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Biodiesel is a renewable fuel source comprising of fatty acid methyl esters (FAMEs). However, biodiesel (FAMEs) is susceptible to autoxidation. Therefore, structural modification of FAMEs through decreasing the number of double bonds (e.g., hydrogenation) or trans isomerization of original double bonds in the cis configuration, could improve the oxidative stability of FAMEs. In this work, the trans isomerization of carbon-carbon double bonds in unsaturated FAMEs obtained from edible oils has been successfully achieved using simple and safe p-toluenesulfinic acid as catalyst. And the effects of reaction time, temperature, catalyst loading, amount of water and antioxidants on the trans isomerization were investigated systematically. The results indicated that the amount of water (0–20 wt%) and antioxidants (

Thermal cis-trans isomerization of azobenzene and azopyridine derivatives was studied. The impact of the structural features of azo derivatives on its isomerization kinetics being important for molecular switching materials was investigated experimentally and theoretically based on the density-functional theory (DFT). Eleven azo-dyes in which the phenyl rings were substituted in the para-position to N N– linkage were prepared. Additionally, their thermal properties were evaluated by thermogravimetric analysis and differential scanning calorimetry. DFT calculations were also used for optimization ground and transition state geometries, the density of states, electronic structures of studied azo-dyes. The kinetics of the dark-return cis to trans-form of azo-compounds was investigated after UV-irradiation. It was found that attaching hydroxyalkoxy-substituents to the azo-dyes structure retarding the photoisomerization process in comparison to the hydroxyl group. Moreover, dark-return cis to trans-isomer was ca. twice times faster for azopyridine derivatives than for their azobenzene analogues. The calculated rate constants were in qualitative agreement with the experimental values.

The synthesis and characterization of dicarbonyl Fe(II) PNP pincer complexes of the type cis-[Fe(PNP-iPr)(CO)2(X)]+ (X = Br, Cl) is described. These complexes are slowly formed when solutions of complexes trans-[Fe(PNP-iPr)(CO)2(X)]+ are kept in the dark for 9 h (X = Br) and 3 days (X = Cl). Upon exposure to visible light these complexes isomerize to the respective trans-dicarbonyl complexes within a few hours. The visible-light reaction seems to involve reversible CO dissociation. The isomerization can be repeated serval times. A mechanistic rationale for this isomerization process is established by means of DFT calculations.

Besides the photochromism isomerization, scientists are likely to focus more and more on the influence on the electrochemical reduction conditions of the azobenzene isomerization process. Diverse azobenzene compounds were chosen to study the behavior of their trans and cis isomers upon electrochemical reduction. Hence, thermal reaction mechanisms for the neutral, cationic and anionic azobenzene derivatives isomerization with a wide variety of electron-donating and electron-withdrawing groups in the para-position of the benzene ring in ground state have been systematically examined applying the density functional theory method. Some prominent changes in structure, energy barrier and reaction rate arose from charge changes of azobenzenes have been detected and interpreted in terms of the strengths of substituents, the polarity of solvents and charge effect. Our findings indicate that the radical anion of the cis isomer can isomerize to the corresponding trans configured counterpart rapidly with a tremendously enhanced rate and obvious decreased energy barriers with respect to the neutral species as the distribution of the electron density changed, signifying that the metastable isomer of a photoswitch is able to isomerize to the more stable one catalytically upon reduction and the charge effect has obvious influence on the reaction mechanism.

Quantum chemical calculations of a potent class of photoswitches acylhydrazones have been carried out with the goal to describe their thermal cis → trans isomerization. The effects of substituents, in particular their number, position and nature of the substituents (electron donating/withdrawing groups (EDGs/EWGs)), on activation energies incorporating solvent effects have been systematically researched with the ultimate goal to illuminate the isomerization process. Our results show that most parameters are highly dependent on the substitution pattern on the crucial positions (R1 and R2 position) of the backbone fragment, which in turn has a significant impact on the absorption spectra, the energy levels of molecular orbitals, the transition properties for the trans/cis isomers, the half-lives τ1/2 and the rate constants for the thermal cis → trans isomerization of the compounds. In summary, both relative strength of the donor/accept groups and solvent polarity have significant impact on the electric properties. Finally, the nature of the transition state(s) and its dependence on substituents and the environment are discussed. An ingenious approach to the construction of reaction path was realized, and energy barriers were determined from two-dimensional potential energy surfaces of the ground states.

In an effort to explore the residual complexity of naturally occurring chalcones from the roots of Glycyrrhiza inflata (Fabaceae), two new licochalcone A (LicA) derivatives were isolated as trace metabolites from enriched fractions. Both constituents contain a dihydrofuran moiety linked to carbons C-4 and C-5 of the retrochalcone core. Compound 1 (LicAF1) represents a new chemical entity, whereas compound 2 (LicAF2) has previously been reported as a Lewis acid catalyzed rearrangement of LicA. Evaluation of chirality revealed that both dihydrofuran derivatives existed as a mixture of R and S enantiomers. Interestingly, when solutions were exposed to sunlight, both dihydrofuran retrochalcones, initially isolated as trans isomers, were found to rapidly isomerize yielding trans and cis isomers. Analysis of the 1D 1H NMR spectra of the photolysis products revealed the presence of two sets of proton resonances ascribed to each of the geometric isomers. An up-field shift of all proton resonances arising from the cis isomer was observed, suggesting that anisotropic shielding effects were introduced through an overall perturbation of the 3-dimensional structure upon photoisomerization. Similar up-field shifts were observed in the 13C spectrum of the cis isomer, except for the C=O, C-α, and C-6 carbons, which experienced downfield shifts. Analogous NMR results were observed for LicA. Hence, the results presented herein encompass the isolation and full characterization of LicAF analogs 1 and 2, and observations of their trans-to-cis photoisomerization through the systematic analysis of their NMR spectra.

Excited-state intramolecular proton transfer (ESIPT) dynamics of the amino-type hydrogen-bonding compound 2-(2′-aminophenyl)benzothiazole (PBT-NH2) as well as its two derivatives 2-(5′-cyano-2′-aminophenyl)benzothiazole (CN-PBT-NH2) and 2-(5′-cyano-2′-tosylaminophenyl)benzothiazole (CN-PBT-NHTs) were studied by the time-dependent density functional theory (TD-DFT) approach with the B3LYP density functional, and their absorption and emission spectra were also explored at the same level of theory. A good agreement is observed between the theoretical simulations and experimental spectra, indicating that the present calculations are reasonably reliable. In addition, it is also found that the energy barriers of the first excited singlet state of the three targeted molecules along the ESIPT reaction are computed to be 0.38, 0.34 and 0.12 eV, respectively, showing the trend of gradual decrease, which implies that the introduction of the electron-withdrawing cyano or tosyl group can facilitate the occurrence of the ESIPT reaction of these amino-type H-bonding systems. Following the ESIPT, both CN-PBT-NH2 and CN-PBT-NHTs dye molecules can undergo the cis-trans isomerization reactions in the ground-state and excited-state potential energy curves along the C2-C3 bond between benzothiazole and phenyl moieties, where the energy barriers of the trans-tautomer → cis-tautomer isomerizations in the ground states are calculated to be 0.83 and 0.34 eV, respectively. According to our calculations, it is plausible that there may exist the long-lived trans-tautomer species in the ground states of CN-PBT-NH2 and CN-PBT-NHTs.

In this study, a novel trans-(1E,2E)-benzil-O,O-dimethylsulfonyl dioxime (BDMDO) monomer was prepared via a one-step dehydrochlorination reaction. BDMDO was characterized by 1H and 13C NMR, UV–Vis, and FT-IR spectroscopies, CHN elemental analysis, mass spectrometry, energy-dispersive X-ray spectroscopy, and TGA. According to the X-ray diffraction data, the BDMDO crystal structure was solved as a trans-isomer dioxime. The lattice structure was stabilized by two types of H-bonds and a sufficient number of interesting non-covalent supramolecular interactions such as the C H⋯H C bonds. Molecular electrostatic potential measurements and Hirschfeld surface analysis were performed to understand these interaction modes. The cis–trans isomerization in BDMDO as well as the structural, vibrational, NMR, highest occupied molecular orbital/lowest unoccupied molecular orbital bandgap, density of states, and electronic properties were computed by the density functional theory and subsequently compared with available experimental 1H and 13C NMR, UV–Vis, and FT-IR spectral data. The thermogravimetric/derivative thermogravimetric behavior of BDMDO was determined experimentally under an open-room atmosphere.

Cis-trans isomerization is one of the most common and well-investigated photoreaction of olefins in solution. This occurs via 180° rotation of one of the substituents on C C bond requiring large space around the double bond. One would expect that in crystals the neighboring molecules would prohibit such a process. In spite of this in early 1960s Schmidt and co-workers reported occurrence of such a process in crystals via 2 + 2 ‘meta cyclobutane’ formation. In this study by examining the photochemistry of four cis-stilbazolium salts we show that the photoconversion to the corresponding trans isomers does not occur via 2 + 2 addition. Large separation that ensures considerable space between neighboring olefins favors cis-trans isomerization in crystals. An aspect that is puzzling is that the product trans isomer phase separates and recrystallizes within the parent cis crystals and photodimerizes to the cyclobutane dimer. Details of this phenomenon are to be understood.

Decomposition of the protonated tripeptide, GlyProAla (H+GPA), through collision-induced dissociation with Xe in a guided ion beam tandem mass spectrometer (GIBMS) is examined. Cross sections for the formation of [b2]+, [y2 + 2H]+, [a1]+, [b3]+, [a3]+, [a2]+, and H+(1-pyrroline) fragment ions were collected as a function of kinetic energy. Analysis of these cross sections include consideration of the effects of multiple collisions, dissociation lifetimes, reactant internal and kinetic energy distributions, competition among channels, and evolution into sequential channels. Structures and molecular parameters of reactants, transition states, intermediates, and products were identified using the B3LYP/6-311+G(d,p) level of theory, with single-point energy calculations performed at the B3LYP, B3P86, and MP2(full) levels of theory using a 6-311+G(2d,2p) basis set. Good agreement between experimental threshold energies and those calculated for rate-limiting transition states allow key reaction pathways for the formation of each product to be identified. The MP2(full) level of theory is found to agree best with the experimental results. Notably, even though H+GPA starts with a cis-peptide bond, it forms an oxazolone as its [b2]+ fragment and the absence of a diketopiperazine [b2]+ fragment is verified by other key observations.

The paper describes the synthesis and characterization of “T-type” polyimides and their precursors – poly(amic acid)s. Investigated polymers contained azobenzene derivatives with no substituent or with fluorine atom in para position to the N N linkage. The influence of the main chain architecture (open or close imide rings), location and content of azo chromophores in polymers chains on physicochemical properties were studied. To the best of our knowledge, for the first time, the cis-trans isomerization reaction was studied for azo poly(amic acid)s, that was 1–18% after 3-minutes of 405 nm-light irradiation. Poly(amic acid)s characterized higher stability of isomerization than their polyimide counterparts. The studied azo polyimides showed high glass transition temperatures (174–206 °C) and high thermal stability with the beginning of the decomposition at 246–345 °C. The imidization kinetic of the studied poly (amic acid)s were investigated by differential scanning calorimetry (DSC) using Kissinger and Ozawa models. It was found that the studied azo polymers characterized high activation energies (275.4–357.1 kJ mol−1) and frequency factors (6.5 × 1029–1.3 × 1037 s−1).

The subject of the research is a series of azo-dyes based on the quinoline structure, which are practically omitted as materials which use photoinduced optical anisotropy for applications. The work presents the kinetics of thermal cis-trans isomerization reaction of 5-azo-8-hydroxy-2-methylquinoline and 5,5'-((1E,1′E)-[1,1′-biphenyl]-4,4′-diylbis(diazene-2,1-diyl))bis(2-methylquinolin-8-ol) compounds. The impact of the structure of the substituent at the azo linkage on the cis-trans isomerization kinetics was investigated experimentally and theoretically using the UV–Vis spectroscopy and the density-functional theory (DFT). The kinetics of the dark-return of the cis-to trans-isomer of azoquinolines was investigated after UV-irradiation. It was found that attaching the nitro substituent to the azoquinolines structure retards the photoisomerization process in comparison to the other substituents (i.e. F, Cl, Br, N atoms or CH3, OH, COOH, OCH3 groups). The calculated rate constants were in qualitative agreement with the experimental values. To the best of our knowledge, the cis-trans isomerization kinetics for azoquinolines has not been studied yet.

A systematic study on the dynamics of trans-cis, thermal cis-trans backward and light induced cis-trans isomerization of some azobismaleimide derivatives was performed. Systems under study were N-[4-(carbonylaminoazobenzene)-phenyl]-maleimide (AAzM) and a styrene-maleimide copolymer containing azo units in the side chain (AAzMC). The isomerization reactions were followed by electronic absorption spectra. The kinetics of trans-cis photoisomerization was investigated in six solvents with different polarities. Results revealed that the content of the photostationary state depended on the solvent nature. In turn, the chemical environment had a minor influence on the photoisomerization rate constants. The kinetic and thermodynamic activation parameters were evaluated according to Arrhenius and Eyring equations. The theoretical calculations based on density functional theory (DFT) were employed to identify the mechanism of the thermal isomerization in AAzM. In our results, the thermal cis-trans isomerization of azobismaleimide AAzM followed an inversion assisted by rotation pathway in the S0 state.

Pin1 is a peptidyl prolyl isomerase that specifically catalyzes cis–trans isomerization of phosphorylated Thr/Ser-Pro peptide bonds in substrate proteins and peptides. Pin1 is involved in many important cellular processes, including cancer progression, so it is a potential target of cancer therapy. We designed and synthesized a novel series of Pin1 inhibitors based on a glutamic acid or aspartic acid scaffold bearing an aromatic moiety to provide a hydrophobic surface and a cyclic aliphatic amine moiety with affinity for the proline-binding site of Pin1. Glutamic acid derivatives bearing cycloalkylamino and phenylthiazole groups showed potent Pin1-inhibitory activity comparable with that of known inhibitor VER-1. The results indicate that steric interaction of the cyclic alkyl amine moiety with binding site residues plays a key role in enhancing Pin1-inhibitory activity.

Steady state absorption, fluorescence and femtosecond transient absorption techniques were employed to investigate the excited state relaxation mechanism of trans-ethyl-3-(1H-indole-3-yl)acrylate (trans-EIA). The measured large value of difference in the static dipole moment from ground state to excited state reflects the involvement of excited state intramolecular charge transfer in EIA. The charge-transfer time constant has been estimated to be 0.35 ps. The charge transfer state depletes to the ground state through internal conversion along double bond torsional coordinate at perpendicular conformation with a 1.8 ps time constant, leading to the trans-cis isomerization and was further confirmed by chromatography and 1H NMR experiments.

In this paper, a detailed isomerization study of a series of 2,4-nitro/cyan substituted azobenzenes is conducted by UV–Vis spectroscopy. The experiments, carried out in both toluene and N, N′-dimethylformamide, reveal an intriguing behavior. Interestingly, the disubstituted azo-derivatives display stable Z isomers in DMF, while in toluene they exhibit very fast relaxation. In addition, the azo-derivatives thermally isomerize from the metastable Z form to the thermodynamically stable E isomer through either inversion route or rotational mechanism, depending on the chemical nature of the substituents and on the solvent polarity.

Alzheimer's disease (AD) is characterized by progressive formation of β-amyloid peptide (Aβ) in the brain. Aβ induced oxidative stress may cause cis-trans isomerization of fatty acids. However, the research on AD rarely discusses the correlation between the formation of endogenous trans fatty acids (TFA) and the progression of AD. Thus, the goal of this research was to study the difference of trans-arachidonic acid (TAA) composition of brain and hippocampus between J20 mice (AD mouse model) and B6 mice (wild type control). Our results showed that the endogenous TAA (8-trans-C20:4) of brain and hippocampus in 9 months old J20 mice was significantly higher than B6 mice which indicated that there was a positive relationship between the formation of endogenous TAA and AD. In addition, this study also revealed that sesamol and ferulic acid could reduce the formation of 8-trans-C20:4. Thus, they might be employed as potential functional food ingredients for the prevention of AD.

We observed unique thermal cis–trans isomerization properties of water-soluble azobenzene derivatives in which a phosphate group was substituted at ortho position of an azobenzene via an amide linkage. Here, we report the effect of the structure of the phosphate groups (the length and the position), pH, and metal ions on thermal cis–trans isomerization. We revealed that the water-soluble azobenzene derivatives, in which di- or tri-phosphate group was substituted at ortho position of the azobenzene via the amide linkage, showed the fast thermal isomerization in low pH condition. In addition, we showed that the effects of the metal ions on the thermal isomerization rate.

One novel 2,2′:6′,2″-terpyridine ligand L and its complexes (Zn(S) 2 L, Cd(S) 2 L) were synthesized and systematically characterized. There were cis-trans isomerization of the ethylene group in compounds of L, Zn(S) 2 L and Cd(S) 2 L though some characterization. Finally it was found that the two-photon absorption cross section values (σ) for the complexes in the near infrared region (∼800 nm) were dramatically enhanced compared to the free D–π–A type ligand.

The first example of non-symmetric isoflavone-based fast photo-switchable liquid crystals with different functional groups at the terminal position were synthesized and characterized. Polarizing optical microscopy study revealed that the compounds showed least ordered nematic phase. Optical photo switching study exhibited very fast photoisomerization effect in solution. The E–Z and Z–E conversion occurred around 3–5 s and 40–700 s respectively. This is also the first example of para-substituted non-symmetric isoflavone liquid crystals exhibiting very fast photo switching property in solution. Argument based on non-symmetrical behaviour might be the reason for the observed behaviour.

A series of ruthenium hydride compounds containing bidentate pyrrole-imine ligands were synthesized and characterized and their properties were fully studied. Reacting [C4H3NH-(2-CH=N-R) (L1H, R = CH2-2-furanyl; L2H, R = CH2-morpholine; L3H, R = CH2-2-pyridyl) with one equivalent of n-BuLi afforded the corresponding lithium reagents (LiL1∼LiL3). Combining RuHCl(CO)(PPh3)2 with LiL1∼LiL3 in THF at 0 °C with stirring at room temperature for 12 h generated pure trans-Pyr-Ru-H 1–3 in moderate yields after repeated fractional recrystallization. When the reactions were performed at reflux temperature, pure cis-Pyr-Ru-H 1–3 were obtained by fractional recrystallization in relatively high yields. A thermal isomerization study showed that pure cis-Pyr-Ru-H 1–3 and trans-Pyr-Ru-H 1–3 can be interconverted in THF after refluxing and equilibrium was attained. Reacting a bulkier ligand [C4H3NLi-(2-CH=N-tBu)] (LiL4) with one equivalent RuHCl(CO)(PPh3)3 at room temperature afforded only cis-Pyr-Ru-H 4 after purification. No trans-Pyr-Ru-H 4 was observed, indicating the bulkier t-butyl group of the L4 ligand altered the stability of cis- and trans-Pyr-Ru-H 4. A catalytic alcohol amination study showed the cis forms of Pyr-Ru-H were the active catalysts for converting benzyl alcohol to N-benzylaniline.

With the aim of better understanding the mechanisms governing the spontaneous cis–trans isomerization of the olefins promoted by palladium(0) centers, we have synthesized several novel complexes bearing (Z)- or (E)-1,2-ditosylethene and a combination of other two spectator ligands including phosphines (PPh3 and 1,3,5-triaza-7-phosphaadamantane), isocyanides (i.e. 2,6-dimethylphenylisocyanide) and N-heterocyclic carbenes (NHCs). The nature of ancillary ligands heavily influences the steric and electronic features of the metal centre and consequently its interaction with olefin determines a different capability of supporting the isomerization process. The kinetic analysis and the determination of the rate constants of the isomerization suggested that both the steric and electronic factors play an important role and only an unpredictable combination can guarantee the success of the reaction. The work is completed by the structural characterization of three different Pd(0) complexes.

The interconversion of cis and trans isomers of dipeptides containing C -terminal proline was studied by dynamic chromatography on zwitterionic chiral stationary phases at temperatures ranging from −15 °C to +45 °C The cis – trans isomers could be separated below 0 °C and above 0–10 °C plateau formation and peak coalescence phenomena occurred, which is characteristic for a dynamic process at the time-scale of partitioning. At and above room temperature, full coalescence was observed, which allowed separations of enantiomers without interference from interconversion effects. Analysis of the dynamic elution profiles of the interconverting peptides allowed the determination of isomerization rate constants and thermodynamic activation parameters (isomerization enthalpy, entropy and activation energy). In accordance with established results, isomerization rates and thermodynamic parameters were found to depend on the nature of the N -terminal amino acid. Isomerization barriers were only slightly lower than values determined with other methods but significant differences in the relative contributions of the activation enthalpy and entropy as well as isomerization rates pointed toward selector-moderated isomerization dynamics.

Em publicação, A series of heterocyclic azo dyes were dispersed in poly(methyl methacrylate) (PMMA) matrix and the photochromic properties of the coloured films studied by UV-Vis spectroscopy. Visible irradiation of the coloured films lead to a fast decrease in the colour intensity due to the trans-cis photoisomerization reaction. When the light source was removed the spontaneous thermal cis-trans reverse isomerization occurred bringing the absorbance back to initial value with variable speed. Contrary to common azobenzenes that after photoisomerization with UV or visible light exhibit very slow thermal cis-trans back re-isomerization, that can last for hours or days, azo dyes bearing pyrrole, thiophene, thiazole or thiadiazole heterocycles display very fast switching between the two isomers (few seconds) even when dispersed in PMMA polymers., Fundação para a Ciência e a Tecnologia (FCT)

In photochemical Hula-twist isomerization of conjugated polyenes, a double bond and an adjacent single bond twist concertedly by 180°. It is here considered to be driven by passing through the last conical intersection (CI), between potentials of the dark covalent S 1 (2A) and the ground S 0 (1A) states. If several such CIs are available, that is, if isomerization of different bonds is possible, regioselectivity can be caused by a sterically induced pre-twist in S 0 , so that the pre-twisted group isomerizes: this deformation is amplified in the Franck–Condon region of the spectroscopic (1B) state; on entering the 2A surface with this geometry, the nearest S 1 /S 0 CI will be chosen, if the reaction is ultrafast. If the reaction is slower, that is, if there is a barrier before each CI, a local pre-twist reduces one of them and thus also selects the site of isomerization. Another pre-twist can be caused in the initially excited (ionic) 1B state by electrostatic effects, thus also giving rise to regioselectivity. The explanations only consider potentials of the molecule, in the majority of cases with no external forces. Also other observations are summarized that support the idea that Hula twist can work without influence of the environment. A variant of Hula twist is also proposed and used, in which the torsion of the two adjacent bonds is not disrotatory as usual but conrotatory.

Three new azo-polyurethanes containing the s-triazolo[3,4-b]thiadiazole heterocycle have been prepared and characterized. The polymers showed glass transition temperatures between 183 °C and 232 °C and a moderate to good thermal stability in air. The thermal cis–trans isomerization around the azo group was analyzed in solution and the kinetic constant of this process was calculated. One of the synthesized polymers was processed in form of thin films (thickness was 1.2 μm) and some experiments of optical laser writing were performed on it: three different surface relief gratings (SRGs) structures were realized at micrometer scale (a square, a concentric square pattern and a grating) indicating this polymer as an interesting candidate for the azo-lithography.

The cis to trans isomerizations during the syntheses of trans-[Ru(NO)(OH)(cyclam)](PF6)2, from cis-[RuCl2(cyclam)]Cl, and [Ru(NO)Cl(cyclam)] (PF6)2, from cis-[RuCl2(dmso)4], are reported. The novel trans-[Ru(NO)(OH)(cyclam)](PF6)2 complex was characterized by X-ray crystallography and vibrational infrared and nuclear magnetic resonance spectroscopies. The Ru–N–O bond angle (176.75o) and ν(NO) (1835 cm− 1) suggest a nitrosonium character for this hydroxo complex. The crystal and molecular structure of trans-[Ru(NO)Cl(cyclam)](ClO4)2·2 H2O is also reported. Results presented here support the cis–trans isomerization observed for the first time with ruthenium cyclam complexes.

H/ACA small nucleolar and Cajal body ribonucleoproteins (RNPs) function in site-specific pseudouridylation of eukaryotic rRNA and snRNA, rRNA processing, and vertebrate telomerase biogenesis. Nhp2, one of four essential protein components of eukaryotic H/ACA RNPs, forms a core trimer with the pseudouridylase Cbf5 and Nop10 that binds to H/ACA RNAs specifically. Crystal structures of archaeal H/ACA RNPs have revealed how the protein components interact with each other and with the H/ACA RNA. However, in place of Nhp2p, archaeal H/ACA RNPs contain L7Ae, which binds specifically to an RNA K-loop motif absent from eukaryotic H/ACA RNPs, while Nhp2 binds a broader range of RNA structures. We report solution NMR studies of Saccharomyces cerevisiae Nhp2 (Nhp2p), which reveal that Nhp2p exhibits two major conformations in solution due to cis/trans isomerization of the evolutionarily conserved Pro83. The equivalent proline is in the cis conformation in all reported structures of L7Ae and other homologous proteins. Nhp2p has the expected α-β-α fold, but the solution structures of the major conformation of Nhp2p with trans Pro83 and of Nhp2p-S82W with cis Pro83 reveal that Pro83 cis/trans isomerization affects the positions of numerous residues at the Nop10 and RNA binding interface. An S82W substitution, which stabilizes the cis conformation, also stabilizes the association of Nhp2p with H/ACA snoRNPs expressed in vivo. We propose that Pro83 plays a key role in the assembly of the eukaryotic H/ACA RNP, with the cis conformation locking in a stable Cbf5–Nop10–Nhp2 ternary complex and positioning the protein backbone to interact with the H/ACA RNA.

The light-induced cis-trans isomerization of rhapontigenin (RHA) and its glucoside rhaponticin (RHA-Glc) were evaluated under ultraviolet (UV) light irradiation. A simple and rapid capillary electrophoresis method was developed for the kinetic study of four stilbenes (both cis and trans form of RHA and RHA-Glc). These analyses were achieved by using β-cyclodextrin (β-CD) modified capillary zone electrophoresis with diode array detector (CZE-DAD). The method provided reliable separations with a short analysis time of 3 min. The purity of individual compound was checked by UV spectral comparisons with known standards, and further confirmed by (1)H and (13)C nuclear magnetic resonance (NMR) spectroscopy. Furthermore, the UV absorbance and the molar absoptivity (ε) values were determined by UV-vis spectrophotometer to be 36824 L mol(-1) cm(-1) at λ(max) 324.5 nm for trans-RHA and 43894 L mol(-1) cm(-1) at λ(max) 325 nm for trans-RHA-Glc in methanol/water mixture solution (50%, v:v), respectively. CZE, UV-vis and NMR spectroscopy studies provided similar conclusions by considering the influence of irradiation time and the influence of irradiation wavelength.

Author Institution: Optical Technology Division, NIST, Gaithersburg, MD 20899-8441, MD, USA; Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan 10617 and Department of Chemistry, University of British Columbia, Vancouver, B.C., Canada V6T 1Z1

The development of a rapid and selective capillary electrophoresis method for the quantitation of ramipril and its eight main impurities in pharmaceutical dosage form is described. Ramipril and three of its impurities contain a proline-similar moiety which causes in solution the presence of interconverting cis–trans isomers with respect to the amide bond. The interplay between electrophoretic migration and isomerization may yield the presence of an undesired interconversion zone between the two isomer peaks in the electropherogram, depending on the experimental conditions. Different capillary electrophoresis operative modes and pseudostationary phases were evaluated, both in normal and reverse polarity, in order to find the essential analytical parameters which could make it possible to overcome this issue and thus accurately quantify the analytes. The best results were obtained by using microemulsion electrokinetic chromatography in reverse polarity, where all the compounds which undergo cis–trans interconversion migrate as a single narrow peak. Experimental design led to identification of the following optimised conditions: background electrolyte, microemulsion made by 88.95% of 90mM phosphate pH 2.5, 1.05% of n-heptane and 10.00% of SDS/n-butanol in 1:2 ratio; voltage, −26 kV; temperature, 17 ◦C. Applying these conditions, the baseline separation of the analytes was obtained in about 10 min. Validation of the method following ICH guidelines was carried out and the procedure was applied to a real sample of ramipril tablets.

The main goal of this work was the synthesis of new azo-functionalized block copolymers (BCP) from epoxidized poly(styrene-b-butadiene-b-styrene) modified with azobenzene groups by one-step facile reaction between the epoxy groups and an azo-amine. The epoxy/amine reaction was verified by Fourier transform infrared spectroscopy. Additionally, we studied the effect of covalent attachment of the azobenzene moieties by analyzing the morphology and the optical anisotropic response of the resulting azo-containing BCP, with respect to solution mixing of the azobenzene as a guest in the BCP host without chemical bonding. Self-assembly of all modified BCP resulted in phase-separated morphologies on the nanometer scale. Nonetheless, segregation of azobenzene aggregates onto the BCP surface was observed in guest–host systems. In relation to the optical anisotropic behaviour of the resulting materials, two distinct optical responses were observed depending on the existence or not of covalent attachment of the azo-chromophores to the BCP.

Carrot is one of the important root crops cultivated worldwide. In many cases, carrot must be dried prior to its use. Although hot air drying is one of the most common methods for drying carrots, it is well known that hot air drying leads to much quality degradation of a dried product. To alleviate the adverse effect of hot air drying many pretreatment methods have been proposed. In this study, the effects of pretreatments with citric acid, which is used as an anti-darkening agent and a texture-modifier, were investigated. Carrots were soaked in citric acid to pH of either 4 or 5 or blanched in citric acid to pH of either 4 or 5 prior to hot air drying. Untreated carrots, carrots soaked in water and carrots blanched in water were used as control samples. The effects of pretreatments, in combination with hot air drying at 70, 80 and 90 °C, on selected physicochemical properties of dried carrots, namely, color, shrinkage, rehydration ability, β-carotene content and cis – trans isomerization of β-carotene, were evaluated. It was noted that carrots, which were soaked and blanched in citric acid, were redder than the untreated sample. Although shrinkage of pretreated carrots was higher, the rehydration ability of the pretreated carrots was higher than that of the untreated sample. The β-carotene content of carrots with no pretreatment decreased continuously, whereas the contents of β-carotene in citric acid soaked and blanched carrots tended to be unchanged. However, isomerization of β-carotene in all samples was not significantly different.

The poly(propargyl acetate) ( A ) having a helical cis – transoid structure was stereospecifically prepared using the Rh complex catalyst, [Rh(norbornadiene)Cl] 2 , in MeOH or NEt 3 solvent at 0 and 40 °C in moderate yield. Electron spin resonance (ESR) analysis of the polymer revealed the formation of the cis ( B ) and trans ( C ) radicals which were produced through the thermal rotational scission of the helical cis C C bonds in the main-chain during the polymerization. The spatial and geometrical structure was successfully deduced using the two analogues’ polymers in which either methyl or methylene group is deuterated, by the aide of computer simulation of the observed ESR spectra together with the calculation of spin density of the two radicals.

The cis–trans isomerization mechanism of N -Methylacetamide was studied by molecular dynamics simulations employing a hybrid QM/MM potential with explicit water. In combination with the umbrella sampling method, we obtained the free-energy barrier between the cis – trans states in accordance with previous experimental and theoretical values. Moreover, we found that the improper dihedral angle, C( O) N H C( H3), correlates well with the degree of the pyramidal conformations. We then investigated the role of the degree of the pyramidal conformation with respect to the isomerization processes and found that it governs the height and peak position of the free-energy barriers and the hydration structure.

The kinetics of trans–cis-isomerization of palladium(II) diglycinate complexes have been studied by the spectrophotometric and potentiometric methods. The reaction of transition of trans-[Pd(Gly)2] to cis-[Pd(Gly)2] is described by a first-order equation. The half-life of the trans-isomer [Pd(Gly)2] and the rate constant of the reaction of its transition to cis-isomeric structure have been determined. A mechanism of transformation of trans-[Pd(Gly)2] into cis-[Pd(Gly)2] is proposed. The spectral characteristics and electrochemical parameters of palladium(II) trans- and cis-diglycinate complexes have been determined. The mechanism of electroreduction of palladium(II) from glycinate electrolyte containing no excess free ligand has been elucidated.

We propose a new scheme of the photoactivation and the photobleaching for Dronpa with molecular dynamics method and density functional theory. These processes can be explained by considering cis-trans isomerization in neutral state of chromophore. The proton transfer from anionic to neutral chromophore makes cis-trans isomerization possible via hula-twist rotation process, since the space for cis-trans isomerization is opened at around the region near chromophore by moving out of the imidazole ring on H193 from the position below the phenol ring on chromophore. Then the cis-trans isomerization can occur through the hula-twist process. The contributions from the protein environment around CRO, especially S142 and H193, are indispensable for photoconversion of Dronpa.

Photo-tunable photonic crystals were prepared by infiltrating a photoresponsive material into SiO2 inverse opal films. Azobenzene-containing polymer liquid crystals, including poly(X-(4-methoxy-4′-oxy-azobenzene) alkyl acrylate), or PMAzXAc, where X is the number of methylene groups in the side chains (3, 6 and 11), were used as the photoresponsive material. PMAzXAc polymers were infiltrated in a spherical void of SiO2 inverse opal films to give photo-tunable photonic crystals that showed a photonic band gap. The photonic band gap of the photonic crystals could be controlled by irradiation with linearly and circularly polarized lights, leading to a reversible change in reflection from the photonic crystals. The change in the reflection band was found to be dependent on the methylene spacer length of the PMAzXAc. PMAz6Ac showed the largest shift of the reflection band (25 nm), while very little shift was observed for PMAz11Ac.

Well-defined platinum-based catalysts were prepared using fourth-generation hydroxyl-terminated poly(amidoamine) (PAMAM) dendrimers as templates. Nanoparticles with an average of 40 Pt atoms and an average diameter of 1.5 nm were produced this way, and dispersed on a homemade sol–gel silica support. Different treatments were tested for the activation of the resulting catalyst, including heating in vacuum or in oxygen or hydrogen atmospheres. A combination of electron microscopy, infrared absorption spectroscopy, X-ray photoelectron spectroscopy, and catalytic kinetic measurements were used to investigate the effect of those treatments on the decomposition and elimination of the material associated with the dendrimer and on the activity and selectivity of the resulting catalysts. Neither heating in vacuum nor burning with oxygen was deemed acceptable, because much graphitic material is deposited with the former treatment and metal oxidation is induced by the latter. Hydrogen pretreatments, on the other hand, lead to fairly active catalysts with high selectivity for the conversion of trans olefins to their cis counterparts.

In this study, we first described the two configurations of cephalosporins by cefozopran, then determined their molecular structure and finally evaluated the stability and biological activity of the two configurations. Our results showed that cefozopran existed as two different configurations in the aqueous solution with acetonitrile. Using mass spectrometry with liquid chromatography, nuclear magnetic resonance with liquid chromatography and optical rotation detection technology, we determined the spatial structures of both configurations and the detailed mechanism for change. By molecular docking and determining their antimicrobial activities, we showed that the biological activity of cis-isomer was stronger than that of trans-isomer.

The free-radical induced reaction between a tri-functional thiol (2-ethyl-(hydroxymethyl)-1,3-propanediol trimercapto propionate) and two 1,2-disubstituted alkenes (methyl oleate and methyl elaidat ...

A novel hyperbranched poly(aryl ether ketone) terminated by azobenzene chromophores (azo-HPAEK) was prepared using 1,3,5-tris[4-(4-fluorobenzoyl)phenoxy] benzene (B3) as a core molecule, 4,4′-(hexafluoroisopropylidene) diphenol as an A2 monomer and 4-(4-hydroxyphenylazo) benzonitrile as an end-capping reagent. It is shown that azo-HPAEK exhibits a high glass transition temperature (Tg), excellent thermal stability and homogeneous photochromic behavior. By exposing the spin-coated film to a laser beam interference pattern, azo-HPAEK can be used for rapid (within 30 s) fabrication of surface relief gratings (SRGs), and the azo-HPAEK gratings cannot be totally erased by heating, even at 300 °C. In particular, azo-HPAEK shows a large photoinduced birefringence intensity and good reversible optical storage upon irradiation with 532 nm Nd:YAG laser.

A series of azobenzene functionalized poly(arylene ether sulfone)s (azo-PAESs) has been synthesized via nucleophilic aromatic substitution polycondensation, and azobenzene chromophores could be introduced into the polymer main chains by copolymerization from azo-monomers. These main chain type azo-PAESs have high glass transition temperatures and good thermal stability. Exposed to an interference pattern laser beam, spin-coated films of the azo-PAESs can form clear and stable surface relief gratings (SRGs). These SRGs show increased thermal stabilities with the content of azo moieties and cannot be fully removed until the temperature increases to 300 °C for azo-PAESs with a content of azo segments above 50%.

An azobenzene-containing supramolecular copolymers, consisted of two polystyrene (PSt) arms and one poly(ethylene oxide) (PEO) arm linked via ionic bond, has been designed and successfully synthesized. Monomethoxy PEO with phenylazobenzenesulfonic acid as the terminus (PEO-N N-SO3H) was utilized to react with polystyrene carrying tertiary amino group at the middle of the polymer chain (PSt2-N(CH3)2) to form ion-bonded supramolecular star copolymers (PSt2-N N-PEO) with an azobenzene group at the core based on the interaction between sulfonic acid group and amino group. The obtained copolymers were characterized by 1H nuclear magnetic resonance (1H NMR) and gel permeation chromatography (GPC) techniques. The self-assembly behavior of the copolymers with different molecular weight of PSt was investigated, which shows solid spherical aggregates in water. The aggregation leads to the lower isomerization degree (54%) at the photostationary state in water compared with that in 1,4-dioxane (82%).

Commercially available Fmoc-protected l -amino acids were employed and coupled to l -allylglycine. Cross metathesis with 2-methyl-2-butene using second generation Grubbs’ catalyst gave l -prenylglycine-containing dipeptides. Treatment with trifluoromethanesulfonic acid resulted in cyclisation and subsequent formation of acid-stable 5,5-dimethyl- l -proline dipeptides for direct insertion into linear peptide sequences.

Thermal cis–trans conversion has been examined in oligothiophene single molecules encapsulated in the molecular vessels of cyclodextrin and Na-mordenite. At high temperatures, optical absorption intensities are enhanced in bithiophene, terthiophene and quinquethiophene, while the intensity of quarterthiophene is temperature independent. Optical absorption comes from uncompensated transition dipole moment due to the thermal torsion between cis and trans forms.

Transient absorption spectra of cis-stilbene in solution, recorded in the range 275–690 nm with 80 fs instrument response, reveal two successive evolving bands. One at 630 nm decays on a subpicosecond scale; it corresponds to vertically-excited nearly planar S1 state. The other, growing and decaying band at 340 nm, is shown to be consistent with theoretically predicted phantom state p∗ of perpendicular molecular geometry. The lifetime of p∗ is determined to be τ p ∗ = 0.4 ps in acetonitrile and 0.3 ps in hexane.

The pH influence on the cis – trans isomerization of the Valine-Proline dipeptide was investigated by means of NMR spectral techniques and quantum chemical calculations at different computational levels. It was found that the process of isomerization is strongly pH dependent. The trans -isomer of Val-Pro is the more abundant isomer when the amino group is protonated, while an approximately equal distribution between the cis and trans -isomers for the neutral and anion forms of the dipeptide was confirmed.

Ag nanoparticle/azopolymer nanocomposites are prepared with controlled concentration of Ag nanoparticles by in situ reduction of Ag(I) β-diketone complexes in an azopolymer matrix. The nanocomposites form an organic–inorganic network-like structure by interactions between the azopolymer matrix and the Ag nanoparticles. The Ag/azopolymer nanocomposites are homogeneous and highly transparent even when the content of Ag is as high as 5.6 wt%. Birefringence of the azopolymer without Ag and the nanocomposites can be optically induced and erased. However, after 5 cycles of optically writing–erasing processes, 9.7% decrease of the birefringence is observed for the azopolymer film without Ag and only 2.9% decrease of the birefringence is observed for the nanocomposite film with 5.6 wt% Ag, showing the stability of photo-induced birefringence of the nanocomposite is improved by introducing Ag nanoparticles into the azopolymer matrix. Relatively high concentration of Ag nanoparticles does not strongly hinder the mobility of azobenzene groups during photoisomerization and one-photon recording. In two-photon optical recording experiments, an image on the nanocomposite with 5.6 wt% Ag can be observed with writing power as low as 6 mW and no such an image can be observed in the azopolymer without Ag under the same two-photon recording condition.