On the Transition from a Biomimetic Molecular Switch to a Rotary Molecular Motor.

The experimental investigation of the unidirectional motion characterizing the photoisomerization of single-molecule rotary motors requires accessible lab prototypes featuring an electronic circular dichroism (ECD) signal that is sensitive to the geometrical and electronic changes occurring during an ultrafast reactive process. Here we report a combined experimental/computational study of a candidate obtained via the asymmetrization of a light-driven biomimetic molecular switch. We show that the achieved motor has an ECD band that is remarkably sensitive to the isomerization motion, and it is therefore suitable for time-resolved ECD studies. However, we also find that, unexpectedly, the synthesized motor isomerizes on a time scale longer than the subpicosecond time measured for the achiral parent, a result that points to alternative candidates conserving a high reaction speed.

Synthesis, spectroscopy and QM/MM simulations of a biomimetic ultrafast light-driven molecular motor.

A molecular motor potentially performing a continuous unidirectional rotation is studied by a multidisciplinary approach including organic synthesis, transient spectroscopy and excited state trajectory calculations. A stereogenic center was introduced in the N-alkylated indanylidene-pyrroline Schiff base framework of a previously investigated light-driven molecular switch in order to achieve the unidirectional C[double bond, length as m-dash]C rotary motion typical of Feringa's motor. Here we report that the specific substitution pattern of the designed chiral molecule must critically determine the unidirectional efficiency of the light-induced rotary motion. More specifically, we find that a stereogenic center containing a methyl group and a hydrogen atom as substituents does not create a differential steric effect large enough to fully direct the motion in either the clockwise or counterclockwise direction especially along the E→Z coordinate. However, due to the documented ultrafast character and electronic circular dichroism activity of the investigated system, we find that it provides the basis for development of a novel generation of rotary motors with a biomimetic framework and operating on a picosecond time scale.

Bile Acid Binding Protein Functionalization Leads to a Fully Synthetic Rhodopsin Mimic.

Rhodopsins are photoreceptive proteins using light to drive a plethora of biological functions such as vision, proton and ion pumping, cation and anion channeling, and gene and enzyme regulation. Here we combine organic synthesis, NMR structural studies, and photochemical characterization to show that it is possible to prepare a fully synthetic mimic of rhodopsin photoreceptors. More specifically, we conjugate a bile acid binding protein with a synthetic mimic of the rhodopsin protonated Schiff base chromophore to achieve a covalent complex featuring an unnatural protein host, photoswitch, and photoswitch-protein linkage with a reverse orientation. We show that, in spite of its molecular-level diversity, light irradiation of the prepared mimic fuels a photochromic cycle driven by sequential photochemical and thermal Z/E isomerizations reminiscent of the photocycles of microbial rhodopsins.

Engineering the vibrational coherence of vision into a synthetic molecular device.

The light-induced double-bond isomerization of the visual pigment rhodopsin operates a molecular-level optomechanical energy transduction, which triggers a crucial protein structure change. In fact, rhodopsin isomerization occurs according to a unique, ultrafast mechanism that preserves mode-specific vibrational coherence all the way from the reactant excited state to the primary photoproduct ground state. The engineering of such an energy-funnelling function in synthetic compounds would pave the way towards biomimetic molecular machines capable of achieving optimum light-to-mechanical energy conversion. Here we use resonance and off-resonance vibrational coherence spectroscopy to demonstrate that a rhodopsin-like isomerization operates in a biomimetic molecular switch in solution. Furthermore, by using quantum chemical simulations, we show why the observed coherent nuclear motion critically depends on minor chemical modifications capable to induce specific geometric and electronic effects. This finding provides a strategy for engineering vibrationally coherent motions in other synthetic systems.

Design, Synthesis, and Dynamics of a Green Fluorescent Protein Fluorophore Mimic with an Ultrafast Switching Function.

While rotary molecular switches based on neutral and cationic organic π-systems have been reported, structurally homologous anionic switches providing complementary properties have not been prepared so far. Here we report the design and preparation of a molecular switch mimicking the anionic p-HBDI chromophore of the green fluorescent protein. The investigation of the mechanism and dynamics of the E/Z switching function is carried out both computationally and experimentally. The data consistently support axial rotary motion occurring on a sub-picosecond time scale. Transient spectroscopy and trajectory simulations show that the nonadiabatic decay process occurs in the vicinity of a conical intersection (CInt) between a charge transfer state and a covalent/diradical state. Comparison of our anionic p-HBDI-like switch with the previously reported cationic N-alkyl indanylidene pyrrolinium switch mimicking visual pigments reveals that these similar systems translocate, upon vertical excitation, a similar net charge in the same axial direction.

Initial excited-state dynamics of an N-alkylated indanylidene-pyrroline (NAIP) rhodopsin analog.

N-Alkylated indanylidene-pyrroline-based molecular switches mimic different aspects of the light-induced retinal chromophore isomerization in rhodopsin: the vertebrate dim-light visual pigment. In particular, they display a similar ultrashort excited-state lifetime, subpicosecond photoproduct appearance time, and photoproduct vibrational coherence. To better understand the early light-induced dynamics of such systems, we measured and modeled the resonance Raman spectra of the Z-isomer of the N-methyl-4-(5'-methoxy-2',2'-dimethyl-indan-1'-ylidene)-5-methyl-2,3-dihydro-2H-pyrrolium (NAIP) switch in methanol solution. It is shown that the data, complemented with a <70 fs excited-state trajectory computation, demonstrate initial excited-state structural dynamics dominated by double-bond expansion and single-bond contraction stretches. This mode subsequently couples with the five-membered ring inversion and double-bond torsion. These results are discussed in the context of the mechanism of the excited-state photoisomerization of NAIP switches in solution and the 11-cis retinal in rhodopsin.

Mechanistic origin of the vibrational coherence accompanying the photoreaction of biomimetic molecular switches.

The coherent photoisomerization of a chromophore in condensed phase is a rare process in which light energy is funneled into specific molecular vibrations during electronic relaxation from the excited to the ground state. In this work, we employed ultrafast spectroscopy and computational methods to investigate the molecular origin of the coherent motion accompanying the photoisomerization of indanylidene-pyrroline (IP) molecular switches. UV/Vis femtosecond transient absorption gave evidence for an excited- and ground-state vibrational wave packet, which appears as a general feature of the IP compounds investigated. In close resemblance to the coherent photoisomerization of rhodopsin, the sudden onset of a far-red-detuned and rapidly blue-shifting photoproduct signature indicated that the population arriving on the electronic ground state after nonadiabatic decay through the conical intersection (CI) is still very focused in the form of a vibrational wave packet. Semiclassical trajectories were employed to investigate the reaction mechanism. Their analysis showed that coupled double-bond twisting and ring inversions, already populated during the excited-state reactive motion, induced periodic changes in π-conjugation that modulate the ground-state absorption after the non-adiabatic decay. This prediction further supports that the observed ground-state oscillation results from the reactive motion, which is in line with a biomimetic, coherent photoisomerization scenario. The IP compounds thus appear as a model system to investigate the mechanism of mode-selective photomechanical energy transduction. The presented mechanism opens new perspectives for energy transduction at the molecular level, with applications to the design of efficient molecular devices.

Photoisomerization and relaxation dynamics of a structurally modified biomimetic photoswitch.

Recent experimental and theoretical studies on N-alkylated indanylidene pyrroline Schiff bases (NAIP) show that these compounds exhibit biomimetic photoisomerization analogous to that in the chromophore of rhodopsin. The NAIP compounds studied previously isomerize rapidly and often evolve coherently on the ground-electronic surface after reaction. We present the results of transient electronic absorption spectroscopy on dMe-OMe-NAIP, a newly synthesized NAIP analogue that differs from other NAIP compounds in the substituents on its pyrrolinium ring. Following excitation with 400 nm light, dMe-OMe-NAIP relaxes from the electronic-excited state in less than 500 fs, which is slower than in other analogues, and does not show the prominent oscillations observed in other NAIP compounds. A reduction in the amount of twisting between the rings caused by removal of the methyl group is likely responsible for the slower isomerization. Measurements in solvents of varying viscosity and structure suggest that intramolecular processes dominate the relaxation of nascent photoproducts.

Toward a stable α-cycloalkyl amino acid with a photoswitchable cationic side chain.

The N-alkylated indanylidenepyrroline (NAIP) Schiff base 3 is an unnatural α-amino acid precursor potentially useful for the preparation of semisynthetic peptides and proteins incorporating charged side chains whose structure can be modulated via Z/E photoisomerization. Here we report that the heteroallylic protons of 3 led to partial loss of ethanol accompanied by formation of the novel heterocyclic system 4 during attempted deprotection. We also show that the same protons catalyze the thermal isomerization of 3, making the light-driven conformational control concept ineffective for times longer than a few hours. These problems are not present in the previously unreported compound 5 where the acidic methyl group is replaced by an H atom. Therefore, 5, rather than 3, constitutes a promising prototype for the design of building blocks capable to modulate the electrostatic potential of a protein in specific locations via light irradiation.

Modeling, preparation, and characterization of a dipole moment switch driven by Z/E photoisomerization.

We report the results of a multidisciplinary research effort where the methods of computational photochemistry and retrosynthetic analysis/synthesis have contributed to the preparation of a novel N-alkylated indanylidene-pyrroline Schiff base featuring an exocyclic double bond and a permanent zwitterionic head. We show that, due to its large dipole moment and efficient photoisomerization, such a system may constitute the prototype of a novel generation of electrostatic switches achieving a reversible light-induced dipole moment change on the order of 30 D. The modeling of a peptide fragment incorporating the zwitterionic head into a conformationally rigid side chain shows that the switch can effectively modulate the fluorescence of a tryptophan probe.

Fluorenylidene-pyrroline biomimetic light-driven molecular switches.

A new family of biomimetic photoactivated molecular switches based in the retinal chromophore is described. Expedient synthesis allows a library of compounds with a different substitution pattern, including chiral substituents, to be obtained. The effect of substitution, solvent, and light source on the photoisomerization step has been assessed. The absorption maximum has been red-shifted ca. 50 nm with respect to related systems and rotation is now easily achieved by using visible light.

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

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.

Heteroaryl-substituted phenols as potential antioxidants.

A series of O-heteroaryl phenols have been synthesised and structurally characterised. Photo-Fries rearrangement of these compounds represents a useful way to access the corresponding C-heteroaryl derivatives. The activity of the new phenolic compounds as radical scavengers towards the 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonate) (ABTS(+*)) has been evaluated. 2-tert-Butyl-4-(4-phenyl-isoxazol-3-ylmethoxy)-phenol (compound 3c) showed the highest scavenger activity (IC50 value (i.e. the concentration that scavenged 50% of the radicals) 3.17 x 10(-6) M), which was one order of magnitude greater than that of the corresponding lead compound tert-butylhydroxy-anisole (BHA) (IC50 1.04 x 10(-5) M). In further experiments, compound 3c showed dose-dependent inhibition of the oxidation of linoleic acid, as well as methaemoglobin formation, promoted by the presence of the radical generator 2,2'-azobis(amidino-propane) hydrochloride (AAPH) and it was markedly more potent than BHA in these assays.

Design and photochemical characterization of a biomimetic light-driven Z/E switcher.

Protonated Schiff bases (PSBs) of polyenals constitute a class of light-driven switchers selected by biological evolution that provide model compounds for the development of artificial light-driven molecular devices or motors. In the present paper, our primary target is to show, through combined computational and experimental studies, that it is possible to approach the design of artificial PSBs suitable for such applications. Below, we use the methods of computational photochemistry to design and characterize the prototype biomimetic molecular switchers 4-cyclopenten-2'-enylidene-3,4-dihydro-2H-pyrrolinium and its 5,5'-dimethyl derivative both containing the penta-2,4-dieniminium chromophore. To find support for the predicted behavior, we also report the photochemical reaction path of the synthetically accessible compound 4-benzylidene-3,4-dihydro-2H-pyrrolinium. We show that the preparation and photochemical characterization of this compound (together with three different N-methyl derivatives) provide both support for the predicted photoisomerization mechanism and information on its sensitivity to the molecular environment.