-spread interest and continues to be studied extensively. But the photochemical changes that occur in biologically important molecules, molecules such as those that serve as the primary light receptors in photosynthesis, vision, phototaxis, and plant regulation, have a special fascination. This chapter documents the complexity of the photochemical process in vision. As with many complex processes, however, the overall efficiency is determined by a single step. In the case of the visual pigments, the efficacy of light quanta in triggering the multistep photochemical process that follows can readily be measured, even though the process itself is not fully understood at the theoretical level.

Molecular dynamics of trans-cis isomerization in bathorhodopsin

Semiempirical molecular dynamics procedures are used to theoretically investigate the trajectories and quantum yields of the rhodopsin leads to bathorhodopsin and bathorhodopsin leads to rhodopsin photoisomerizations. The calculations are based on the semiclassical trajectory formalism and rhodopsin binding site model proposed by Birge and Hubbard (1980. J. Am. Chem. Soc. 102: 2195-2205). The rhodopsin leads to bathorhodopsin photoisomerization is predicted to occur in approximately 2.2 ps with a quantum yield of 0.62 in reasonable agreement with experiment (less than 6 ps, phi = 0.67). The bathorhodopsin leads to rhodopsin photoisomerization is predicted to occur in approximately 1.8 ps with a quantum yield of 0.48. The latter number is in good agreement with the observed quantum yield for cattle bathorhodopsin (phi = 0.5) but in poor agreement with the observed value for squid bathorhodopsin (phi = 0.36). Our calculations suggest that the observed photochemical preference of the chromophore in cattle bathorhodopsin to isomerize to form rhodopsin (phi = 0.5), instead of isorhodopsin (phi - 0.054), is associated with a significant out-of-plane distortion (9-17 degrees) of the 11,12-trans dihedral angle in the batho chromophore.

A note on the evolution of the transducer mechanism of the vertebrate retinal rod.

The Ca2+-gating mechanism that is the key component of membrane excitability in paramecium has recently been localized on the cilia of this animal. Such a finding shows how the change in Ca2+ permeability (which is the probable consequence of the photoisomerization of rhodopsin) of the discs of the outer segments of the retinal rods could have evolved.

Molecular Mechanism for the Initial Process of Visual Excitation. I. Model of Photoisomerization in Rhodopsin and Its Theoretical Basis by a Quantum Mechanical Calculation of Adiabatic Potential

A model for explaining the photoisomerization of the chromophore in rhodopsin molecule with a high quantum yield is proposed on the basis of the strain effect. Necessary conditions assumed in the model are shown to be satisfied by the calculation using a self-consistent HMO theory which is recently developed by one of the authors. It is also shown that when a double-bond is twisted by about 40°, a steep change occurs in the molecular geometry, and that calculated adiabatic potentials, absorption wavelengths and transition dipole moments show some anomalies, corresponding to the change. Calculated energy barriers for isomerization around single-bonds of Schiff-bases of retinal in the ground state are very small.

Photoconduction and semiconduction in dried receptors of sheep eyes

Both photoconduction and semiconduction have been measured in dried sheep rods. The semiconduction activation energy is 2.3 e.v. The photocurrent decreases under prolonged irradiation and shows a slight recovery in the dark. This is related to the photoisomerization of rhodopsin. The photoconduction excitation spectrum was roughly determined to correspond to absorption in the α and β bands of rhodopsin. These results are in good agreement with the photoconductive theory of the visual receptor process.