On the Fluorescence Properties and Nonradiative Transitions in Medium-Sized All-Trans Polyenes

The nonadiabatic photodynamics of all-trans linear polyenes with N = 4-8 conjugated double bonds is studied from an electronic structure perspective. Excitation energies and stationary points for the 1Bu and 2Ag singlet states have been computed by using the state-average complete active space (SA-CASSCF) method and its second-order perturbation theory variant (MS-CASPT2). The dependence of the two low-lying excited states on the "chain length" N has been elucidated. In addition, the 1Bu-2Ag crossing seam has been mapped out in a suitable two-dimensional coordinate space and its minimum within the subspace has been determined. This minimum is found to increase substantially and monotonously in energy with increasing N. This increase is discussed and interpreted in relation to the fluorescence properties of these systems. In particular, it allows to understand the crossover from S1(2Ag) fluorescence for smaller N to S2(1Bu) (or dual) fluorescence for larger N.

From a one-mode to a multi-mode understanding of conical intersection mediated ultrafast organic photochemical reactions

This review discusses how ultrafast organic photochemical reactions are controlled by conical intersections, highlighting that decay to the ground-state at multiple points of the intersection space results in their multi-mode character.

Combining Photodeprotection and Ligation into a Dual-Color Gated Reaction System

We report a photochemical reaction system which requires activation by two colors of light. Specifically, a dual wavelength gated system is established by fusing the visible light mediated deprotection of a dithioacetal with the UV light activated Diels–Alder reaction of an o‐methylbenzaldehyde with n‐ethylmaleimide. Critically, both light sources are required to achieve the Diels–Alder adduct, irradiation with visible or UV light alone does not lead to the target product. The introduced dual gated photochemical system is particularly interesting for application in light driven 3D printing, where two color wavelength activated photoresists may become reality.

Acid-base equilibrium of the chromophore counterion results in distinct photoisomerization reactivity in the primary event of proteorhodopsin

Proteorhodopsin (PR) is a proton-pumping rhodopsin, and it is known to exhibit a multi-phasic decay of the excited-state population in the primary process. So far, this complex excited-state decay has been attributed to the branching of the relaxation pathway on the excited-state potential energy surface. However, a recent ultrafast spectroscopic study on a sodium-pumping rhodopsin suggested that such a complex decay may originate from the heterogeneity in the ground state due to the acid-base equilibrium of the counterion of the protonated retinal Schiff base (PRSB). In this study, we studied the excited-state dynamics of PR at pH 11 and 4, in which the counterion of the PRSB, Asp97, is completely deprotonated and protonated, respectively. The obtained time-resolved absorption data revealed that the excited-state lifetime is decisively governed by the protonation state of Asp97, and the photoisomerization of the PRSB chromophore proceeds faster and more efficiently when Asp97 is deprotonated. This conclusion was further supported by high similarity of the excited-state dynamics between PR at pH 4 and the D97N mutant in which Asp97 is replaced with neutral Asn. The results of this study suggest that the protonation state of the PRSB counterion plays a decisive role in determining the excited-state dynamics and the photoisomerization reactivity of rhodopsins in general, by making a significant influence on the exited-state potential energy surface of the PRSB chromophore.

Substituent Dependence Charge Transfer and Photochemical Properties of Donor-Acceptor Substituted Ethenyl Thiophenes

Donor-acceptor conjugated molecules with efficient light induced properties represent interesting material for electronic device application. In this context, we have calculated excited state dipole moment of three ethenyl thiophenes (1-3) bearing varied electron donor-acceptor substituent in p-phenyl unit using Lippert-Mataga, Bakhshiev and Kawski method. It is found that 1 with strong electron-withdrawing nitro substituent, is exhibiting charge transfer and highly dipolar excited state as compared to 2 and 3. Photochemical studies of 1-3, indicate towards the charge transfer dependence trans-cis photoisomerization under direct irradiation condition. Compound 1 exhibits charge transfer and less efficient towards photoisomerization, whereas 2 and 3 undergo efficient photoisomerization. Graphical Abstract Substituent dependence charge transfer and photochemical properties of donor-acceptor substituted ethenyl thiophenes.

Development of Bacteriorhodopsin Analogues and Studies of Charge Separated Excited States in the Photoprocesses of Linear Polyenes†

Development of bacteriorhodopsin (bR) analogues employing chromophore substitution technique for the purpose of characterizing the binding site of bR and generating bR analogues with novel opto-electronic properties for applications as photoactive element in nanotechnical devices are described. Additionally, the photophysical and photochemical properties of variously substituted diarylpolyenes as models of photobiologically relevant linear polyenes are discussed. The role of charge separated dipolar excited states in the photoprocesses of linear polyenes is highlighted.

The retinal chromophore/chloride ion pair: structure of the photoisomerization path and interplay of charge transfer and covalent states

Ab initio multi-reference second-order perturbation theory computations are used to explore the photochemical behavior of two ion pairs constituted by a chloride counterion interacting with either a rhodopsin or bacteriorhodopsin chromophore model (i.e., the 4-cis-gamma-methylnona-2,4,6,8-tetraeniminium and all-trans-nona-2,4,6,8-tetraeniminium cations, respectively). Significant counterion effects on the structure of the photoisomerization paths are unveiled by comparison with the paths of the same chromophores in vacuo. Indeed, we demonstrate that the counterion (i) modulates the relative stability of the S0, S1, and S2 energy surfaces leading to an S1 isomerization energy profile where the S1 and S2 states are substantially degenerate; (ii) leads to the emergence of significant S1 energy barriers along all of the isomerization paths except the one mimicking the 11-cis --> all-trans isomerization of the rhodopsin chromophore model; and (iii) changes the nature of the S1 --> S0 decay funnel that becomes a stable excited state minimum when the isomerizing double bond is located at the center of the chromophore moiety. We show that these (apparently very different) counterion effects can be rationalized on the basis of a simple qualitative electrostatic model, which also provides a crude basis for understanding the behavior of retinal protonated Schiff bases in solution.

QM/MM study of energy storage and molecular rearrangements due to the primary event in vision

The energy storage and the molecular rearrangements due to the primary photochemical event in rhodopsin are investigated by using quantum mechanics/molecular mechanics hybrid methods in conjunction with high-resolution structural data of bovine visual rhodopsin. The analysis of the reactant and product molecular structures reveals the energy storage mechanism as determined by the detailed molecular rearrangements of the retinyl chromophore, including rotation of the (C11-C12) dihedral angle from -11 degrees in the 11-cis isomer to -161 degrees in the all-trans product, where the preferential sense of rotation is determined by the steric interactions between Ala-117 and the polyene chain at the C13 position, torsion of the polyene chain due to steric constraints in the binding pocket, and stretching of the salt bridge between the protonated Schiff base and the Glu-113 counterion by reorientation of the polarized bonds that localize the net positive charge at the Schiff-base linkage. The energy storage, computed at the ONIOM electronic-embedding approach (B3LYP/6-31G*:AMBER) level of theory and the S0-->S1 electronic-excitation energies for the dark and product states, obtained at the ONIOM electronic-embedding approach (TD-B3LYP/6-31G*//B3LYP/6-31G*:AMBER) level of theory, are in very good agreement with experimental data. These results are particularly relevant to the development of a first-principles understanding of the structure-function relations in prototypical G-protein-coupled receptors.

Femtosecond primary events in bacteriorhodopsin and its retinal modified analogs: Revision of commonly accepted interpretation of electronic spectra of transient intermediates in the bacteriorhodopsin photocycle

Femtosecond primary events in bacteriorhodopsin (BR) and its retinal modified analogs are discussed. Ultrafast time resolved electronic spectra of the primary intermediates induced in the BR photocycle are discussed along with spectral and kinetic inconsistencies of the previous models proposed in the literature. The theoretical model proposed in this paper based on vibrational coupling between the electronic transition of the chromophore and intramolecular vibrational modes allows us to calculate the equilibrium electronic absorption band shape and the hole burning profiles. The model is able to rationalize the complex pattern of behavior for the primary events in BR and explain the origin of the apparent inconsistencies between the experiment and the previous theoretical models. The model presented in the paper is based on the anharmonic coupling assumption in the adiabatic approximation using the canonical transformation method for diagonalization of the vibrational Hamiltonian instead of the commonly used perturbation theory. The electronic transition occurs between the Born-Oppenheimer potential energy surfaces with the electron involved in the transition being coupled to the intramolecular vibrational modes of the molecule (chromophore). The relaxation of the excited state occurs by indirect damping (dephasing) mechanisms. The indirect dephasing is governed by the time evolution of the anharmonic coupling constant driven by the resonance energy exchange between the intramolecular vibrational mode and the bath. The coupling with the intramolecular vibrational modes results in the Franck-Condon progression of bands that are broadened due to the vibrational dephasing mechanisms. The electronic absorption line shape has been calculated based on the linear response theory whereas the third order nonlinear response functions have been used to analyze the hole burning profiles obtained from the pump-probe time-resolved measurements. The theoretical treatment proposed in this paper provides a basis for a substantial revision of the commonly accepted interpretation of the primary events in the BR photocycle that exists in the literature.

Retinoid cycle in the vertebrate retina: experimental approaches and mechanisms of isomerization

Retinoid cycle describes a set of chemical transformations that occur in the photoreceptor and retinal pigment epithelial cells. The hydrophobic and labile nature of the retinoid substrates and the two-cell chromophore utilization-regeneration system imposes significant constraints on the experimental biochemical approaches employed to understand this process. A brief description of the recent developments in the investigation of the retinoid cycle is the current topic, which includes a review of novel results and techniques pertaining to the retinoid cycle. The chemistry of the all-trans-retinol to 11-cis-retinol isomerization is also discussed.

Isomerization around a CN double bond and a CC double bond with a nitrogen atom attached: thermal and photochemical routes

The Longuet-Higgins phase change theorem is used to show that, in certain photochemical reactions, a single product is formed via a conical intersection. The cis-trans isomerization around the double bond in the formaldiminium cation and vinylamine are shown to be possible examples. This situation is expected to hold when the reactant can be converted to the product via two distinct elementary ground-state reactions that differ in their phase characteristics. In one, the total electronic wavefunction preserves its phase in the reaction; in the other, the phase is inverted. Under these conditions, a conical intersection necessarily connects the first electronic excited state to the ground state, leading to rapid photochemical isomerization following optical excitation. Detailed quantum chemical calculations support the proposed model. The possibility that a similar mechanism is operative in other systems, among them the rapid photo-induced cis-trans isomerization of longer protonated Schiff bases (the parent chromophores of rhodopsins), is discussed.

Introduction to the symposium-in-print: photoisomerization pathways, torsional relaxation and the hula twists

A review of recent literature on volume-conserving cis-trans photoisomerization reaction mechanism, including hula twist, is presented. Differences between substrates trapped in amorphous solids and chromophores that are protein bound are discussed.

Confronting complexity: the interlink of phototransduction and retinoid metabolism in the vertebrate retina

Absorption of light by rhodopsin or cone pigments in photoreceptors triggers photoisomerization of their universal chromophore, 11-cis-retinal, to all-trans-retinal. This photoreaction is the initial step in phototransduction that ultimately leads to the sensation of vision. Currently, a great deal of effort is directed toward elucidating mechanisms that return photoreceptors to the dark-adapted state, and processes that restore rhodopsin and counterbalance the bleaching of rhodopsin. Most notably, enzymatic isomerization of all-trans-retinal to 11-cis-retinal, called the visual cycle (or more properly the retinoid cycle), is required for regeneration of these visual pigments. Regeneration begins in rods and cones when all-trans-retinal is reduced to all-trans-retinol. The process continues in adjacent retinal pigment epithelial cells (RPE), where a complex set of reactions converts all-trans-retinol to 11-cis-retinal. Although remarkable progress has been made over the past decade in understanding the phototransduction cascade, our understanding of the retinoid cycle remains rudimentary. The aim of this review is to summarize recent developments in our current understanding of the retinoid cycle at the molecular level, and to examine the relevance of these reactions to phototransduction.

Computational evidence in favor of a two-state, two-mode model of the retinal chromophore photoisomerization

In this paper we use ab initio multiconfigurational second-order perturbation theory to establish the intrinsic photoisomerization path model of retinal chromophores. This is accomplished by computing the ground state (S(0)) and the first two singlet excited-state (S(1), S(2)) energies along the rigorously determined photoisomerization coordinate of the rhodopsin chromophore model 4-cis-gamma-methylnona-2,4,6,8-tetraeniminium cation and the bacteriorhodopsin chromophore model all-trans-hepta-2,4, 6-trieniminium cation in isolated conditions. The computed S(2) and S(1) energy profiles do not show any avoided crossing feature along the S(1) reaction path and maintain an energy gap >20 kcal small middle dotmol(-1). In addition, the analysis of the charge distribution shows that there is no qualitative change in the S(2) and S(1) electronic structure along the path. Thus, the S(1) state maintains a prevalent ionic (hole-pair) character whereas the S(2) state maintains a covalent (dot-dot) character. These results, together with the analysis of the S(1) reaction coordinate, support a two-state, two-mode model of the photoisomerization that constitutes a substantial revision of the previously proposed models.

Low-temperature trapping of early photointermediates of alpha-isorhodopsin

Alpha-Isorhodopsin, an artificial visual pigment with a 9-cis-4,5-dehydro-5,6-dihydro(alpha)retinal chromophore, was photolyzed at low temperatures and absorption difference spectra were collected as the sample was warmed. A bathorhodopsin (Batho)-like intermediate absorbing at ca 495 nm was detected below 55 K,a blue-shifted intermediate (BSI)-like intermediate absorbing at ca 453 nm was observed when the temperature was raised to 60 K and a lumirhodopsin (Lumi)-like intermediate absorbing at ca 470 nm was found when the sample was warmed to 115 K. Photointermediates from this pigment were compared to those of native rhodopsin and 5,6-dihydroisorhodopsin. As in native rhodopsin, Batho is the first intermediate detected in alpha-isorhodopsin, though unlike native rhodopsin at low temperatures BSI is observed prior to Lumi formation. Alpha-Isohodopsin behaves similarly to 5,6-dihydroisorhodopsin, with the same early intermediates observed in both artificial visual pigments lacking the C5-C6 double bond. The transition temperature for BSI formation is higher in alpha-isorhodopsin, suggesting an interaction involving the chromophore ring in BSI formation. The transition temperature for Lumi formation is similar for these two pigments as well as for native rhodopsin, suggesting comparable changes in the protein environment in that transition.

Vibrationally coherent photochemistry in the femtosecond primary event of vision

Femtosecond pump-probe experiments reveal the impulsive production of photoproduct in the primary event in vision. The retinal chromophore of rhodopsin was excited with a 35-femtosecond pulse at 500 nanometers, and transient changes in absorption were measured with 10-femtosecond probe pulses. At probe wavelengths within the photo-product absorption band, oscillatory features with a period of 550 femtoseconds (60 wavenumbers) were observed whose phase and amplitude demonstrate that they are the result of nonstationary vibrational motion in the ground state of the photoproduct. The observation of coherent vibrational motion of the photoproduct supports the idea that the primary step in vision is a vibrationally coherent process and that the high quantum yield of the cis-->trans isomerization in rhodopsin is a consequence of the extreme speed of the excited-state torsional motion.

The first step in vision: femtosecond isomerization of rhodopsin

The kinetics of the primary event in vision have been resolved with the use of femtosecond optical measurement techniques. The 11-cis retinal prosthetic group of rhodopsin is excited with a 35-femtosecond pump pulse at 500 nanometers, and the transient changes in absorption are measured between 450 and 580 nanometers with a 10-femtosecond probe pulse. Within 200 femtoseconds, an increased absorption is observed between 540 and 580 nanometers, indicating the formation of photoproduct on this time scale. These measurements demonstrate that the first step in vision, the 11-cis----11-trans torsional isomerization of the rhodopsin chromophore, is essentially complete in only 200 femtoseconds.

Applications of ultrafast laser spectroscopy for the study of biological systems

The discovery of mode-locked laser operation now nearly two decades ago has started a development which enables researchers to probe the dynamics of ultrafast physical and chemical processes at the molecular level on shorter and shorter time scales. Naturally the first applications were in the fields of photophysics and photochemistry where it was then possible for the first time to probe electronic and vibrational relaxation processes on a sub-nanosecond timescale. The development went from lasers producing pulses of many picoseconds to the shortest pulses which are at present just a few femtoseconds long. Soon after their discovery ultrashort pulses were applied also to biological systems which has revealed a wealth of information contributing to our understanding of a broadrange of biological processes on the molecular level.It is the aim of this review to discuss the recent advances and point out some future trends in the study of ultrafast processes in biological systems using laser techniques. The emphasis will be mainly on new results obtained during the last 5 or 6 years. The term ultrafast means that I shall restrict myself to sub-nanosecond processes with a few exceptions.

Motion of proton in double-well potential. Its possible role in the process of vision

A quantum-mechanical model that allows direct calculation of the rate of proton transfer with possible application to the early events in vision is presented. The rate is obtained in closed form in the linear-response approximation in terms of four parameters. The model permits a simple calculation of these parameters and shows modest agreement with the experimentally determined rate constant.

Quantum efficiencies of the reversible photoreaction of octopus rhodopsin

The classic method of photometric curves for photosensitivity determination has been extended to the case of photoreversible reactions and applied to the octopus rhodopsin --> acid metarhodopsin photoreaction. In such cases, measurements at one irradiation wavelength yield the sum of the photosensitivities of the forward and reverse processes. However, by using different irradiation wavelengths, together with appropriate molar extinction coefficients, the quantum efficiencies for both reactions may be resolved. For detergent-solubilized octopus rhodopsin at room temperature, pH 7, the quantum yields are found to be 0.69 (+/- 0.03) for rhodopsin --> metarhodopsin, in line with values observed in a range of vertebrate visual pigments, and 0.43 (+/- 0.02) for the reverse photoregeneration process. The similarities in overall photosensitivities of the forward and reverse reactions in the visible region are consistent with a significant physiological role for photoreversal in the cephalopod visual cycle.

Fourier-transform infrared difference spectroscopy of rhodopsin and its photoproducts at low temperature

Fourier-transform infrared difference spectroscopy has been used to detect the vibrational modes in the chromophore and protein that change in position or intensity between rhodopsin and the photoproducts formed at low temperature (70 K), bathorhodopsin and isorhodopsin. A method has been developed to obtain infrared difference spectra between rhodopsin and bathorhodopsin, bathorhodopsin and isorhodopsin, and rhodopsin and isorhodopsin. To aid in the identification of the vibrational modes, we performed experiments on deuterated and hydrated films of native rod outer segments and rod outer segments regenerated with either retinal containing 13C at carbon 15 or 15-deuterioretinal. Our infrared measurements provide independent verification of the resonance Raman result that the retinal in bathorhodopsin is distorted all-trans. The positions of the C = N stretch in the deuterated pigment and the deuterated pigments regenerated with 11-cis-15-deuterioretinal or 11-cis-retinal containing 13C at carbon 15 are indicative that the Schiff-base linkage is protonated in rhodopsin, bathorhodopsin, and isorhodopsin. Furthermore, the C = N stretching frequency occurs at the same position in all three species. The data indicate that the protonated Schiff base has a C = N trans conformation in all three species. Finally, we present evidence that, even in these early stages of the rhodopsin photosequence, changes are occurring in the opsin and perhaps the associated lipids.