An artificial molecular switch that mimics the visual pigment and completes its photocycle in picoseconds

Abstract

Single molecules that act as light-energy transducers (e.g., converting the energy of a photon into atomic-level mechanical motion) are examples of minimal molecular devices. Here, we focus on a molecular switch designed by merging a conformationally locked diarylidene skeleton with a retinal-like Schiff base and capable of mimicking, in solution, different aspects of the transduction of the visual pigment Rhodopsin. Complementary ab initio multiconfigurational quantum chemistry-based computations and time-resolved spectroscopy are used to follow the light-induced isomerization of the switch in methanol. The results show that, similar to rhodopsin, the isomerization occurs on a 0.3-ps time scale and is followed by <10-ps cooling and solvation. The entire (2-photon-powered) switch cycle was traced by following the evolution of its infrared spectrum. These measurements indicate that a full cycle can be completed within 20 ps.CASPT2//CASSCF ͉ mid-IR ͉ photochemical switch ͉ time resolved spectroscopy ͉ UV-vis M olecular switches based on photochemical E/Z isomerizations have been used in different contexts to convert light energy into ''mechanical'' motion at the molecular level (1-3). For instance, switches based on azobenzene have been used to control ion complexation (4, 5), electronic properties (6), catalysis (7), and the folding of peptides (8-13) whereas diarylidenes have provided the framework for the construction of rotary motors and transmissions (14). The computer modeling of switches that differ in size, polarity, and isomerization mechanism represents an attractive research target (15) yielding building blocks to be used in diverse molecular environments. However, this cannot be limited to the computation of equilibrium properties but requires the description of the entire photocycle. In other words, one needs to compute the potential energy surfaces controlling the switch E 3 Z and Z 3 E excited-state evolution, its decay and ground state relaxation, and the competing thermal E/Z isomerization in the proper environment (e.g., in solution or in a biomolecule backbone). The complexity of these calculations impedes the study of candidates that are intractable with accurate quantum chemical methods (allowing comparison with spectroscopic data) or that feature, as for azobenzene and diarylidenes (16), more than 1 low-lying excited state, leading to a plethora of reaction paths to be computed.The retinal protonated Schiff-base chromophore of rhodopsins (17-19) constitutes an example of an E/Z switch shaped by biological evolution that can be modeled with quantitative computations (20). In bovine rhodopsin (Rh), a selective photoisomerization of the 11-cis chromophore (PSB11) occurs via evolution of a single 3 * excited state (S 1 ) that survives for only 150 fs and yields, upon decay, the all-trans ground state (S 0 ) product with a 67% quantum yield (16,20). Although these properties make Rh an excellent reference for the design of E/Z switches, irradiation of PSB11 in solution (26, 27) features an unselec...

Coherent ultrafast torsional motion and isomerization of a biomimetic dipolar photoswitch

“…This yields the "true" absorption spectra of photo-excited ZW-NAIP molecules from the moment of excitation up to the completion of the photoreaction; these spectra are shown in The complete disappearance of the 1570 cm -1 band upon photoexcitation is consistent with ab initio minimum energy path calculations, which predict loss of C=C double bond character in the S 1 excited state to occur prior to torsional motion about the thus weakened C1'-C4 bond 27,28 . Its sub-picosecond reappearance is thus interpreted as a signature for the return of photo-excited molecules into the electronic ground state, as previously observed for MeO-NAIP 26,27 . At delays longer than 2 ps the essentially constant integrated signal in Fig.…”

“…Figure 1A shows the structure and the electronic absorption spectrum of ZW-NAIP (Z), wherein the negative counterion is carried by a deprotonated carboxylate group (COO-) covalently bound to the pyrroline moiety. A racemic mixture has been synthesized according to procedures described in details elsewhere [27][28][29] . The absorption spectrum of the E form is blue-shifted by 6 nm and slightly more intense than the one of the Z isomer 28 (Fig.…”

“…The biomimetic photoswitches have a single isomerizing bond, present one low-lying excited state, dissolve well in water and in other polar solvents and are small enough to be entirely manageable by state-of-the-art quantum chemistry methods. A methoxylated form of NAIP (MeO-NAIP) has been studied by quantum mechanics/molecular mechanics (QM/MM) computations, wavepacket dynamics simulations, and ultrafast spectroscopy, showing that, in methanol (MeOH), MeO-NAIP displays excited state properties similar to those of the 11-cis PSBR embedded in rhodopsin 26,27 . In particular, upon light excitation a barrierless excited state profile drives the molecular system through a CI to the isomer within 0.3 ps.…”

“…This is a time delay significantly larger than the average fluorescence lifetime and occurring about 150 fs after the maximum of ESA3. Hence we assign it to ground state photoproduct absorption (PA), also in analogy with the non zwitterionic variant 27 . PA designates hereafter pump-induced absorption features in the ground state, regardless of them being due to either the Z or E isomer.…”

A voltage-dependent fluorescent indicator for optogenetic applications, archaerhodopsin-3: Structure and optical properties from in silico modeling

“…They are presented in Table 1. From these results we can conclude that the best matching result was obtained using I-TASSER suite with AlignMe 24 alignment ( Figure 1). …”