A photochemical mechanism for homochirogenesis

Theoretical analysis of one-step and multiple-step photoreactions initiated with circularly polarized light shows that the enantiomeric excess of a chiral reactant approaches +/- 1 as the amount of unreacted reactant approaches 0. The final product never has a large enantiomeric excess at any stage of its formation and slowly decreases to 0 at the completion of the reaction. For multiple-step reactions the behavior of the intermediate photoproducts is much more interesting. During certain stages of the overall reaction both the size of the enantiomer excess and the amount of a given intermediate photoproduct are large. Furthermore, the sign of the enantiomeric excess of an intermediate may change during the course of the reaction. Multiple-step photoreactions initiated with circularly polarized light may be a method by which the exogenous and endogenous synthesis of optically active molecules occurred in the prebiotic universe.     

Magnetic isotope effect in the photochemical reduction of uranyl nitrate

A magnetic isotope effect was found in the photochemical reduction of uranyl nitrate by p-methoxyphenol. The initial²³⁸U/²³⁵U ratio equal to 8.93±0.008 was altered during the reaction to 8.945±0.008-8.970±0.005 in the reaction product, namely, uranium tetrafluoride.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1902–1903, August, 1990.     

A radical mechanism for the photochemical iron pentacarbonyl catalyzed hydrogenation of octenes

A high pressure IR and UV spectroscopic study of the catalysis of hydrogenation of 1-octene by Fe(CO)₅ under UV irradiation reveals that Fe(CO)₅ and (olefin)Fe(CO)₄ are the light absorbing species. The primary photolysis product Fe(CO)₄ reacts with H₂Fe(CO)₄ to give HFe(CO)₄ radicals, which are suggested to be the active hydrogenation promoters.     

Probing the photochemical mechanism in photoactive yellow protein

Selectively bridged model compounds related to the chromophore in photoactive yellow protein have been synthesized where the single bond adjacent to the benzene ring (bond 1) and where both bond 1 and the adjacent double bond (bond 2) are bridged. They were compared to the nonbridged reference compound regarding their photophysical properties using steady-state and time-resolved fluorescence at various temperatures. Quantum chemical calculations were additionally performed and showed that several conformers are populated in the ground state. The neutral model compounds show that the nonradiative deactivation channel is linked to both single- and double-bond twisting. The relative importance of single-bond twisting is increased for the corresponding deprotonated hydroxy compounds with an enhanced donor character. The simultaneous photochemical activity of both single and double bonds explains the ease of photochemical isomerization in the confined environment of the natural PYP protein and also of the primary step in the vision process in rhodopsin.     

Molecular mechanism of the photochemical generation of gold nanoparticles in ethylene glycol: support for the disproportionation mechanism

It is found that replacement of the chloride ions in tetrachloroauric acid with bulky bromide ions inhibits the formation of gold nanoparticles in the photochemical reduction in ethylene glycol. However, the addition of silver ions to either the bromide or the chloride auric acid solution is found to enhance the rate of gold nanoparticle formation. These results are found to be accounted for by the previously proposed mechanism (Eustis, S.; Hsu, H.-Y.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 4811) which involves disproportionation of the chloroauric complexes to generate free gold atoms and chloride ions. The steric effects of the bulky bromide ions inhibit the formation of the Au-Au bond needed in the electron transfer process of the disproportionation reaction. The addition of Ag+ ions results in the formation of insoluble silver halide, which shifts the disproportionation reaction toward the formation of gold atoms and thus the formation of gold nanoparticles.     

The determination and role of peroxyacetil nitrate in photochemical processes in atmosphere

Peroxyacetilnitrates (PAN) is the most characteristic photoxidant of a range of secondary pollutants formed by the photochemical reaction of hydrocarbons with nitrogen oxides in the atmosphere: it is phytotoxic and shows an increasing role in human health effects due to ambient air exposure, especially in presence of high ozone concentrations. Because of the similarity of the conditions required for their photochemical production PAN is observed in conjunction with elevated ozone concentrations. PAN has very low natural background concentrations so it is the very specific indicator of anthropogenic photochemical air pollution. In this paper we report PAN concentrations determined in Rome urban area during winter- and summer-period. PAN measurements were carried out by means of a gas-chromatograph equipped with an Electron Capture Detector (ECD) detector. For identifying the acute episodes of atmospheric photochemical pollutants the relationship between PAN and the variable Ox (=NO2+O3) which describes the oxidation process evolution is investigated. The role of Volatile Organic Compounds and PAN in the ozone formation is investigated as well the issue of taking in account the autovehicular emissions for checking the NOx fraction in fuel.     

Polymorphous transformation of a clathrate in the pyridinecadmium nitrate system

The polymorphous transformation of the [CdPy ₄(NO₃)2]·2Py clathrate (Py is pyridine) at 221 K is investigated by analyzing the temperature dependences of the Raman spectra and the lattice parameters in the range 100–293 K The crystal structures of the high- and low-temperature modifications of the clathrate were determined. Above the transition point the clathrate crystallizes in space group Ccca (at 293 K: a = 12.360(4), b = 15.456(3), c = 16.931(3) å, V = 3234(1) å³, Z = 4, R = 0.050 for 879 reflections and 126 refined parameters). Below 221K the unit cell of the clathrate is transformed into the primitive cell, space group Pcca (at 215 K: a = 11.953(3), b = 15.129(3), c = 17.234(3) A, V = 3234(1) å³, Z =4, R = 0.057 for 1596 reflections and 210 refined parameters). In both modifications, the host complex involves four pyridine ligands and two monodentate trans-nitrato groups forming a distorted octahedron around the central atom. The guest pyridine molecules lie in the cavities of the molecular framework of the complex. As the clathrate is cooled, the a and b parameters decrease and the c parameter increases; at transition point, each of these parameters abruptly changes by about 0.2 A. The nitrate ligand undergoes ordering and inclination to the equatorial plane of the complex by 20?.     

Chemiluminescence determination of nitrate with photochemical activation in a flow injection system

A chemiluminescence flow injection system is described for the determination of nitrate, involving use of a laboratory-built flow-through photochemical reactor. Optimum analytical conditions were established. The linear range for nitrate is 7 x 10(-8)-1 x 10(-4)M. The sampling frequency is 60 samples per hour. The relative standard deviation for 1 x 10(-7), 1 x 10(-6) and 1 x 10(-5)M nitrate is 0.97, 0.84 and 0.76%, respectively. The method has been applied to the determination of nitrate in natural water samples, and recoveries of 96-103% have been attained.     

Toward understanding the mechanism of chromophore-assisted laser inactivation--evidence for the primary photochemical steps

Chromophore-assisted laser inactivation (CALI) is a light-mediated technique used to selectively inactivate proteins of interest to elucidate their biological function. CALI has potential applications to a wide array of biological questions, and its efficiency allows for high-throughput application. A solid understanding of its underlying photochemical mechanism is still missing. In this study, we address the CALI mechanism using a simplified model system consisting of the enzyme beta-galactosidase as target protein and the common dye fluorescein. We demonstrate that protein photoinactivation is independent from dye photobleaching and provide evidence that the first singlet state of the chromophore is the relevant transient state for the initiation of CALI. Furthermore, the inactivation process was shown to be dependent on oxygen and likely to be based on photooxidation of the target protein via singlet oxygen. The simple model system used in this study may be further applied to identify and optimize other CALI chromophores.     

The role of nitrate free radicals in the photochemical side-chain nitrooxylation of alkylbenzenes by cerium(IV) ammonium nitrate in acetonitrile

It is suggested that the reactive species in the CAN-induced photochemical side-chain nitrooxylation of alkylbenzenes is the nitrate radical, which probably acts as one-electron oxidant.     

H-vinyl conical intersections for dienes: a mechanism for the photochemical hula twists

The potential role of "H-vinyl" conical intersections in a photochemical process known as the hula twist has been evaluated. The H-vinyl conical intersections of butadiene are explored with complete active space self-consistent field calculations, and proposals about the geometries involved in the hula-twist mechanism are discussed and contrasted with the conventional one-bond flip mechanism of cis-trans isomerization.     

Cerium (IV) ammonium nitrate catalyzed photochemical autoxidation of alkylbenzenes

The autoxidation of alkylbenzenes can be promoted photochemically in the presence of catalytic amounts of cerim (IV) ammonium nitrate (CAN) under very mild conditions, the efficiency of the process being significantly increased by added acids. It is suggested that the reaction is promoted by NO₃, radicals formed in the light induced decomposition of CAN and that Ce(III) may be reoxidized to Ce(IV) by benzylperoxy radicals.     

A theoretical insight into the reaction mechanism of photochemical transposition from pyrazole to imidazole

The mechanisms of the photochemical isomerization reactions were investigated by using a model system of 1,3,5-trimethylpyrazole ( 1) with the CASSCF (eight-electron/six-orbital active space) and MP2-CAS methods and the 6-311G(d) basis set. Three reaction pathways were examined in the present work. They are denoted as the ring-contraction-ring-expansion path (path I), the internal-cyclization-isomerization path (path II), and the conical-intersection path (path III). Our model investigations suggest that the preferred reaction route for the pyrazoles is as follows: reactant --> Franck-Condon region --> conical intersection --> photoproduct. In particular, the conical-intersection mechanism (path III) found in this work gives a better explanation than the previously proposed two other mechanisms (paths I and II). The theoretical findings also indicate that path III-1 should be favored over path III-2 from a kinetic point of view. This suggests that the quantum yield of 1,2,4-trimethylimidazole ( 2) should be greater than that of 1,2,5-trimethylimidazole ( 3), which supports the available experimental observations. Additionally, we propose a simple p-pi orbital topology model, which can be used as a diagnostic tool to predict the location of the conical intersections, as well as the geometries of the phototransposition products of various heterocycles.     

Kinetics and mechanism of the photochemical hydrolysis of phosphoenolpyruvic acid

It was established that phosphoenolpyruvic acid, photoexcited by UV light at 20°C, loses its phosphoryl group under the action of acids and bases. Raising the temperature to 50°C induces dissociation of the photoexcited molecule into enolpyruvate and metaphosphate, which in turn phosphorylates HPO₄ ^2– and H₂P₂O₇ ^2– ions and AMP.Institute of Bioorganic Chemistry and Petrochemistry, National Academy of Sciences of Ukraine, 1 Murmanskaya St., 253094 Kiev-94, Ukraine. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 32, No. 5, pp. 287–290, September–October, 1996. Original article submitted December 1, 1995.     

Parahalogenated phenols accelerate the photochemical release of nitrogen oxides from frozen solutions containing nitrate

The photolysis of nitrate anion (NO(3)(-)) contained in surface ice and snow can be a regionally significant source of gas-phase nitrogen oxides and affect the composition of the planetary boundary layer. In this study, the photochemical release of nitrogen oxides from frozen solutions containing NO(3)(-) in the presence of organic compounds was investigated. Gas-phase nitrogen oxides were quantified primarily by NO-O(3) chemiluminescence detection of NO and NO(y) (=NO + NO(2) + HONO + HNO(3) + ∑PAN + ∑AN ...) and cavity ring-down spectroscopy of NO(2) and total alkyl nitrates (∑AN). The photochemical production of gas-phase NO(y) was suppressed by the presence of formate, methanesulfonate, toluene, or phenol. In contrast, para-halogenated phenols (in the order of Cl > Br > F) promoted the conversion of NO(3)(-) to gas-phase NO(y), rationalized by acidification of the ice surface.     

Thermal and photochemical transformation of conformational chirality into configurational chirality in the crystalline state

Because they crystallize in chiral conformations in which abstraction of only one of two diastereotopic gamma-hydrogen atoms is possible, salts formed between achiral keto-acids possessing the tricyclo[4.4.1.0]undecane ring system and optically pure amines undergo Norrish type II cleavage in the solid state in enantiomeric excesses as high as 95% at 98% conversion, following removal of the ionic chiral auxiliaries. Thermal enolene rearrangement of the same salts results in optical yields approximately half those observed for the photochemical reaction.     

Solid state and solution nitrate photochemistry: photochemical evolution of the solid state lattice

We examined the deep UV 229 nm photochemistry of NaNO(3) in solution and in the solid state. In aqueous solution excitation within the deep UV NO(3)ˉ strong π → π* transition causes the photochemical reaction NO(3)ˉ → NO(2)ˉ + O·. We used UV resonance Raman spectroscopy to examine the photon dose dependence of the NO(2)ˉ band intensities and measure a photochemical quantum yield of 0.04 at pH 6.5. We also examined the response of solid NaNO(3) samples to 229 nm excitation and also observe formation of NO(2)ˉ. The quantum yield is much smaller at ～10(-8). The solid state NaNO(3) photochemistry phenomena appear complex by showing a significant dependence on the UV excitation flux and dose. At low flux/dose conditions NO(2)ˉ resonance Raman bands appear, accompanied by perturbed NO(3)ˉ bands, indicating stress in the NaNO(3) lattice. Higher flux/dose conditions show less lattice perturbation but SEM shows surface eruptions that alleviate the stress induced by the photochemistry. Higher flux/dose measurements cause cratering and destruction of the NaNO(3) surface as the surface layers are converted to NO(2)ˉ. Modest laser excitation UV beams excavate surface layers in the solid NaNO(3) samples. At the lowest incident fluxes a pressure buildup competes with effusion to reach a steady state giving rise to perturbed NO(3)ˉ bands. Increased fluxes result in pressures that cause the sample to erupt, relieving the pressure.