Ultrafast Photoisomerization Research Articles

Ultrafast Photoisomerization of Photoactive Yellow Protein Chromophore Analogues in Solution: Influence of the Protonation State

We investigate solvent viscosity and polarity effects on the photoisomerization of the protonated and deprotonated forms of two analogues of the photoactive yellow protein (PYP) chromophore. These are trans-p-hydroxybenzylidene acetone and trans-p-hydroxyphenyl cinnamate, studied in solutions of different polarity and viscosity at room temperature, by means of femtosecond fluorescence up-conversion. The fluorescence lifetimes of the protonated forms are found to be barely sensitive to solvent viscosity, and to increase with increasing solvent polarity. In contrast, the fluorescence decays of the deprotonated forms are significantly slowed down in viscous media and accelerated in polar solvents. These results elucidate the dramatic influence of the protonation state of the PYP chromophore analogues on their photoinduced dynamics. The viscosity and polarity effects are, respectively, interpreted in terms of different isomerization coordinates and charge redistribution in S(1). A trans-to-cis isomerization mechanism involving mainly the ethylenic double-bond torsion and/or solvation is proposed for the anionic forms, whereas "concerted" intramolecular motions are proposed for the neutral forms.

Ab initio nonadiabatic quantum dynamics of cyclohexadiene/hexatriene ultrafast photoisomerization

Reaction mechanisms of the ultrafast photoisomerization between cyclohexadiene and hexatriene have been elucidated by the quantum dynamics on the ab initio potential energy surfaces calculated by multireference configuration interaction method. In addition to the quantum wave-packet dynamics along the two-dimensional reaction coordinates, the semiclassical analyses have also been carried out to correctly estimate the nonadiabatic transition probabilities around conical intersections in the full-dimensional space. The reaction time durations of radiationless decays in the wave-packet dynamics are found to be generally consistent with the femtosecond time-resolution experimental observations. The nonadiabatic transition probabilities among the ground (S0), first (S1), and second (S2) excited states have been estimated by using the semiclassical Zhu-Nakamura formula considering the full-dimensional wave-packet density distributions in the vicinity of conical intersections under the harmonic normal mode approximation. The cyclohexadiene (CHD) ring-opening process proceeds descending on the S1(1 1B) potential after the photoexcitation. The major part of the wave-packet decays from S1(1 1B) to S1(2 1A) by the first seam line crossing along the C2-symmetry-breaking directions. The experimentally observed ultrafast S1-S0 decay can be explained by the dynamics through the S1-S0 conical intersection along the direction toward the five-membered ring. The CHD: hexatriene (HT) branching ratio is estimated to be approximately 5:5, which is in accordance with the experiment in solution. This branching ratio is found to be mainly governed by the location of the five-membered ring S1-S0 conical intersection along the ground state potential ridge between CHD and HT.

The trans–cis isomerization reaction dynamics in sensory rhodopsin II by femtosecond time‐resolved midinfrared spectroscopy: Chromophore and protein dynamics

Transient infrared (IR) vibrational spectroscopy at subpicosecond time resolution on sensory rhodopsin II from Natronomonas pharaonis, NpSRII, has been performed for the first time. The experiments yield three time constants for the description of the primary photoinduced reaction dynamics, i.e. 0.5, 3.7-4.4, and 11 ps. The data are consistent with a sequential reaction scheme, with the isomerization taking place within 0.5 ps, succeeded by an electronic ground state relaxation. The 11 ps component, observed at 1550 and 1530 cm(-1), is attributed to dynamics of protein vibrational bands, possibly amide II bands of the protein backbone, perturbed by the ultrafast retinal photoisomerization. Similar observations, yet not as strongly expressed, have been made earlier in bacteriorhodopsin and halorhodopsin.

A semiclassical 1D model of ultrafast photoisomerization reactions

A simple model for ultrafast and efficient photoisomerization reactions is presented. In this model, the relevant region of the potential results from double crossing of two harmonic 1D diabatic potentials with opposite curvature. The eigenvalue problem is solved within a semiclassical approximation using the method of connection matrices, and the propagation of wave packets, expanded over these semiclassical eigenfunctions, is calculated. Within this model, fast double non-adiabatic transitions through both crossing points circumvent slow tunneling through the potential barrier. The extension to real systems makes this model relevant for the design of bi-stable photochromic materials.

Deuterium NMR structure of retinal in the ground state of rhodopsin.

The conformation of retinal bound to the G protein-coupled receptor rhodopsin is intimately linked to its photochemistry, which initiates the visual process. Site-directed deuterium ((2)H) NMR spectroscopy was used to investigate the structure of retinal within the binding pocket of bovine rhodopsin. Aligned recombinant membranes were studied containing rhodopsin that was regenerated with retinal (2)H-labeled at the C(5), C(9), or C(13) methyl groups by total synthesis. Studies were conducted at temperatures below the gel to liquid-crystalline phase transition of the membrane lipid bilayer, where rotational and translational diffusion of rhodopsin is effectively quenched. The experimental tilt series of (2)H NMR spectra were fit to a theoretical line shape analysis [Nevzorov, A. A., Moltke, S., Heyn, M. P., and Brown, M. F. (1999) J. Am. Chem. Soc. 121, 7636-7643] giving the retinylidene bond orientations with respect to the membrane normal in the dark state. Moreover, the relative orientations of pairs of methyl groups were used to calculate effective torsional angles between different planes of unsaturation of the retinal chromophore. Our results are consistent with significant conformational distortion of retinal, and they have important implications for quantum mechanical calculations of its electronic spectral properties. In particular, we find that the beta-ionone ring has a twisted 6-s-cis conformation, whereas the polyene chain is twisted 12-s-trans. The conformational strain of retinal as revealed by solid-state (2)H NMR is significant for explaining the quantum yields and mechanism of its ultrafast photoisomerization in visual pigments. This work provides a consensus view of the retinal conformation in rhodopsin as seen by X-ray diffraction, solid-state NMR spectroscopy, and quantum chemical calculations.

Femtosecond Fluorescence and Absorption Dynamics of an Azobenzene with a Strong Push−Pull Substitution

The ultrafast photoisomerization of a push−pull substituted azobenzene (4-nitro-4‘-(dimethylamino)azobenzene, NA) is studied by means of femtosecond fluorescence and absorption spectroscopy. The fluorescence dynamics is biphasic. The initial fluorescence with a narrow and intense spectrum decays in ∼100 fs. This decay is accompanied by the rise of broad red-shifted and much weaker emission. The same time constants recur in the transient absorption spectra which hold additional information on the ground-state dynamics. The ground state recovers in 0.8 ps, demonstrating that only the longer time constant is associated with an internal conversion process. Small spectral changes occurring thereafter (∼5 ps) point to vibrational cooling in the ground state. The results are analyzed in comparison with the behavior of the parent compound azobenzene. Though the push−pull substitution of azobenzene strongly alters the character of its excited states, the photodynamics are surprisingly robust with respect to that s...

Direct measure of functional importance visualized atom-by-atom for photoactive yellow protein: application to photoisomerization reaction.

Photoreceptor proteins serve as efficient nano-machines for the photoenergy conversion and the photosignal transduction of living organisms. For instance, the photoactive yellow protein derived from a halophilic bacterium has the p-coumaric acid chromophore, which undergoes an ultrafast photoisomerization reaction after light illumination. To understand the structure-function relationship at the atomic level, we used a computational method to find functionally important atoms for the photoisomerization reaction of the photoactive yellow protein. In the present study, a "direct" measure of the functional significance was quantitatively evaluated for each atom by calculating the partial atomic driving force for the photoisomerization reaction. As a result, we revealed the reaction mechanism in which the specific role of each functionally important atom has been well characterized in a systematic manner. In addition, we observed that this mechanism is strongly conserved during the thermal fluctuation of the photoactive yellow protein. We compared the experimental data of fluorescence decay constant of several different mutants and the present analysis. As a result, we found that the reaction rate constant is decreased when a large positive driving force is missing.

Low-Frequency Vibrations and Their Role in Ultrafast Photoisomerization Reaction Dynamics of Photoactive Yellow Protein

Low-frequency vibrational modes of the native photoactive yellow protein (PYP) and its several mutant and analogue systems have been investigated in “time” (femtosecond fluorescence up-conversion) ...

Time and Frequency Domain Investigations on Ultrafast Photoreaction Dynamics of Photoactive Yellow Protein (PYP)

Low vibrations of the native PYP and related systems in solution have been investigated in (fs fluorescence up-conversion) and frequency (resonance Raman) domains to elucidate their role in ultrafast photoisomerization reaction dynamics of PYP. Tentative assignments of the oscillatory components to particular vibrations are proposed supported by normal mode calculations based on DFT and ab initio MO methods. It is concluded that the out-of-plane skeleton bending mode of the chromophore, γ16, is responsible for the observed oscillations in native and all mutant PYPs (f1 ∼ 135 cm-1), while in-plane ν'42 and ν'43 modes are probably responsible for oscillations observed in the PYP analogue with locked chromophore. A dynamic model called trigger mode mediated guidance has been proposed to explain in simple terms the ultrafast primary process initiating PYP's photocycle. Furthermore, first successful fluorescence dynamics experiments on PYP single crystals with femtosecond time resolution are presented and discussed.

Ultrafast photoreactions in protein nanospaces as revealed by fs fluorescence dynamics measurements on photoactive yellow protein and related systemsDedicated to Professor Dr Z. R. Grabowski and Professor Dr J. Wirz on the occasions of their 75th and 60th birthdays.

We have investigated primary processes of ultrafast photoreactions of various photoresponsive proteins by means of fs fluorescence dynamics measurements. Based on these studies, the effects of the protein nanospace (PNS), containing the chromophore, on the dynamics and mechanisms of the ultrafast and highly efficient reactions of these proteins have been elucidated. In this article, we discuss mainly the results of our studies on ultrafast photoisomerization of photoactive yellow protein (PYP), its mutants and analogues. The chromophore of the PYP, deprotonated coumaric acid thioester (O−-phenyl-CHCH–CO–S–CH2–), fixed in PNS by hydrogen (H) bonding interactions at the head part, O−-phenyl-, and by covalent bonding at the tail part, –CO–S–, undergoes ultrafast twisting by flipping the thioester bond, owing to the intrachromophore head to tail charge transfer caused by the photoexcitation. In the site-directed mutants where the PNS structure is looser and more disordered, the photoinduced twisting reaction becomes slowed compared with the wild-type PYP and moreover, the twisting becomes much slower in the denatured PYP, showing the supreme importance of more regulated PNS for the fast twisting. We found also coherent vibrations in the fluorescence decay curves which might be coupled with the twisting. Furthermore, we found for a PYP analogue with replaced chromophore ultrafast dynamic Stokes shift of fluorescence rather than the quenching due to twisting, indicating the importance of chromophore-PNS fine adjustment for the ultrafast twisting.

Ultrafast transient lens spectroscopy of photoisomerization dynamics of azocompounds in confined nanospace of cyclodextrins

The ultrafast photoisomerization dynamics of azocompounds encapsulated in the cavity of α-, β-, and γ-cyclodextrin (CD) was investigated by the ultrafast transient lens method regarding effects of special restriction and intermolecular interactions. As expected, the spatial restriction reduced the yield of photoisomerization, but the effect was not so remarkable, indicating that the host and guest were relatively freely bounded. This effect was more prominent in azo: γ-CD=2:2 system, where the two guest molecules were packed in parallel as a dimer. From the viewpoint of the confined nanospace as a new reaction field, we found that the azo: γ-CD=2:2 system induced a specific intermediate having a long lifetime, which was not observed in free solutions. We also found that the formation of hydrogen-bonding between CD and guest remarkably elongated the trans–cis transformation of guest molecules in Orange II/CD systems.

Ultrafast Photoinduced Reaction Dynamics and Coherent Oscillations in Photoresponsive Proteins.

We have studied primary processes of ultrafast photoisomerization reactions of photoactive yellow protein (PYP) and visual rhodopsin (Rh) as well as ultrafast photoinduced electron transfer reactions of flavoproteins (FP) by means of the fs fluorescence up-conversion method. On the basis of these studies, we have examined the effects of the protein nanospace (PNS), where the chromophore is held, on the dynamics and mechanisms of the ultrafast and highly efficient photoinduced reactions of these proteins. In this article, we discuss mainly results of our comparative fluorescence dynamics studies on wild type (w. -t.) PYP, it’s site-directed mutants, analogues with replaced chromophore and FP, including the coherent vibrations in the ultrafast fluorescence decays of w. -t. PYP and mutants in the course of the photoisomerization, ultrafast dynamic Stokes shift of fluorescence of PYP analogues, and ultrafast electron transfer of FP.

Effect of Potential Energy Gap between the n-π* and the π-π* State on Ultrafast Photoisomerization Dynamics of an Azobenzene Derivative

We considered the trans → cis photoisomerization dynamics of the S2-excited azobenzene derivatives in terms of the potential energy gap between the n-π* (S1) and the π-π* (S2) state. The photoisomerization dynamics of trans-4-aminoazobenzene (trans-4-AAB) which has an energy gap (3000−4000 cm-1) rather smaller than that of trans-azobenzene (∼10000 cm-1) due to an amino-substitution was investigated using UV−vis transient absorption spectroscopy. The recorded transient absorption spectra of the π-π* (S2) excited trans-4-AAB in ethanol and heptanol indicated that the π-π* state of the trans-4-AAB decays to the n-π* state with a time constant of 0.2 ps. Following this process, the n-π* state decays to the ground state via two reaction pathways with time constants of 0.6 and 1.9 ps, and finally vibrational cooling of the ground state occurs with ∼15 ps. The π-π* state decay dynamics of the π-π* excited trans-4-AAB showed good agreement with that of the π-π* excited trans-azobenzene. On the other hand, the n-π...

Ultrafast photoisomerization in DCM dye observed by new femtosecond Raman spectroscopy

Ultrafast trans–cis photoisomerization in DCM solutions has been investigated using new time-resolved Raman spectroscopy. The new method measures stimulated Raman scattering in photoinduced transient species using a Raman pump pulse with narrow bandwidth and a femtosecond supercontinuum probe pulse. The temporal and spectral resolutions obtained in this study are 250 fs and 25 cm −1, respectively. The observed Raman signals are separated to two groups from the temporal dependence and assigned to an intermediate state between trans- and cis-forms and a charge-transfer state in DCM.

Ultrafast Radiationless Deactivation of Organic Dyes: Evidence for a Two-State Two-Mode Pathway in Polymethine Cyanines

CASSCF quantum chemical calculations (including dynamics) have been used to investigate the ultrafast photoisomerization of three symmetric cyanine dye models of different chain lengths. For the “model” trimethine cyanine, the photochemical isomerization path can be divided into two phases: initial barrierless skeletal stretching coupled with torsional motion and the decay process that takes place in the region of the twisted intramolecular charge-transfer (TICT) minimum state with an adjacent conical intersection. The path is consistent with both biexponential decay of fluorescence without rise time at short wavelengths and the rise time followed by monoexponential decay at long wavelengths observed in time-resolved experiments. For penta- and heptamethine cyanines, the photoisomerization about different C−C bonds is shown to be an activated process, where the torsional reaction path terminates, again, at a TICT state and the decay takes place at a twisted S1/S0 conical intersection. In agreement with t...

Molecular Dynamics Simulation of the Excited‐State Dynamics of Bacteriorhodopsin

Abstract— The excited‐state dynamics of bacteriorhodopsin was studied by molecular dynamics simulation. For the purpose of suppressing large displacement of amino acid residues on the surface of bacteriorhodopsin, positional restraints were imposed on these residues. A new method was developed to investigate the movement of amino acid residues upon photoexcitation and their role on the ultrafast photoisomerization of the chromophore. The structural change of bacteriorhodopsin was then traced up to 200 fs, i.e. until the formation of the intermediate I. We found that when all the conjugated bonds of the chromophore were allowed to twist freely in the excited state, many bonds including the C13=C14 bond twist in large scale within 100 fs. When only the C13=C14 bond and the single bonds were allowed to twist freely, the twisting took place at most 20° within 200 fs. From these results, it is claimed that a special potential surface is provided for the C13=C14 bond twisting by the protein environment in the course of the isomerization reaction, giving rise to the specific, ultrafast photoisomerization of bacteriorhodopsin. As a trace of such a mechanism, we observed that several functionally important residues incuding Asp85, Asp212 and Tyr185 responded quickly to the photoexcitation of the chromophore.

Ultrafast photoisomerization of azobenzene compounds

The spectroscopy and dynamics of the azobenzene compound 4-(4′-aminophenylazo) benzoic acid sodium salt (APB) is described and its applicability as a reversible fast photoswitch for conformational control in small peptides is discussed. Addition. of an electron withdrawing (carboxy-) and an electron donating (amino-) group on the phenyl rings of azobenzene shifts the absorption maximum of the π-π ∗ band of the resulting APB molecule from 320 to 420 nm. Reversible photoisomerization is detected by UV-vis, MR and NMR spectroscopy. Femtosecond time resolved ground and excited state dynamics were recorded, indicating that excited trans-APB reaches the electronic ground state within ≈ 1 ps. This ultrafast photoisomerization is followed by a wavelength dependent process in the 10 ps range which is interpreted as vibrational cooling of the product molecules. At delay times longer than 200 ps the absorption characteristics, as expected from the cw-difference spectrum, can be seen, indicating the completion of the photoprocess.

Vibrational cooling after ultrafast photoisomerization of azobenzene measured by femtosecond infrared spectroscopy

The vibrational cooling of azobenzene after photoisomerization is investigated by time resolved IR spectroscopy with femtosecond time resolution. Transient difference spectra were obtained in a frequency range where phenyl ring modes and the central N=N-stretching mode absorbs. The experimental data are discussed in terms of a simple theoretical model which was derived in order to account for the off-diagonal anharmonicity between the investigated high-frequency modes and the bath of the remaining low-frequency modes in a polyatomic molecule. It is shown that these off-diagonal anharmonic constants dominate the observed transient absorbance changes while the anharmonicity of the high-frequency modes themselves (diagonal anharmonicity) causes only minor effects. Based on the transient IR spectra, the energy flow in the azobenzene molecule can be described as follows: After an initial ultrafast intramolecular energy redistribution process, the decay of the related intramolecular temperature occurs via intermolecular energy transfer to the solvent on a time scale of ca. 20 ps.