Cis Isomerization Research Articles

Trans 18-carbon monoenoic fatty acid has distinct effects from its isomeric cis fatty acid on lipotoxicity and gene expression in Saccharomyces cerevisiae

Epidemiological studies have suggested that an excess intake of trans-unsaturated fatty acids increases the risk of coronary heart disease. However, the mechanisms of action of trans-unsaturated fatty acids in eukaryotic cells remain unclear. Since the budding yeast Saccharomyces cerevisiae can grow using fatty acids as the sole carbon source, it is a simple and suitable model organism for understanding the effects of trans-unsaturated fatty acids at the molecular and cellular levels. In this study, we compared the physiological effects of Δ9 cis and trans 18-carbon monoenoic fatty acids (oleic acid and elaidic acid) in yeast cells. The results obtained revealed that the two types have distinct effects on the expression of OLE1, which encodes Δ9 desaturase, and lipotoxicity in are1Δare2Δdga1Δlro1Δ and gat1Δ cells. Our results suggest that cis and trans 18-carbon monoenoic fatty acids exert different physiological effects in the regulation of gene expression and processing of excess fatty acids in yeast.

Dynamics of Inter-DNA Chain Interaction of Photoresponsive DNA.

Photoresponsive DNA modified with azobenzene is an attractive design molecule for efficient photoregulation of DNA hybridization, which may be used for controlling DNA functions. Although the essential step of photocontrolling DNA is the initial isomerization of the azobenzene, the dissociation/association kinetics remain unknown. Here, the time-resolved diffusion method was used to trace the dissociation/association processes of photoresponsive DNA. Although the isomerization of azobenzene occurs in picoseconds, the dissociation of the double-stranded DNA to single-stranded DNA triggered by the trans to cis isomerization takes place ∼10(7) times slower, with a time constant of 670 μs at 200 μM. From the concentration dependence, the dissociation and association rates were determined. Furthermore, the reaction rate from the single- to double-stranded DNA after the cis to trans isomerization was measured to be 3.6 ms at 200 μM. The difference in the melting temperatures of DNA between tethered trans- and cis-azobenzene is explained by the different rate of dissociation of the double-stranded form.

Crystal Structure of the 11-cis Isomer of Pharaonis Halorhodopsin: Structural Constraints on Interconversions among Different Isomeric States.

Like other microbial rhodopsins, the light driven chloride pump halorhodopsin from Natronomonas pharaonis (pHR) contains a mixture of all-trans/15-anti and 13-cis/15-syn isomers in the dark adapted state. A recent crystallographic study of the reaction states of pHR has shown that reaction states with 13-cis/15-syn retinal occur in the anion pumping cycle that is initiated by excitation of the all-trans isomer. In this study, we investigated interconversions among different isomeric states of pHR in the absence of chloride ions. The illumination of chloride free pHR with red light caused a large blue shift in the absorption maximum of the retinal visible band. During this "red adaptation", the content of the 11-cis isomer increased significantly, while the molar ratio of the 13-cis isomer to the all-trans isomer remained unchanged. The results suggest that the thermally activated interconversion between the 13-cis and the all-trans isomers is very rapid. Diffraction data from red adapted crystals showed that accommodation of the retinal chromophore with the 11-cis/15-syn configuration was achieved without a large change in the retinal binding pocket. The measurement of absorption kinetics under illumination showed that the 11-cis isomer, with a λmax at 565 nm, was generated upon excitation of a red-shifted species (λmax = 625 nm) that was present as a minor component in the dark adapted state. It is possible that this red-shifted species mimics an O-like reaction state with 13-cis/15-syn retinal, which was hypothesized to occur at a late stage of the anion pumping cycle.

Photo-responsive polysiloxane-based azobenzene liquid crystalline polymers prepared by thiol-ene click chemistry

ABSTRACTIn this work, a novel polysiloxane-based azobenzene-containing liquid crystalline polymer (LCP) PMMS-A44V6 has been designed and synthesized via a facile thiol-ene click chemistry method by grafting a side-on azobenzene mesogenic group A44V6 onto poly-[3-mercaptopropylmethylsiloxane] (PMMS) backbone. 1H NMR, 13C NMR, GPC, TGA, DSC, POM and WAXS were used to investigate the mesogenic properties and photo-responsive behaviour of PMMS-A44V6. Taking advantage of the azobenzene’s trans–cis isomerization effect, PMMS-A44V6 can perform an isotropization process under UV irradiation in its nematic phase. Initially, the isotropization process starts in a linearly decreasing manner with a rate of ca −6.5 × 105 intensity/s, and eventually finishes in an exponential decrease regime to form cis-azobenzene moieties. The reversible UV-response behaviour of PMMS-A44V6 can be performed in a relatively low temperature range of 30 ~ 75°C, which might help this azobenzene-containing LCP material to find potential application in control devices.

Light-induced self-assembly of gold nanoparticles with a photoresponsive polymer shell

The light-induced self-assembly of gold nanoparticles was studied systematically. A methacrylate type monomer with an azobenzene sidechain was polymerized in a reversible addition–fragmentation chain transfer (RAFT) polymerization. The resulting light responsive polymer was grafted to gold nanoparticles via the RAFT group. UV-light induced trans to cis isomerization of the azobenzene moieties triggers the aggregation of the polymer–gold hybrid particles in toluene dispersion. The thermally induced cis to trans relaxation was found to be significantly slower than for small molecules at gold surfaces. The self-assembly was followed by dynamic light scattering (DLS), UV/vis spectroscopy and transmission electron microscopy (TEM). The density of primary gold particles within the self-assembled aggregates can be tuned by varying the molar mass of the grafted polymer.

A photoresponsive azobenzene-bridged cubic silsesquioxane network

An azobenzene (AZO)-bridged cubic silsesquioxane network exhibiting reversible photoisomerization behavior in nonpolar solvents has been prepared via hydrosilylation reaction between 4,4′-diallyloxy-azobenzene and octahydridosilsesquioxane (H8Si8O12; H-POSS). Approximately 70 % of the corner Si–H groups of H-POSS are reacted to form a three-dimensional gel network while maintaining the cubic siloxane structure. The dried gel has a high thermal stability, which is attributed to the highly cross-linked cubic silsesquioxane network where AZOs are covalently incorporated in the main chain. The gel exhibits reversible swelling behavior in nonpolar solvents during wetting–drying cycles. In toluene, a large extent of reversible trans–cis isomerization of the AZO moiety is observed. These results are promising for the design of a new class of photoresponsive materials applicable in host–guest chemistry.

Azobenzene-containing bent-core liquid crystals: an overview

ABSTRACTAzo-functionalised materials are of special interest due to their photochromic nature, i.e. reversible trans–cis isomerisation upon photoirradiation. The combination of photosensitivity and liquid crystalline properties in the same molecule allows the material to be exploited for optical and optoelectronic devices. Azobenzene-based bent-core liquid crystals (BCLCs) have attracted considerable attention in recent years due to their rich mesomorphism. In this review, the main research directions and different molecular structures of bent-core molecules incorporating azobenzene unit and its subtype the so-called hockey-stick molecules are summarised. Additionally, azobenzene-based U-shaped molecules, hydrogen-bonded bent-shaped liquid crystalline materials and some selected examples of two different types of photoswitchable mesogenic dimers are provided. The nature, number and position of the lateral substitutions able to modify the phase behaviour of such BCLCs, affording in turn interesting liquid crystalline phases are discussed. Finally, the isomerisation process of these photosensitive BCLCs in solutions or in mesophases under the effect of UV–visible irradiation is summarised.

The Origin of Bond Selectivity and Excited-State Reactivity in Retinal Analogues.

The effect of different conformations and substitutions on the photoisomerization of a retinal protonated Schiff base model is investigated by nonadiabatic molecular dynamics simulations. Three groups of retinal analogues are studied: (i) conformational isomers, (ii) methyl-substituted retinals, and (iii) C11-C12 bond locked retinals. In total 259 trajectories are calculated in the gas phase starting from different initial conditions. The effect on bond selectivity, the directionality of the isomerization, excited-state lifetime, and product distribution is derived from the ensemble of trajectories. Among the group of four isomers (9-, 11-, 13-cis, and all-trans) the 11-cis analogue is the most selective in terms of isomerizing double bond, while the other three produce a mixture of isomers. However, there is no preference for isomerization directionality and the product formation for the 11-cis isomer. In the group of analogues with different methylation patterns, it is found that a methyl group at position C10 can introduce unidirectionality. This methyl group also speeds the photoisomerization. In case of the analogue that is demethylated at the positions C10 and C13, all trajectories isomerize successfully from cis to trans conformation. The three C11-C12 bond locked retinals are found to have very different properties, which depend on the number of methylene units bridging this bond. The five-membered ring imposes a too-large restriction; hence, all trajectories remain on the excited state in the simulation time of 300 fs. The seven-membered ring is more flexible with preference for isomerization of the C9-C10 bond. Interestingly, the eight-membered ring leads to the fastest isomerization time and full directionality of C11-C12 bond isomerization. The trends observed in these simulations can help to understand whether the effects are intrinsic to the chromophore or are induced by the protein environment, by comparing to the trends from experiment. Furthermore, the derived understanding can support design of molecular motors to achieve high product yield and unidirectionality.

Oxidative trans to cis Isomerization of Olefins in Polyketide Biosynthesis

AbstractGeometric isomerization can expand the scope of biological activities of natural products. The observed chemical diversity among the pseurotin‐type fungal secondary metabolites is in part generated by a trans to cis isomerization of an olefin. In vitro characterizations of pseurotin biosynthetic enzymes revealed that the glutathione S‐transferase PsoE requires participation of the bifunctional C‐methyltransferase/epoxidase PsoF to complete the trans to cis isomerization of the pathway intermediate presynerazol. The crystal structure of the PsoE/glutathione/presynerazol complex indicated stereospecific glutathione–presynerazol conjugate formation is the principal function of PsoE. Moreover, PsoF was identified to have an additional, unexpected oxidative isomerase activity, thus making it a trifunctional enzyme which is key to the complexity generation in pseurotin biosynthesis. Through the study, we identified a novel mechanism of accomplishing a seemingly simple trans to cis isomerization reaction.

Oxidative trans to cis Isomerization of Olefins in Polyketide Biosynthesis.

Geometric isomerization can expand the scope of biological activities of natural products. The observed chemical diversity among the pseurotin-type fungal secondary metabolites is in part generated by a trans to cis isomerization of an olefin. In vitro characterizations of pseurotin biosynthetic enzymes revealed that the glutathione S-transferase PsoE requires participation of the bifunctional C-methyltransferase/epoxidase PsoF to complete the trans to cis isomerization of the pathway intermediate presynerazol. The crystal structure of the PsoE/glutathione/presynerazol complex indicated stereospecific glutathione-presynerazol conjugate formation is the principal function of PsoE. Moreover, PsoF was identified to have an additional, unexpected oxidative isomerase activity, thus making it a trifunctional enzyme which is key to the complexity generation in pseurotin biosynthesis. Through the study, we identified a novel mechanism of accomplishing a seemingly simple trans to cis isomerization reaction.

Optical Input/Electrical Output Memory Elements based on a Liquid Crystalline Azobenzene Polymer.

Responsive polymer materials can change their properties when subjected to external stimuli. In this work, thin films of thermotropic poly(metha)acrylate/azobenzene polymers are explored as active layer in light-programmable, electrically readable memories. The memory effect is based on the reversible modifications of the film morphology induced by the photoisomerization of azobenzene mesogenic groups. When the film is in the liquid crystalline phase, the trans → cis isomerization induces a major surface reorganization on the mesoscopic scale that is characterized by a reduction in the effective thickness of the film. The film conductivity is measured in vertical two-terminal devices in which the polymer is sandwiched between a Au contact and a liquid compliant E-GaIn drop. We demonstrate that the trans → cis isomerization is accompanied by a reversible 100-fold change in the film conductance. In this way, the device can be set in a high- or low-resistance state by light irradiation at different wavelengths. This result paves the way toward the potential use of poly(metha)acrylate/azobenzene polymer films as active layer for optical input/electrical output memory elements.

Synthesis of a new photoresponsive molecular carcerand

Abstract The synthesis of a covalently bonded new photoresponsive molecular capsule formed from two resorcin[4]arene cavitands by fourfold ether linkages using azobenzene subunits is reported. The new molecular capsule undergoes trans to cis isomerization upon irradiation with 365 nm light and can be reversed back from cis to trans form by means of exposure to visible light (530 nm) in benzene solution. The photo-switchable behaviour was also investigated by 1 H NMR spectroscopy which further substantiated the UV–vis experiment. The quantum chemical calculations both at PM6 as well as at the DFT (B97-D) level of approximation for various stereoisomers starting from all- trans to all- cis indicated an increasing flexibility of these molecular capsules.

Excited state relaxation pathways of 4-dimethylamino-β-nitrostyrene: Effect of solvent polarity and donor–acceptor conjugation

Abstract Solvent polarity dependent relaxation pathways of the excited state of 4-dimethylamino-β-nitrostyrene (DANS) have been investigated using steady state and transient absorption spectroscopic techniques in femtosecond and nanosecond timescale. The molecule undergoes ultrafast twisted intramolecular charge transfer (TICT) relaxation involving the rotation of dimethylaniline group in polar solvents. In the medium and low polarity solvents, TICT relaxation rate is retarded due to larger barrier of twisting along TICT coordinate. In nonpolar solvents, the S1 state population undergo efficient intersystem crossing (ISC) to triplet surface without any conformational change. However, trans → cis isomerisation in nonpolar solvents takes place from the excited triplet manifold, as monitored by steady state photo-irradiation experiments. The ISC efficiency decreases with the increase in solvent polarity, which is explained by invoking solvent polarity dependent stabilization of charge transfer S1 state relative to a receiver triplet state. Comparison of the excited state dynamics of DANS with less conjugated para-dialkylamino nitrobenzene derivatives, reported previously, suggests that additional π-conjugation between the donor and acceptor groups stabilizes planar ICT state resulting to slower TICT relaxation.

Photoluminescence intensity and polarization modulation of a light emitting liquid crystal via reversible isomerization of an α-cyanostilbenic derivative

We found reversible photo/thermal isomerization of the α-cyanostilbenic derivative (Z)-2-(4-aminophenyl)-3-(4-(dodecyloxy)phenyl)-acrylonitrile, (Z)-CN-APHP, which is accompanied by a large decrease in compatibility when mixed with a nematic liquid crystal. Moreover, a novel optical switch using a CN-APHP doped liquid crystal that emits partially linearly polarized light is investigated. By controlling the trans–cis isomerization of CN-APHP, it is possibly to modulate not only photoluminescence intensity but also its polarization. This could open possibilities to design improved optical devices.

Exciton Splitting of Adsorbed and Free 4-Nitroazobenzene Dimers: A Quantum Chemical Study.

Molecular photoswitches such as azobenzenes, which undergo photochemical trans ↔ cis isomerizations, are often mounted for possible applications on a surface and/or surrounded by other switches, for example, in self-assembled monolayers. This may suppress the isomerization cross section due to possible steric reasons, or, as recently speculated, by exciton coupling to neighboring switches, leading to ultrafast electronic quenching (Gahl et al., J. Am. Chem. Soc. 2010, 132, 1831). The presence of exciton coupling has been anticipated from a blue shift of the optical absorption band, compared to molecules in solution. From the theory side the need arises to properly analyze and quantify the change of absorption spectra of interacting and adsorbed switches. In particular, suitable methods should be identified, and effects of intermolecule and molecule-surface interactions on spectra should be disentangled. In this paper by means of time-dependent Hartree-Fock (TD-HF), various flavors of time-dependent density functional theory (TD-DFT), and the correlated wave function based coupled-cluster (CC2) method we investigated the 4-nitroazobenzene molecule as an example: The low-lying singlet excited states in the isolated trans monomer and dimer as well as their composites with a silicon pentamantane nanocluster, which serves also as a crude model for a silicon surface, were determined. As most important results we found that (i) HF, CC2, range-separated density functionals, or global hybrids with large amount of exact exchange are able to describe exciton (Davydov) splitting properly, while hybrids with small amount of exact exchange fail producing spurious charge transfer. (ii) The exciton splitting in a free dimer would lead to a blue shift of the absorption signal; however, this effect is almost nullified or even overcompensated by the shift arising from van der Waals interactions between the two molecules. (iii) Adsorption on the Si "surface" leads to a further, strong red shift for the present system. (iv) At a next-nearest neighbor distance (of ∼3.6 Å), the exciton splitting is ∼0.3 eV, with or without "surface", suggesting a rapid quenching of the molecular π → π* excitation. At larger distances, exciton splitting decreases rapidly.

Active Demulsification of Photoresponsive Emulsions Using Cationic-Anionic Surfactant Mixtures.

The influence of ultraviolet (UV) light irradiation on the emulsification properties of mixtures of an anionic surfactant, sodium dodecyl sulfate (SDS), and a photoresponsive cationic surfactant, 2-(4-(4-butylphenyl)diazenylphenoxy)ethyltrimethylammonium bromide (C4AzoTAB), containing an azobenzene group has been investigated. When mixtures of n-octane and aqueous SDS/trans-C4AzoTAB solution are homogenized, stable emulsions are obtained in regions of specific surfactant concentrations and molar ratios of the mixed surfactants. The stable emulsions are stable for over a week and found to be of the oil-in-water (O/W) type. UV light irradiation of the stable O/W emulsions leads to the coalescence of smaller oil droplets into larger ones in the emulsions, i.e., demulsification. As a result, the oil and aqueous surfactant solution phases are fully separated by UV light irradiation for 90 min, even shorter than our previous result (6 h; Langmuir 2014 , 30 , 41 - 47 ). The use of a microreactor shortens the time required for the photoinduced demulsification into 3.5 min. When mixtures of octane and aqueous SDS/cis-C4AzoTAB solution are homogenized, no emulsions are obtained. The interfacial tension (IFT) between octane and aqueous SDS/cis-C4AzoTAB solution is higher than that between octane and aqueous SDS/trans-C4AzoTAB solution, indicating that the IFT of SDS/trans-C4AzoTAB mixtures increases with the cis photoisomerization of the trans isomer. These results suggest that cis isomerization of the SDS/trans-C4AzoTAB mixtures due to UV light irradiation causes Ostwald ripening of the octane droplets in the emulsions, thereby reducing the interfacial area between the octane and water phases as the IFT between octane and the aqueous surfactant solution increases. Subsequently, the octane and aqueous solution phases separate.

A novel azo-thiourea based visible light switchable anion receptor

A novel azo-thiourea based anion receptor can be synthesized by coupling a bis-azoaniline motif with butyl isothiocyanate with 45% yield under mild conditions. It was found that azo-thiourea was an excellent receptor to recognize various di-carboxylate anions. It can serve as a colorimetric sensor for azelate ion and led to a significant change in the absorption spectrum. Furthermore, a reversible trans–cis isomerization of azo-thiourea induced by visible lights was confirmed by 1H NMR spectroscopy.

Synthesis and characterization of azobenzene-based gold nanoparticles for photo-switching properties

A series of new azobenzene based thiolated liquid crystals modified with gold nanoparticles were synthesized and characterized using a different mode of delegated tools e.g. FTIR, NMR and FESEM–EDX measurements for the structural properties of synthesized compounds. Polarized optical microscopy studies have revealed all the studied compounds having liquid crystalline properties, as a typical nematic phase. These liquid crystal capped gold nanoparticles' size was determined by TEM experiment. In addition, azobenzene-based gold nanoparticles containing flexible spacers showed photochromic behavior upon UV irradiation. These molecules exhibited strong photoisomerisation behavior in solutions and their trans to cis isomerisation took about 44s whereas the reverse process almost took place ranging from 82 to 125min.

Application of spectroscopic and theoretical methods in the studies of photoisomerization and photophysical properties of the push-pull styryl-benzimidazole dyes.

The synthesis and spectroscopic properties of a series of substituted 1,3-dimethyl-2-aminostyrylbenzimidazolium iodides are described and discussed. The products were identified by NMR, IR and UV-Vis spectroscopy and elemental analysis. Their electronic absorption and fluorescence band positions are affected by the character of the substituent and by the solvent polarity. The fluorescence decay of the dyes shows two lifetimes interpreted in terms of emission from two forms of the dye in the excited state. Moreover, the photochemical trans→cis isomerization is reported for these compounds. It occurs from the first excited singlet state of the trans isomer to the cis isomer following a trans-S0→ S1 excitation. The electron-donating character of the substituent in a styrene moiety is one of the crucial factors influencing the photoisomerization process. The structure of the cis isomer was established by (1)H and (15)N NMR. Experimental studies are supported by the results of quantum chemical calculations.

Self-Healing in Epoxy Thermoset Polymer Films Triggered by UV Light

Self-healing of damaged structures can occur in three processes: capsule-based healing, vascular healing and intrinsic healing of polymers. The latest concept has a tremendous potential to repair damaged polymers and composite structures. Until now self-healing of composite materials has been addressed using capsule based resin with an initiator that polymerises in the damaged region. In this study, self-healing behaviour of cured epoxy thermoset resins modified with an epoxy-functionalised photoresponsive azobenzene molecule has been addressed by UV light without the use of any resin capsules or other component that is not chemically joined with the composites. The study was executed by nanoindentation and atomic force microscopy (AFM). Artificially damaged thermoset films exhibited good self-healing behaviour under UV irradiation only. This effect is attributed to the intrinsic healing mechanism triggered by UV-induced trans→cis isomerization of the azobenzene chromophore which enables the damaged polymeric matrix to recover.