PHOTOCHEMISTRY OF ALL-TRANS- AND 13-CIS-14,15-DIDEUTERIORETINAL

Abstract— All- trans - and 13- cis -14,15-dideuterioretinal were synthesized and their solution photochemistry examined. Quantum yields of trans → cis or cis → trans photoisomerization and the number and ratio of primary photoproducts, determined by high pressure liquid chromatographic analysis, are essentially identical to that of their corresponding retinal isomer. The C-14, C-15 carbon-hydrogen modes play no particularly important role in the deactivation of electronically excited linear polyenes related to retinal.     

TIME-RESOLVED RESONANCE RAMAN SPECTRA OF THE PHOTOCYCLE INTERMEDIATES OF ACID AND DEIONIZED BACTERIORHODOPSIN

Abstract— The photocycle of bacteriorhodopsin (bR) and its perturbed forms are investigated by a time-resolved resonance Raman study. These experiments were performed in the C=C stretching and in the fingerprint spectral regions for the acid blue, acid purple and deionized forms of bR.
The main observations are as follows: (1) isomerization of the retinal, from all- trans to 13- cis , occurs in native bR and in all of the acid and deionized perturbed bR species; (2) formation of the early intermediates (the K₆₁₀ and L₅₅₀ analogues) also occur in native bR and in all of the perturbed species; and (3) deprotonation of the protonated Schiff base (PSB), to give the M₄₁₂ type intermediate, occurs in native bR, but is inhibited in all of the perturbed bR species on the time-scale of the native bR photocycle.
The results show that isomerization alone is not a prerequisite for the PSB deprotonation process. The observed photocycle, initiated with retinal isomerization, is found to occur from all- trans to 13- cis in all of the perturbed forms of bR. In addition, the results imply that removal of the cations, of an increase in the hydrogen ion concentration, prevent only the PSB deprotonation process and not the formation of earlier cycle intermediates. Some attention is focused on the two blue forms of bR (acid and deionized) due to the fact that their ground-state absorption maximum, unphotolyzed Raman spectra, and Raman spectra changes during the photocycle are all very similar. The similarities between the acid blue and deionized blue forms in the fingerprint region support previous suggestions that both blue species have nearly the same retinal active site.     

PHOTOCHEMICAL REACTION OF 13-CIS-BACTERIORHODOPSIN STUDIED BY LOW TEMPERATURE SPECTROPHOTOMETRY

Abstract— The photoreaction cycle of 13- cis -bacteriorhodopsin (13- cis -bR) was investigated by low temperature spectrophotometry using two different preparations; 13- cis -bR constituted from bacterioopsin and 13- cis -retinal, and dark-adapted bacteriorhodopsin (bR^D), which is an equi-molar mixture of 13- cis -bR and trans -bR.
By irradiation with 500 nm light at — 190°C, 13- cis -bR was converted to its batho-product, batho-13- cis -bR (batho-bR¹³), which is different from batho-product from trans -bR, batho-bR^t. On warming batho-bR¹³ to -5°C in the dark, it completely changed to trans -bR. We estimated the composition of 13- cis -bR and trans -bR in the warmed sample spectrophotometrically and then the absorption spectrum of batho-bR¹³ was calculated. The absorption maximum lies at 608 nm, 1250 cm⁻¹ longer than that of 13- cis -bR; the molar extinction coefficient (ε) is about 74000 M ⁻¹ cm⁻¹, larger than that of 13- cis -bR (52000 M ⁻¹ cm⁻¹).
On the warming the sample containing batho-bR¹³ formed by irradiating 13- cis -bR or bR^D at — 190°C, we could not detect other intermediates such as the lumi- or meta-intermediates seen in trans-bR system.     

ALL-trans RETINAL PHOTOISOMERIZATION AND PHOTOOXIDATION FROM UV LASER RADIATION. VIBRATIONAL ASSIGNMENTS OF ALL-trans 5,8-PEROXYRETINAL

Abstract— Photoisomerization and photooxygenation of all- trans retinal in acetonitrile, illuminated by laser radiations (λ_act, = 333.6 or 350.7 nm), were investigated under various experimental conditions. In deoxygenated solutions, the major photoproducts are 13- cis and 9- cis retinal. All- trans and 13- cis 5,8-peroxyretinal are obtained in large amounts in oxygenated solutions. 11-cis derivatives have not been detected in any of these solutions. The photoproducts were identified by UV, NMR, mass and vibrational spectroscopies and HPLC chromatography. All- trans and 13- cis 5,8-peroxyretinal were isolated. Their vibrational spectra (IR and Raman) are analyzed. Most of the bands are assigned by comparison with previous studies on all- trans and 13- cis retinal, ( E,E,E )-3-methyl-2,4,6-octatrienal and peroxides.     

Darzens reaction of acyl phosphonates with alpha-bromo ketones: selective synthesis of cis- and trans-epoxyphosphonates

Acyl phosphonates with alpha-halo ketones in the presence of bases at room temperature afford cis- and trans-epoxyphosphonates in good chemical yields and high selectivities using different bases. The diastereoselectivity of this reaction is easily controlled by changing the base. Changing the base from Cs2CO3 to DBU changed the diastereomeric ratio (trans/cis) from 3/2 to 9/1. Moreover, the treatment of the trans isomer with DBU showed a complete conversion to the corresponding cis isomer.     

Donor atom dependent geometric isomers in mononuclear oxo-molybdenum(V) complexes: implications for coordinated endogenous ligation in molybdoenzymes

We have previously demonstrated that the complex [(L1O)MoOCl(2)], where L1OH = (2-hydroxy-3-tert-butyl-5-methylphenyl)bis(3,5-dimethylpyrazolyl)methane, exists as both cis and trans isomers (Kail, B.; Nemykin, V. N.; Davie, S. R.; Carrano, C. J.; Hammes, B. S.; Basu, P. Inorg. Chem. 2002, 41, 1281-1291). Here, the cis isomer is defined as the geometry with the heteroatom in the equatorial position, and the trans isomer is designated as the geometry with the heteroatom positioned trans to the terminal oxo group. The trans isomer represents the thermodynamically more stable geometry as indicated by its spontaneous formation from the cis isomer. In this report, we show that for complexes of [(LO)MoOCl(2)], where LOH is the sterically less restrictive (2-hydroxyphenyl)bis(3,5-dimethylpyrazolyl)methane, only the trans isomer could be isolated, while in the corresponding thiolate containing ligand (2-dimethylethanethiol)bis(3,5-dimethylpyrazolyl)methane (L3SH) only the cis isomer could be observed. In addition, we have isolated and structurally characterized the complex [(L1O)MoO(OPh)(Cl)], a rare example of a species possessing both cis and trans phenolates. Using DFT calculations, we have investigated the origins of the differences in stability between the cis and trans isomers in these complexes and suggest that they are related to the trans influence of the oxo-group. Crystal data for [(LO)MoOCl(2)] (1) include that it crystallizes in the triclinic space group P(-)1 with cell dimensions a = 8.9607 (12) A, b = 10.596 (4) A, c = 13.2998 (13) A, alpha = 98.03 (2) degrees, beta = 103.21 (2) degrees, gamma = 110.05(2) degrees, and Z = 2. [(L1O)MoO(OPh)Cl].2CH(2)Cl(2) (2.2CH(2)Cl(2)) crystallizes in the triclinic space group P(-)1 with cell dimensions a = 12.2740 (5) A, b = 13.0403 (5) A, c = 13.6141 (6) A, alpha = 65.799 (2) degrees, beta = 64.487 (2) degrees, gamma = 65.750 (2) degrees, and Z = 2. [(L3S)Mo(O)Cl(2)] (3) crystallizes in the orthorhombic space group Pna2(1), with cell dimensions a = 13.2213 (13) A, b = 8.817 (2) A, c = 15.649 (4) A, and Z = 4. The implications of these results on the function of mononuclear molybdoenzymes such as sulfite oxidase, and the DMSO reductase, are discussed.     

Molecular structure of the trans and cis isomers of metal-free phthalocyanine studied by gas-phase electron diffraction and high-level quantum chemical calculations: NH tautomerization and calculated vibrational frequencies

The molecular structure of the trans isomer of metal-free phthalocyanine (H2Pc) is determined using the gas electron diffraction (GED) method and high-level quantum chemical calculations. B3LYP calculations employing the basis sets 6-31G**, 6-311++G**, and cc-pVTZ give two tautomeric isomers for the inner H atoms, a trans isomer having D2h symmetry and a cis isomer having C2v symmetry. The trans isomer is calculated to be 41.6 (B3LYP/6-311++G**, zero-point corrected) and 37.3 kJ/mol (B3LYP/cc-pVTZ, not zero-point corrected) more stable than the cis isomer. However, Hartree-Fock (HF) calculations using different basis sets predict that cis is preferred and that trans does not exist as a stable form of the molecule. The equilibrium composition in the gas phase at 471 degrees C (the temperature of the GED experiment) calculated at the B3LYP/6-311++G** level is 99.8% trans and 0.2% cis. This is in very good agreement with the GED data, which indicate that the mole fraction of the cis isomer is close to zero. The transition states for two mechanisms of the NH tautomerization have been characterized. A concerted mechanism where the two H atoms move simultaneously yields a transition state of D2h symmetry and an energy barrier of 95.8 kJ/mol. A two-step mechanism where a trans isomer is converted to a cis isomer, which is converted into another trans isomer, proceeds via two transition states of C(s) symmetry and an energy barrier of 64.2 kJ/mol according to the B3LYP/6-311++G** calculation. The molecular geometry determined from GED is in very good agreement with the geometry obtained from the quantum chemical calculations. Vibrational frequencies, IR, and Raman intensities have been calculated using B3LYP/6-311++G**. These calculations indicate that the molecule is rather flexible with six vibrational frequencies in the range of 20-84 cm(-1) for the trans isomer. The cis isomer might be detected by infrared matrix spectroscopy since the N-H stretching frequencies are very different for the two isomers.     

PHOTOREGULATION OF α-CHYMOTRYPSIN ACTIVITY BY ISOMERIZABLE INHIBITORS. STUDY OF THE ACTION MECHANISM*

Abstract— The N- p -phenylazophenyl-N-phenylcarbamyl chloride (PAPC) in its cis form is five times more active as inhibitor of α-chymotrypsin than in the trans form. In the present work, derivatives of PAPC have been synthesized. Each of these new compounds is photoisomerizable and is an inhibitor (in the cis and in the trans form) of α-chymotrypsin. The cis isomer is always more active than the trans. The m -methyl derivative is 17.5 times more active in the cis form than in the trans , whereas, for the para -substituted compound, this ratio is only 3.5.
Several hypotheses can explain this difference of activity between the cis and trans isomers: (1) steric hindrance towards the trans isomer, (2) lower affinity of the enzyme for the trans isomer, (3) higher reactivity of the complex formed between the enzyme and the cis form of the inhibitor. These hypotheses are discussed.     

Practical synthesis of a neuropeptide Y antagonist via stereoselective addition to a ketene

[Reaction: see text]. The synthesis of neuropeptide Y antagonist 1, currently under clinical investigation for the treatment of obesity, is described. The convergent synthesis from trans-spirolactone carboxylic acid intermediate 2a and aminopyrazole 3 is predicated on a stereoselective route to the former. The coupling reaction of ethyl 4-oxocyclohexanecarboxylate (10a) with lithiated isonicotinamide 11 was investigated in detail, but even optimized conditions only provided a 45:55 ratio of trans:cis isomers (12a:12b). While selective crystallization schemes were developed to isolate the thermodynamically less stable trans isomer 2a, improved stereocontrol was subsequentially achieved by the application of ketene chemistry. The ketene formation and quench was investigated under a variety of conditions aimed at maximizing the trans:cis ratio. Reacting a mixture of carboxylic acids 2a and 2b with POCl3 in THF, followed by concomitant addition of tert-butyl alcohol in the presence of TMEDA at 35 degrees C provided a 4:1 ratio of trans:cis tert-butyl esters (18a:18b) via in situ ketene formation. Ester hydrolysis, followed by selective crystallization of undesired 2b as the HCl salt, led to isolation of 2a in 47% overall yield. Aminopyrazole intermediate 3 was synthesized via the condensation reaction of 2-fluorophenylhydrazine hydrochloride (4a) with acrylonitrile derivative 5 in 65-70% yield. Coupling of advanced intermediates 2a and 3b via activation with thionyl chloride gave a 92% yield of 1.     

SPECTROSCOPIC BEHAVIOUR OF RETINYL POLYENES IN SOLID FILMS

Abstract— The absorption spectroscopic study of retinyl polyenes, i.e. all- trans retinal, 9- cis retinal, 13- cis retinal, all- trans retinol, 13- cis retinol and all- trans retinyl acetate in solid films is discussed. The spectra of the films with low surface coverages obtained from dilute solutions are red shifted relative to their solution spectra. This shift is interpreted as due to the weak interaction between the surface and polyene molecules. With increased surface coverages, i.e. the films obtained from concentrated solutions, the retinals show a large red shift whereas a blue shift is observed with the retinols. Chromophore–chromophore interactions in addition to the surface effects are believed to be responsible for such observations. Effect of surface causing permanent changes in molecule is also discussed.     

Temperature dependence of aggregation and dynamic surface tension in a photoresponsive surfactant system

The response of a nonionic photoresponsive surfactant system to changes in temperature is reported. This surfactant contains the light-sensitive azobenzene group, and when exposed to light, a solution of this surfactant contains a mixture of the cis and trans photoisomers of this group. The temperature of the surfactant solution has a strong impact on the time needed for the surfactant to diffuse and adsorb to a freshly formed interface. At surfactant concentrations that give rise to trans aggregates but not to cis aggregates, the transport of cis and of trans isomers to the surface of a pendant bubble have quite different temperature dependencies, owing largely to the difference in their aggregation states in bulk solution. Diffusion and adsorption of the cis isomer are described reasonably well by a simple diffusion model that accounts for the effect of temperature on the diffusion coefficient. The trans isomer, which was primarily bound in aggregates during these measurements, exhibits a stronger dependence of this adsorption time scale on the temperature of the solution. This temperature dependence of trans diffusion and adsorption is quantitatively consistent between samples containing only the trans isomer and samples containing a mixture of isomers. Fluorescence studies were done to determine the effect of temperature on the cmc of the surfactant. The critical concentration associated with the formation of cis-dominant aggregates increases modestly with increasing temperature. The cmc of the trans isomer also increases with increasing temperature, most significantly when the temperature exceeds about 35 degrees C. These trans cmc temperature-dependence data were incorporated into diffusion models that account for the potential roles of aggregates in the adsorption process. The observed temperature dependency of the trans adsorption time scale is consistent with a model that includes the effect of temperature on both the diffusivity and the supply of monomer via its effect on the cmc. Specifically, the results suggest that the dissolution of trans-dominant aggregates is important to the trans adsorption process. Further fluorescence studies were performed in which surfactant solutions containing aggregates were diluted rapidly, and the rate of dissolution of these aggregates was inferred from fluorescence decay. Aggregate breakup in colder trans samples is slower than in warmer samples, but these dissolution time scales are significantly shorter than those associated with the adsorption process. This is consistent with the assumption that aggregation kinetics do not contribute to the observed adsorption kinetics.     

Theoretical study of the mechanism of NO2 production from NO + ClO

The reaction of NO with ClO has been studied theoretically using density-functional and wave function methods (B3LYP and CCSD(T)). Although a barrier for cis and trans additions could be located at the RCCSD(T) and UCCSD(T) levels, no barrier exists at the B3LYP/6-311+G(d) level. Variational transition state theory on a CASPT2(12,12)/ANO-L//B3LYP/6-311+G(d) surface was used to calculate the rate constants for addition. The rate constant for cis addition was faster than that for trans addition (cis:trans 1:0.76 at 298 K). The rate constant data summed for cis and trans addition in the range 200-1000 K were fit to a temperature-dependent rate in the form kdi) = 3.30 x 10(-13)T(0.558) exp(305/T) cm3.molecule(-1).s(-1), which is in good agreement with experiment. When the data are fit to an Arrhenius plot in the range 200-400 K, an activation barrier of -0.35 kcal/mol is obtained. The formation of ClNO2 from ONOCl has a much higher activation enthalpy from the trans isomer compared to the cis isomer. In fact, the preferred decomposition pathway from trans-ONOCl to NO2 + Cl is predicted to go through the cis-ONOCl intermediate. The trans --> cis isomerization rate constant is kiso = 1.92 x 10(13) exp(-4730/T) s(-1) using transition state theory.     

CIS-TRANS PHOTOISOMERIZATION OF ABSCISIC ACID

Abstract— An important regulator of numerous physiological processes in higher plants is abscisic acid (ABA), which is photoisomerized from the more biologically active cis isomer to the nearly inactive trans isomer by natural sunlight. It is possible that this photoisomerization is a UV control mechanism in functions regulated by ABA.
The quantum yields of both the cis to trans and trans to cis photoisomerizations were measured by an initial velocities method under various conditions of pH and oxygen concentration at room temperature. The yield for photoisomerization of cis-ABA ranged from 0.25 at pH 3.0 to 0.11 at pH 7.0. Oxygen partially quenched the process. The quantum yield varies only slightly with wavelength.
The quantum yield of photolysis of cis-ABA is reported for pH 3.0 as 0.06. This yield also varies slightly with wavelength and is relatively insensitive to oxygen. This relatively high yield explains the loss of potency of ABA during UV irradiation.
Phosphorescence of cis - and trans-ABA is observed in methanol at 77 K. Onset of the emission is at 350 nm. The emission spectra is the same for both isomers.
From these results a mechanism of UV action on plants based on the photoisomerization of the inactive trans-ABA to the biologically active cis isomer is proposed.     

PHOTOREACTION OF THE ACIDIFIED FORM OF BACTERIORHODOPSIN AND ITS 9-CIS DERIVATIVE IN PURPLE MEMBRANE AT LOW TEMPERATURES

Abstract— The photoreaction of the acidified form of bacteriorhodopsin and its 9-cis derivative was studied by low temperature spectroscopy.
A short exposure of the acidified form of bacteriorhodopsin, which was prepared by adding 2 m M HC1 to purple membrane suspension in 67% glycerol at 0°C, to red light at – 72°C resulted in the blue-shift of the spectrum. The feature of the shift was very similar to that accompanied by the formation of stable 9- cis acidified form of bacteriorhodopsin at 0°C, but only 13- cis - and all- trans -retinals were found in the extract from this product. No blue-shifted product was found on irradiation at – 190°C.
Irradiation of the 9- cis form of acidified bacteriorhodopsin at -72°C with blue light caused the isomerization of its 9- cis -retinylidene chromophore to 13- cis and all- trans forms without a significant spectral change. It became greater only after the sample was warmed above – 24°C. These results indicate the presence of the light-induced product which has trans configuration on the 9-10 double bond and exhibits the 9- cis type spectrum.     

Concerted reductive coupling of an alkyl chloride at Pt(IV)

Oxidation of a doubly cyclometallated platinum(II) complex results in two isomeric platinum(IV) complexes. Whereas the trans isomer is robust, being manipulable in air at room temperature, the cis isomer decomposes at -20 °C and above. Reductive coupling of an alkyl chloride at the cis isomer gives a new species which can be reoxidised. The independence of this coupling on additional halide rules out the reverse of an S(N)2 reaction, leaving a concerted process as the only sensible reaction pathway.     

RECONSTITUTION OF SQUID RHODOPSIN IN RHABDOMAL MEMBRANES

Abstract— Squid opsin which is capable of combining with 11- cis or 9- cis retinal to reconstitute photo-pigment has been prepared by irradiation of rhabdomal membranes with orange light (> 530 nm) in the presence of 0.2 M hydroxylamine. When the irradiation is carried out either at concentrations of hydroxylamine higher than 0.2 M or with light of wavelength shorter than 530 nm, rhodopsin in the membranes is bleached quickly, but the ability of the resultant opsin to form rhodopsin is greatly reduced.
The optimum pH for rhodopsin regeneration in rhabdomal membranes was found to be between 6.5 and 8.5. The rate of regeneration of rhodopsin increases with raising temperature, and at about 20°C it is almost the same as that of isorhodopsin. Even after solubilization in digitonin solution, opsin still preserves the ability to reform rhodopsin.
All- trans retinal can be incorporated into retinochrome-bearing membranes, in which it is isomerized into 11- cis isomer by the photoisomerase activity of retinochrome. Rhabdomal membranes retaining active opsin can take up 11- cis retinal from retinochrome membranes so as to synthesize rhodopsin.     

1.1](3,3')-azobenzenophane: novel crystal structure and cis-trans isomerization of distorted azobenzene

[formula: see text] [1.1](3,3')-Azobenzenophane, in which two azobenzenes are cyclically connected by -CH2- chains at the meta positions, has been synthesized. The crystal structures of all isomers have been revealed. This is the first report on the crystal structure of the cis isomer of macrocyclic azobenzenes. The trans,trans isomer was slightly distorted, the trans,cis isomer highly deformed, and the cis,cis isomer unstrained. The thermal stability of cis isomers in solutions are deducible from the crystal structures.     

WAVELENGTH DEPENDENCE OF THE INTERSYSTEM CROSSING EFFICIENCY FOR RETINAL ISOMERS

Abstract— The quantum efficiencies of intersystem crossing (_ISC) fur four isomers of retinal, the all- trans , 9- cis , 11- cis and 13- cis , have been measured using both 265 nm and 353 nm excitation. The values for the all- trans and 9- cis isomers are independent of the excitation wavelength but the values for the 11- cis and 13- cis isomers show a marked increase in the efficiency of ISC for 353 nm excitation compared with the 265 nm excitation.     

Crosslinking investigation of polybutadiene thermal degradation by carbon-13 nuclear magnetic resonance

Thermal degradation of polybutadiene (PBD) in anaerobic atmosphere at 250 °C had been studied by carbon-13 nuclear magnetic resonance spectroscopy (¹³C NMR) before complete crosslinking. In this investigation four types of low molecular weight PBD with different 1,2-vinyl isomer content had been chosen, then pure and mixed samples of PBD were heated in different time periods. ¹³C NMR spectra showed that two kinds of crosslinking mechanisms occur that both of them produce methyl groups. The first mechanism is a reaction between 1,2-vinyl isomers of two PBD chains, and the second one occurs between 1,2-vinyl isomer of one chain via methylene carbon of cis or trans isomer in another chain. Also ¹³C NMR results showed that the presence of 1,2-vinyl isomer in the PBD structure is necessary and without it none of the mentioned reactions will occur. Furthermore isomers sequence is another important parameter which affects crosslinking. Results show that cis or trans isomer which is not adjacent to 1,2-vinyl isomer does not take part in crosslinking reaction. Moreover such cis or trans isomer can take part in second mechanism of crosslinking that 1,2-vinyl isomer was attached from head to cis or trans isomer, thus in this arrangement of isomers second mechanism of crosslinking will become dominant rather than first mechanism of crosslinking.     

THE PHOTOREACTION OF THE DEIONIZED FORM OF THE PURPLE MEMBRANE INVESTIGATED BY FTIR DIFFERENCE SPECTROSCOPY

Abstract— Deionization of the purple membrane of Halobacterium halobium shifts the visible absorption maximum from 570 to 605 nm and inhibits proton transport. FTIR-difference-spectra of this blue membrane at 280 K reveal that the retinal chromophore adopts a 13 -cis and all -trans geometry in a light dependent ratio. In contrast to purple membrane the 13-cu isomer forms much faster in the dark. The all- trans component produces an L-intermediate which can be stabilized at 170 K. Spectral characteristics are similar to normal L. including comparable changes of internal aspartic acids of the opsin. However, stronger changes in the amide-I absorption are observed. IR bands of the chromo-protein states are assigned to retinal normal modes by the use of bacteriorhodopsin regenerated with'C-labeled retinals.