Photochemical E (trans)-->Z (cis) isomerization in substituted 1-naphthylacrylates.

Substituted naphthylacrylates, 1-3, not showing rotamerism have been synthesized with a view to study photochemical E (trans)-->Z (cis) isomerization. Photostationary state composition of the isomers upon direct excitation, triplet sensitized isomerization, quantum yield of isomerization, and steady state and time-resolved fluorescence behavior have been studied for these naphthylacrylates. The direct excitations of the compounds yield high Z (approximately 80%) isomer composition, whereas the triplet sensitization results in less Z (approximately 20%) isomer composition. This indicates that the singlet pathway is very efficient in converting the E isomer to the Z isomer. The naphthylacrylates 1 and 2 exhibit structured fluorescence at room temperature in hexane and upon changing the solvent to CH3CN; the structure of the fluorescence is lost, indicating that the singlet excited-state develops a polar character in a polar environment. The polar nature of the singlet excited state becomes more clear in the case of 3 from its fluorescence solvatochromism. The naphthylacrylates did not exhibit excitation wavelength-dependent fluorescence at room temperature suggesting that the ground state conformers (rotamers) are not involved. Fluorescence lifetimes measured for these compounds displayed biexponential behavior, which is explained using a two-state model.     

Kinetics and equilibria of cis/trans isomerization of backbone amide bonds in peptoids

The biological activities of N-substituted glycine oligomers (peptoids) have been the subject of extensive research. As compared to peptides, both the cis and trans conformations of the backbone amide bonds of peptoids can be significantly populated. Thus, peptoids are mixtures of configurational isomers, with the number of isomers increasing by a factor of 2 with each additional monomer residue. Here we report the results of a study of the kinetics and equilibria of cis/trans isomerization of the amide bonds of N-acetylated peptoid monomers, dipeptoids, and tripeptoids by NMR spectroscopy. Resonance intensities indicate the cis conformation of the backbone amide bonds of the peptoids studied is more populated than is generally the case for the analogous secondary amide bond to proline residues in acyclic peptides. Rate constants were measured by inversion-magnetization transfer techniques over a range of temperatures, and activation parameters were derived from the temperature dependence of the rate constants. The rate of cis/trans isomerization by rotation around the amide bonds in the peptoids studied is generally slower than that around amide bonds to proline residues and takes place on the NMR inversion-magnetization transfer time scale only by rotation around the amide bond to the C-terminal peptoid residue.     

Kinetics of the cis,cis to trans,trans isomerization of 1,1,2,2,5,5,6,6-octamethyl-1,2,5,6-tetrasilacycloocta-3,7-diene

The kinetics of the ruthenium-promoted cis,cis to trans,trans isomerization of 1,1,2,2,5,5,6,6-octamethyl-1,2,5,6-tetrasilacycloocta-3,7-diene were investigated. Incubation of a ruthenium alkylidene complex, (Cy(3)P)RuCl(2)(==CHPh)Ru(p-cymene)Cl(2), in CD(2)Cl(2) for 5 days at 40 degrees C afforded a catalytically active ruthenium species that was shown to be responsible for promoting the isomerization. The isomerization was observed to proceed in two steps: (1) conversion of the starting cis,cis isomer to a proposed cis,trans intermediate and (2) subsequent conversion of the intermediate to the product trans,trans isomer. Kinetic studies demonstrated that the two steps are first-order with respect to the concentrations of the cis,cis isomer, the intermediate, and the ruthenium alkylidene complex. The data were further consistent with a mechanism involving bimolecular hydride addition-elimination during the two isomerization steps.     

Unexpectedly fast cis/trans isomerization of Xaa-Pro peptide bonds in disulfide-constrained cyclic peptides

Acyclic dithiol and cyclic disulfide forms of the peptides Ac-Cys-Pro-Xaa-Cys-NH2 (Xaa = Phe, His, Tyr, Gly, and Thr) and Ac-Cys-Gly-Pro-Cys-NH2 and the peptide Ac-Ala-Gly-Pro-Ala-NH2 were synthesized and characterized by mass spectrometry and NMR spectroscopy. Rate constants kct and ktc for cis-to-trans and trans-to-cis isomerization, respectively, across the Cys-Pro or Gly-Pro peptide bonds were determined by magnetization transfer NMR techniques over a range of temperatures, and activation parameters were derived from the temperature dependence of the rate constants. It was found that constraints imposed by the disulfide bond confer an unexpected rate enhancement for cis/trans isomerization, ranging from a factor of 2 to 13. It is proposed that the rate enhancements are a result of an intramolecular catalysis mechanism in which the NH proton of the Pro-Xaa peptide bond hydrogen bonds to the proline nitrogen in the transition state. The peptides Ac-Cys-Pro-Xaa-Cys-NH2 and Ac-Cys-Gly-Pro-Cys-NH2 are model compounds for proline-containing active sites of the thioredoxin superfamily of oxidoreductase enzymes; the results suggest that the backbones of the active sites of the oxidized form of these enzymes may have unusual conformational flexibility.     

Properties of astaxanthin/Ca2+ complex formation in the deceleration of cis/trans isomerization

Deceleration of the regioselective cis/trans isomerization of all-trans-astaxanthin (ASTX) in the presence of Ca2+ was shown by HPLC analysis. The NMR and ITC analyses provided evidence for complexation of ASTX with Ca2+ in 1:2 stoichiometry via chelation at both carbonyl and hydroxyl groups. The rotation across torsion omega6 (C5-C6-C7-C8) upon complexation is consistent with the NOE between 7-H and 5-CH3. This study supports the inhibitory effect of ASTX on calcium-induced turbidity of lens crystallins.     

Thermal- and UV-induced cis/trans isomerization of undoped polyacetylene films. An ESR and IR investigation

The UV- and thermal-induced cis→trans isomerization of undoped polyacetylene (PA) films has been investigated. The results have shown that temperature and UV light promote the isomerization of PA with a similar mechanism. We suggest that the formation of paramagnetic defects in trans PA takes place by bond rehybridization, in agreement with previous hypotheses, and that both spin concentration and spin delocalization depend on the temperature of isomerization. It was found that under UV irradiation thermal cis→trans isomerization of polyacetylene also takes place at temperatures at which the sole thermal treatment isomerization is much lower or nil, and we suggest that the possibility of preparing trans PA under relatively mild conditions may lead to a better material.     

Theoretical research of isomerization mechanisms from cis to trans form in polyacetylene

The characteristics of photo, thermal and doping isomerizations of polyacetylene were discussed. A new biradical mechanism for photo and thermal isomerizations is proposed, which is consistent with the ESR results. The CNDO/2 and INDO molecular orbital calculation results show that the mechanism is energetically the most favorable. The reliability of applying general semi-empirical MO methods in polymers is estimated and showed to be valuable in predicting properties.     

In-solution and on-plate light-catalyzed E/Z isomerization of cyclic chalcone analogues. Lipophilicity of E- and Z-2-(X-benzylidene)-1-benzosuberones

Some cyclic chalcone analogues [E-2-(X-benzylidene)-1-indanones, -tetralones, and -benzosuberones], on-plate UV light-catalyzed formation of new chromatographic spots, can be observed during thin-layer chromatography (TLC). Gas chromatographic (GC) analysis of selected derivatives indicates the formation of one new substance in each case. GC coupled with mass spectrometry and 1H NMR analysis of the samples reveals that the compounds formed are the respective Z-2-(X-benzylidene)-1-indanones, -tetralones, and -benzosuberones. Two-dimensional TLC shows that the E/Z isomerization is a reversible process. By means of the RP-TLC, the logarithm of n-octanol-water partition coefficient (log P) values of E- and Z-isomeric pairs of selected 2-(X-benzylidene)-1-benzosuberones is determined. The Z-isomers are less lipophilic than the E-isomers.     

Unconventional thermodynamically stable cis isomer and trans to cis thermal isomerization in reversibly photoresponsive [0.0](3,3')-azobenzenophane

Sterically hindered [0.0](3,3')-azobenzenophane exhibits thermal trans-to-cis isomerization to the thermodynamically stable cis-cis isomer, and reversible photochemical isomerization with good fatigue resistance.     

Regioselective E(trans)-Z(cis) photoisomerization in naphthyldiene derivatives

Naphthyldiene derivatives,1-4, carrying electron-donating groups at one end and electron-withdrawing groups at the other, were synthesized to study the photoisomerization process. All the compounds showed efficient photoisomerization upon direct excitation leading to the formation of 4-Z isomer with high selectivity. Triplet sensitization studies indicated inefficientE-Z isomerization process. Room temperature fluorescence of1 and2 displayed fine structure in hexane solvent and the same was replaced by broad or structureless fluorescence in acetonitrile and methanol solvents. A mechanism involving a polarized or charge transfer singlet excited state is proposed for the observed photoisomerization in these naphthyldiene derivatives.     

Peptidyl-Prolyl cis/trans Isomerases and Their Effectors

Changes in the conformation of the units within the peptide chain are the elementary processes in the folding of a protein into its native three-dimensional structure. Even a few years ago protein folding in vivo was considered to be an autonomous process not requiring the help of enzymes or auxiliary substances. Recently an increasing number of proteins that assist in the folding process have been found; these include enzymes that catalyze conformational interconversions. The cis/trans isomerization of the petptide bond N-terminal to a praline residue is catalyzed by peptidyl-prolyl cis/trans isomerases (PPIases). Two families of these ubiquitous and phylogenetically highly conserved enzymes are known, the cyclophilins and FK506-binding proteins. Their catalytic activity is extremely highmdash;the rate constants for the bimolecular reactions they catalyze approach the diffusion-controlled limit for enzyme-substrate reactions. These enzymes increase the rate of isomerization in oligopeptides as well as in intermediate in protein folding. It is not yet known which structural units in the cell serve as substrates and exactly which reactions are catalyzed. However, these isomerases have been shown to interact with the heat shock proteins of the nonactivated steroid receptors and with the gag polyprotein of the AIDS HIV-1 virus. The immunosuppressive agents cyclosporine A and FK506 are highly effective inhibitors for PPIases. Surprisingly these compounds affect the signal cascade of T cells but not through enzyme inhibition; the inhibitor-enzyme complexes themselves are the active agents. These complexes exhibit properties not displayed by the individual components and thus are able to affect other cellular components. The current model of the suppression of the antigen- and mitogen-stimulated clonal expansion of T cells is presented here.     

Structural evolution of the chromophore in the primary stages of trans/cis isomerization in photoactive yellow protein

We have studied the structural changes induced by optical excitation of the chromophore in wild-type photoactive yellow protein (PYP) in liquid solution with a combined approach of polarization-sensitive ultrafast infrared spectroscopy and density functional theory calculations. We identify the nuC8-C9 marker modes for solution phase PYP in the P and I0 states, from which we derive that the first intermediate state I0 that appears with a 3 ps time constant can be characterized to have a cis geometry. This is the first unequivocal demonstration that the formation of I0 correlates with the conversion from the trans to the cis state. For the P and I0 states we compare the experimentally measured vibrational band patterns and anisotropies with calculations and find that for both trans and cis configurations the planarity of the chromophore has a strong influence. The C7=C8-(C9=O)-S moiety of the chromophore in the dark P state has a trans geometry with the C=O group slightly tilted out-of-plane, in accordance with the earlier reported structure obtained in an X-ray diffraction study of PYP crystals. In the case of I0, experiment and theory are only in agreement when the C7=C8-(C9=O)-S moiety has a planar configuration. We find that the carboxylic side group of Glu46 that is hydrogen-bonded to the chromophore phenolate oxygen does not alter its orientation on going from the electronic ground P state, via the electronic excited P state to the intermediate I0 state, providing conclusive experimental evidence that the primary stages of PYP photoisomerization involve flipping of the enone thioester linkage without significant relocation of the phenolate moiety.     

Direct measurement indicates a slow cis/trans isomerization at the secondary amide peptide bond of glycylglycine.

Spectral differences between the cis and the trans isomer of a secondary amide peptide bond were used to follow the time course of the cis/trans isomerization of Gly-Gly, Gly-Ala, Ala-Gly, and Ala-Ala dipeptides in the UV/vis region at 220 nm. Isomerization rates and Eyring activation energies were calculated from pH- and LiCl-mediated solvent jump experiments. Rate constants were found to be in a narrow range of 0.29 to 0.64 s(-)(1) for the zwitterionic dipeptides at 25 degrees C. The isomerization rate is about 2-fold higher for the monoionic forms of Gly-Gly. The zwitterionic Gly-Gly has an activation enthalpy DeltaH() of 71.6 +/- 4.9 kJ mol(-)(1) that is in the range of the rotational barriers of aromatic side chain dipeptides that have been measured by (1)H NMR magnetization transfer experiments. Late stages of protein backbone rearrangements often involve crossing the energy barrier for rotational isomerization of imidic peptide bonds. Our findings are consistent with the idea that a wide range of secondary amide peptide bonds are also able to induce slow rate-limiting steps in protein restructuring.     

cis/trans photoisomerization of secondary thiopeptide bonds

The reversible cis/trans photoisomerization of secondary thiopeptide bonds has been systematically studied with UV-visible absorption, capillary electrophoresis, 1H NMR spectroscopy, and circular dichroism methods. It was found that the concentration of the cis conformers could be increased from less than 1 % in the thermal equilibrated solution to up to 20 % in the photostationary state. The rotational barriers of the thiopeptide bond and the pH dependence of the isomerization rates were also studied. The quantum yields of the trans-->cis and the cis-->trans processes were determined from photokinetic analysis.     

A Diastereoselective Synthesis of trans/cis Octahydronaphthoquinolizine

The tetracyclic system of 2,3,6,7,7a,8,12b,12c-octahydro-lH,5H-naphtho[1,2,3,i,j]quinolizine with trans/cis fusions of the B/D and B/C rings has been diastereospecifically synthesized in nine steps and in 23% total yield.     

Mechanism of the photochemical cis⇌trans isomerization of thioindigo and 6,6′-diethoxy-thioindigo in solution

The light induced isomerization of thioindigo (I) and 6,6′-diethoxy-thioindigo(II) has been investigated by means of nanosecond flash photolysis and steady state methods. A mechanism is proposed in which a common triplet state generated from both the trans and cis isomers is responsible for the isomerization. Quantum yields at different oxygen concentrations     

E—1、Z—2（1—咪唑基）—O—（α—甲基对卤苄基）2′，4′—二氯苯乙酮肟硝酸盐的合成与结构表征

首先以2,4－二氯氯代苯乙酮、咪唑、羟胺和对卤苯乙酮为原料,分别合成E－、Z－2－1（1－咪唑基）－2′,4－′二氯苯乙酮肟（IE、IZ）和对卤α－氯乙苯（ⅡA和ⅡB）,ⅠE和ⅠZ分别与ⅡA和ⅡB反应,生成E－、Z－2－1（1－咪唑基）－O－（α－甲基对卤苄基）－2′,4′－二氯苯乙酮肟硝酸盐四个新化合物,产率炒52．0％－60．0％,并经元素分析、IR和^1HNMR进行了表征。