Amir Wand

Mapping multidimensional excited state dynamics using pump-impulsive-vibrational-spectroscopy and pump-degenerate-four-wave-mixing

Pump-impulsive vibrational spectroscopy (pump-IVS) is used to record excited state vibrational dynamics following photoexcitation of two carotenoids, β-carotene and lycopene, with <30 fs temporal resolution, and covering the full vibrational spectrum of the investigated chromophores. The results record the course of S2-S1 internal conversion, followed by vibrational relaxation and decay to the electronic ground state. This interpretation is corroborated by comparison with pump-degenerate-four-wave-mixing (pump-DFWM) experiments on the same systems. The results demonstrate the potential of both time-domain spectroscopic techniques to resolve photochemical dynamics, including fingerprint frequencies which directly reflect changes in bonding and structure in the nascent sample. The exclusive strengths and limitations of these two methods are compared with those presented by the frequency-domain Femtosecond Stimulated Raman Scattering (FSRS) technique, highlighting the complementary nature of the three, and the benefits of using them in concert to investigate vibrational dynamics in reactive species.

Asymmetric toggling of a natural photoswitch: ultrafast spectroscopy of Anabaena sensory rhodopsin.

Photochemistry in retinal proteins (RPs) is determined both by the properties of the retinal chromophore and by its interactions with the surrounding protein. The initial retinal configuration, and the isomerization coordinates active in any specific protein, must be important factors influencing the course of photochemistry. This is illustrated by the vast differences between the photoisomerization dynamics in visual pigments which start 11-cis and end all-trans, and those observed in microbial ion pumps and sensory rhodopsins which start all-trans and end in a 13-cis configuration. However, isolating these factors is difficult since most RPs accommodate only one active stable ground-state configuration. Anabaena sensory rhodopsin, allegedly functioning in cyanobacteria as a wavelength sensor, exists in two stable photoswitchable forms, containing all-trans and 13-cis retinal isomers, at a wavelength-dependent ratio. Using femtosecond spectroscopy, and aided by extraction of coherent vibrational signatures, we show that cis-to-trans photoisomerization, as in visual pigments, is ballistic and over in a fraction of a picosecond, while the reverse is nearly 10 times slower and kinetically reminiscent of other microbial rhodopsins. This provides a new test case for appreciating medium effects on primary events in RPs.

Shedding New Light on Retinal Protein Photochemistry

The ultrafast spectroscopic investigation of novel retinal proteins challenges existing notions concerning the course of primary events in these natural photoreceptors. We review two illustrations here. The first demonstrates that changes in the initial retinal configuration can alter the duration of photochemistry by nearly an order of magnitude in Anabaena sensory rhodopsin, making it as rapid as the ballistic photoisomerization in visual pigments. This prompted a reinvestigation of the much studied bacteriorhodopsin, leading to a similar trend as well, contrary to earlier reports. The second involves the study of xanthorhodopsin, an archaeal proton pump that includes an attached light-harvesting carotenoid. Pump-probe experiments demonstrate the efficient transfer of energy from carotenoid to retinal, providing a first glimpse at a cooperative multichromophore function, which is probably characteristic of many other proteins as well. Finally, we discuss measures required to advance our knowledge from kinetics to mode-specific dynamics concerning this expanding family of biological photoreceptors.

Probing Ultrafast Photochemistry of Retinal Proteins in the Near-IR: Bacteriorhodopsin and Anabaena Sensory Rhodopsin vs Retinal Protonated Schiff Base in Solution

Photochemistry of bacteriorhodopsin (bR), anabaena sensory rhodopsin (ASR), and all-trans retinal protonated Schiff base (RPSB) in ethanol is followed with femtosecond pump-hyperspectral near-IR (NIR) probe spectroscopy. This is the first systematic probing of retinal protein photochemistry in this spectral range. Stimulated emission of the proteins is demonstrated to extend deep into the NIR, and to decay on the same characteristic time scales previously determined by visible probing. No signs of a transient NIR absorption band above λpr > 1.3 μm, which was recently reported and is verified here for the RPSB in solution, is observed in either protein. This discrepancy demonstrates that the protein surroundings change photochemical traits of the chromophore significantly, inducing changes either in the energies or couplings of photochemically relevant electronic excited states. In addition, low-frequency and heavily damped spectral modulations are observed in the NIR signals of all three systems up to 1.4 μm. By background subtraction and Fourier analysis they are shown to resemble wave packet signatures in the visible, stemming from multiple vibrational modes and by analogy are assigned to torsional wave packets in the excited state of the retinal chromophore. Differences in the vibrational frequencies between the three samples and the said discrepancy in transient spectra are discussed in terms of opsin effects on the RPSB electronic structure.

Solvent Tuning of a Conical Intersection: Direct Experimental Verification of a Theoretical Prediction

We report an ultrafast study of a merocyanine molecule, whose fluorescence lifetime was tuned by changing the solvents polarity. A recent theoretical prediction that the fluorescence lifetime is considerably shortened upon lowering the polarity of the solvent, due to tuning of the conical intersection properties, is fully confirmed (Xu et al. J. Phys. Chem. A 2009, 113, 9779-9791). This constitutes a direct measurement of a previously predicted tunable property of a conical intersection.

Ultrafast Photochemistry of Light-Adapted and Dark-Adapted Bacteriorhodopsin: Effects of the Initial Retinal Configuration

Femtosecond spectroscopy is used to compare photochemical dynamics in light-adapted and dark-adapted bacteriorhodopsin (BR). The retinal prosthetic group is initially all-trans in the former, while it is nearly a 1:1 mixture with 13-cis in the latter. Comparing photochemistry in both serves to assess how the initial retinal configuration influences internal conversion and photoisomerization dynamics. Contrary to an earlier study, our results show that after excitation of the 13-cis form it crosses back to the ground state much more rapidly than the biologically active all-trans reactant. A similar result was recently obtained for another microbial retinal protein, Anabaena Sensory Rhodospin (ASR), which can be toggled by light between two analogous ground state configurations. Together, these studies suggest that this disparity in rates may be a general trend in the photochemistry of microbial retinal proteins. This may bear as well on the well-known enhancement in photoisomerization rates going from microbial retinal proteins to the visual pigments, as the latter also start the course of photoreception in a cis retinal configuration, in that case 11-cis. In lieu of indications for pretwisting or straining of the 13-cis retinal forms of BR and ASR, akin to those reported for rhodopsin, current results challenge many of the mechanisms held responsible for the ballistic photochemical dynamics observed in visual pigment.

A new spectral window on retinal protein photochemistry.

A VIS pump/hyperspectral NIR probe study of all-trans-retinal protonated Schiff base (RPSB) in ethanol is presented. Upon irradiation, a short-lived absorption band covers the recorded range of λ = 1-2 μm. It decays to reveal the tail of S(1) emission at λ < 1.3 μm, along with a residual absorption at longer wavelengths, both of which decay with the known kinetics of internal conversion to S(0). The existence of this hitherto unrecorded excited-state absorption deep in the NIR will require a revision of current models for RPSB electronic structure. The phenomenological similarity of these observations with ultrafast NIR studies of carotenoids raises the question of whether three, and not two, electronic states participate in RPSB photochemistry as well. The relevance of these observations to retinal protein photochemistry is discussed.

pH dependence of Anabaena sensory rhodopsin: retinal isomer composition, rate of dark adaptation, and photochemistry.

Microbial rhodopsins are photoactive proteins, and their binding site can accommodate either all-trans or 13-cis retinal chromophore. The pH dependence of isomeric composition, dark-adaptation rate, and primary events of Anabaena sensory rhodopsin (ASR), a microbial rhodopsin discovered a decade ago, are presented. The main findings are: (a) Two pKa values of 6.5 and 4.0 assigned to two different protein residues are observed using spectroscopic titration experiments for both ground-state retinal isomers: all-trans, 15-anti (AT) and 13-cis, 15-syn (13C). The protonation states of these protein residues affect the absorption spectrum of the pigment and most probably the isomerization process of the retinal chromophore. An additional pKa value of 8.5 is observed only for 13C-ASR. (b) The isomeric composition of ASR is determined over a wide pH range and found to be almost pH-independent in the dark (>96% AT isomer) but highly pH-dependent in the light-adapted form. (c) The kinetics of dark adaptation is recorded over a wide pH range, showing that the thermal isomerization from 13C to AT retinal occurs much faster at high pH rather than under acidic conditions. (d) Primary photochemical events of ASR at pH 5 are recorded using VIS hyperspectral pump-probe spectroscopy with <100 fs resolution and compared with the previously recorded results at pH 7.5. For AT-ASR, these are shown to be almost pH-independent. However, photochemistry of 13C-ASR is pH-dependent and slowed down in acidic environments.

Opening a New Spectral Window on Retinal Protein Photochemistry

Probing the spectroscopy of the active chromophore in retinal proteins in the NIR for the first time shows new absorption features which support a three-state model for the photochemical dynamics of retinal proteins.

Probing Photodynamics of Retinal Protonated Schiff-Base with 7 fs Impulsive Vibrational Spectroscopy

Two and three pulses experiments are conducted to record the elusive S1 vibrational spectrum of Retinal Protonated Schiff-Base in solution. We find a reduction in C=C stretching frequency and other shifted bands in the fluorescent state, whose relevance are discussed.