THE EFFECT OF pH ON THE AEROBIC and ANAEROBIC PHOTOLYSIS OF TRYPTOPHAN AND SOME TRYPTOPHAN-CONTAINING DIPEPTIDES

Abstract— Rates of photolysis, quantum yields of fluorescence, and fluorescence emission maxima for the dipeptides glycyltryptophan (Gly-Trp) and tryptophylglycine (Trp-Gly) and for free tryptophan (Trp) were determined under both degassed and aerated conditions in the pH range 4.5-10.0. The photolyses were performed at 25°C using 290 nm radiation from a 1000 W xenon lamp. Photolysis rates were determined by monitoring tryptophan fluorescence loss with time. It was found that Trp-Gly and free Trp showed similar behavior in that their fluorescence quantum yields and photolysis rates increased significantly above neutral pH. In contrast, the Gly-Trp fluorescence yield was smaller than that of Trp or Trp-Gly, showing no significant increase at high pH and the photolysis rate for Gly-Trp decreased with increasing pH. In comparing aerated to degassed samples, it was found that degassing had a far greater effect on the photolysis rates of Trp and Trp-Gly than on the photolysis rate of Gly-Trp especially at higher pH. But, degassing did not change the relative fluorescence quantum yields or fluorescence emission maxima of any of the three compounds. Possible mechanisms for photolysis under various experimental conditions were examined in light of the data.     

Emission spectrum of CH3S radical

The emission spectrum of the CH₃S radical is obtained in the photolysis of CH₃SCH₃ by using a xenon resonance lamp. The emission is also obtained in the photolysis of CH₃SSCH₃ at ca. 200 nm irradiation as well as at 147.0 nm irradiation. The excitation threshold to produce CH₃S fluorescence in the photolysis of CH₃SSCH₃ is determined to be 202 ± 3 nm or 6.14 ± 0.09 eV. The ratios of electronic quenching rate to fluorescence rate of CH₃S^* with N₂, H₂, D₂ and CH₄ are determined.     

ACRYLAMIDE QUENCHING OF TRYPTOPHAN PHOTOCHEMISTRY AND PHOTOPHYSICS

Studies of acrylamide quenching of tryptophan (Trp) fluorescence, photochemistry, and photoionization have been conducted. Quenching of Trp fluorescence in aqueous solution by addition of acrylamide in the concentration range 0.0-0.5 M was measured and resulted in a Stern-Volmer quenching constant of KSV = 21 +/- 3 M-1. Photolysis experiments were performed in which Trp was photolyzed at 295 nm in the presence of varying concentrations of acrylamide. The loss of Trp was monitored using reverse-phase high performance liquid chromatography (RP-HPLC) and was observed to follow first order kinetics. Production of N-formylkynurenine (NFK) was observed by RP-HPLC in irradiated Trp samples both in the presence and absence of added acrylamide. In addition, no new photochemical product was detected. This was taken as evidence that acrylamide did not alter the photochemical pathway but just reduced the reaction rate as expected for a physical quenching mechanism. Plotting the reciprocal of photolysis rate constant versus acrylamide concentration produced a Stern-Volmer constant for quenching of Trp photochemistry of KSV = 6 +/- 2 M-1. The KSV values for both fluorescence quenching and photolysis quenching were thus large, implying efficient quenching of both processes by acrylamide. Assuming an excited singlet state lifetime of 2.8 ns, the calculated second-order quenching rate constants for fluorescence and photolysis were kq = 7.5 x 10(9) and 2.1 x 10(9) M-1 s-1 respectively. The possible involvement of photoionization in the photolysis mechanism was investigated by studies of acrylamide quenching of voltage transients produced by xenon flash lamp excitation of Trp at aqueous/teflon or aqueous/mica interfaces.(ABSTRACT TRUNCATED AT 250 WORDS)     

Ozone initiated oxidation of long chained n -alkanes in the presence of activated charcoal or silica gel

Photolysis behavior of acetamiprid in different kinds of surfactant solutions were investigated. The results showed that acetamiprid was prone to photolysis when exposed to the irradiation of a mercury lamp, and the photolysis kinetics fit in well with the first-order kinetic equation. Surfactants did not participate in the acetamiprid’s photolysis process directly, whereas they all had obvious inhibiting effects on it.     

Determination of liposome permeability of ionizable carbamates of zidovudine by steady state fluorescence spectroscopy

In the present paper the relative permeabilities of AZT-Pyp and AZT-Ethy across a phospholipid bilayer were estimated by the means of fluorescence spectroscopy.

The center of spectral mass of both non-encapsulated AZT-derivatives (AZT-der) emission spectra increased as a function of the illumination time inside the spectrofluorimeter cell. This phenomenon was even more evident when drugs were incubated under an UV mercury lamp, suggesting its photolytic origin. AZT-der were protected from photolysis inside liposomes and decomposed upon irradiation when they were free in the aqueous phase. The time-dependent decrease in the fluorescence intensity at a constant wavelength was fitted to a two-exponential equation and the values of rate constants for permeability and photolysis were calculated. It was concluded that AZT-Pyp but not AZT-Ethy diffused across the bilayer. This behavior correlated with the molecular volumes of AZT-Pyp (379.6 Å³) and AZT-Ethy (450.5 Å³), determined from the minimum energy conformations but not with previously reported log P values. These results reinforce the concept that not only lipophilicity but also membrane structure and AZT-der molecular size had a critical influence in passive diffusion across bilayers and may help in future refinements of other AZT-der molecular design.     

Kinetics and mechanism of the photolysis of CF<Subscript>2</Subscript>ClBr exposed to light with a wavelength of 253.7 nm

CF₂ClBr mixed with oxygen was photolyzed using a low pressure mercury lamp, and the kinetics of photolysis was studied. The absorption spectra of the starting material and products of photolysis were recorded in the wavelength range from 200 to 900 nm on an Agilent 8453 spectrophotometer. The concentrations of the main photolysis products at different irradiation times were calculated by the mathematical processing of the absorption spectra. The scheme of CF₂ClBr photolysis was suggested, the model calculations according to this scheme were performed, and the results of simulation were compared with experimental data.     

硝基酚类化合物在海水中的光化学氧化研究

酚类化合物尤其是硝基苯酚属于国家严格控制排放的有毒物质 .由于工业排污等原因 ,在近岸水体中能检测到许多酚类化合物 .我国胶州湾和杭州湾表层海水中酚的含量分别高达 3和 9mg/L[1] .酚的毒性会影响海水生物的生长和繁殖 ,危及人类健康 .由于硝基苯酚难于生化降解 ,因此研究其在水体中的光降解十分重要 [2～ 6 ] .文献中关于有机污染物的光化学研究大多侧重于废水处理 ,且降解方法也多采用 Ti O2 催化降解 [7,8] .对于硝基苯酚在天然水体中的光降解则知之甚少 .本文通过模拟海水条件下硝基苯酚的光化学反应及其影响因素 ,更加深入地了…     

SPECTRAL QUALITY OF TWO FLUORESCENT UV SOURCES DURING LONG-TERM USE

The characteristics of a fluorescent ultraviolet (UV) lamp (UVB-313), UV-B transmitting cellulose diacetate (CA) and UV-B absorbing polyester (PE) films were determined during actual use. Although lamp emission was stable between 70 and 386 h of burn time (longer times were not investigated), the absorbance of UV-B and UV-A radiation by CA and PE films, respectively, increased with time when wrapped around lamps. As a result, the irradiance of lamp/filter combinations decreased steadily (even when CA films were presolarized for 10 h), making it necessary to compensate by adjusting the height of the lamp bank or by changing filters frequently. Note that corrective action is required for UV-A controls (PE films) as well as UV-B experimental treatments (CA films). Changing filters is preferable, since aging of CA filters caused shifts in the ratio of UV-B to UV-A. However, in spite of these shifts, the normalized spectrum of weighted biologically effective UV-B radiation did not change to a large extent.     

The laser vs. the lamp: Photochemistry of long-chained bis-dibenzylketones

A comparison of products formed during photolysis of 1,12-bis-[4-(2-oxo-3-phenylpropyl)phenyl]dodecane (1) reveals a difference in laser and lamp photolysis in that the use of a laser increases the yield of macrocyclic paracyclophane rings (2) by a factor of three.     

Postcolumn formation of fluorophores from nitrogenous pesticides by UV photolysis

The ultraviolet (UV) photolysis of several classes of nitrogenous pesticides was examined with a view to photo-induced fluorescence detection in flow-injection analysis (FIA) and liquid chromatography. The solvents evaluated as typical reversed-phase mobile phases included water, methanol, and 1:1 mixtures of methanol/water and acetonitrile/water, and methanol/acetonitrilelwater mixtures. Acetone, acetophenone, the surfactant triton X-100, and the photocatalyst titanium dioxide were assessed as photosensitizers to enhance the UV photolysis and fluorescence responses. FIA and liquid chromatographic separations of several pesticides were followed by post-column UV photolysis for the fluorescence detection. Ultraviolet photolysis produces some fluorescent products. The type of photolytic solvent seems to play a significant role. The presence of photosensitizers also affects the fluorescence response of some pesticides. The photochemical transformation products of some of the pesticides are suggested. Analytical figures of merit were evaluated for determination of several pesticides in ground water. The post-column UV photolysis approach for fluorescence detection in liquid chromatography was assessed for several nitrogenous pesticides in ground water samples at ng/g concentrations.     

Phototransformations of Phenols in Aqueous Solutions under Different Excitation Modes

The photolysis of aqueous solutions of two widespread environmental organic ecotoxicants—phenol and 4-chlorophenol—on excitation by UV radiation from a KrCl exciplex laser and a glow-discharge KrCl excilplex lamp was studied. The irradiated solutions were studied by spectroscopic techniques. The laser and lamp photolyses were compared.     

Chromatographic investigations of the photolysis products of nitrobenzenes

Summary By using gas, liquid, column and thin-layer chromatography and spectroscopy the main products of the photolysis of nitrobenzene in ethereal solutions have been identified. These are: azobenzene, azoxybenzene, nitroazoxybenzene, 4-aminoazobenzene, 3- and 4-nitrobenzene, 2-, 3- and 4-nitrophenol, 1,4-quinone, phenol, aniline and 4-nitroaniline. The photolysis was carried out by irradiation with a mercury lamp. The removal of oxygen from the irradiated solution was found to affect both the qualitative and quantitative composition of the photolysis products. The mechanism of the photochemical processes occurring in the irradiated nitrobenzene solutions is discussed.     

ON THE MECHANISMS OF PHOTOLYSIS OF COMPOUNDS I AND II OF HORSERADISH PEROXIDASE AT 77 K*

Abstract—When Compounds I and II of horseradish peroxidase in glycerol/water glasses at 77 K were irradiated with the light of a mercury lamp, some changes in optical spectra as well as the development of strong EPR signals of a free-radical type were detected. In case of photolysis of Compound I the light of wavelengths around 400 nm was the most effective, while only short wavelength (≥ 280 nm) light affected Compound II. The results of experiments with thawing and freezing the product of photolysis of Compound I (called Intermediate Y) provide further evidence for the suggestion that the ferric state of the heme iron is generated in the course of the photolysis. The quantum yields of the photolysis of Compound I at different wavelengths of irradiating light were obtained and mechanisms of the photolysis of Compounds I and II are proposed.     

Reaction CH(3) + OH Studied over the 294-714 K Temperature and 1-100 bar Pressure Ranges

Reaction of methyl radicals with hydroxyl radicals, CH(3) + OH → products (1) was studied using pulsed laser photolysis coupled to transient UV-vis absorption spectroscopy over the 294-714 K temperature and 1-100 bar pressure ranges (bath gas He). Methyl radicals were produced by photolysis of acetone at 193.3 nm. Hydroxyl radicals were generated in reaction of electronically excited oxygen atoms O((1)D), produced in the photolysis of N(2)O at 193.3 nm, with H(2)O. Temporal profiles of CH(3) were recorded via absorption at 216.4 nm using xenon arc lamp and a spectrograph; OH radicals were monitored via transient absorption of light from a dc discharge H(2)O/Ar low pressure resonance lamp at ca. 308 nm. The absolute intensity of the photolysis light inside the reactor was determined by an accurate in situ actinometry based on the ozone formation in the presence of molecular oxygen. The results of this study indicate that the rate constant of reaction 1 is pressure independent within the studied pressure and temperature ranges and has slight negative temperature dependence, k(1) = (1.20 ± 0.20) × 10(-10)(T/300)(-0.49) cm(3) molecule(-1) s(-1).     

Enhancement of the Photolysis of Nucleic Acid Monomers by Phosphates

Abstract— In the tentative simulation of prebiotic synthesis, it was found that the photolysis yield of nucleic acid bases, nucleosides and nucleotides (NA) undergoing UV irradiation was sharply enhanced by added orthophosphate. Filter experiments demonstrated that the enhancement is due to UV quanta of wavelength less than 215 nm in the emission spectrum of a medium pressure mercury lamp. The phosphate-induced enhancement of the photolysis of NA constitutes an additional channel of nucleic acid monomer degradation through their interaction with photoionized phosphate radicals.     

Photolysis by UVA–Visible Light of TNT in Ethanolic,Aqueous-Ethanolic,and Aqueous Solutions According to Electrospray and Aerodynamic Thermal Breakup Droplet Ionization Mass Spectrometry

The mechanism of photolytic degradation of 2-4-6-trinitrotoluene (TNT) by UVA–visible light (>320 nm) in ethanolic, aqueous-ethanolic, and aqueous solutions was investigated by electrospray and aerodynamic thermal breakup droplet ionization mass-spectrometric analyses. For the photolysis, a DRK-120 mercury-quartz lamp was used. Products of the photolysis reaction were compared with known products of TNT transformation in the environment. Because the photochemistry of some compounds in alcohols (in contrast to aqueous solutions) features a transfer of electrons from the solvent to the light-excited compound, we believe that the efficiency of photolysis (polymerization) of TNT in ethanol and aqueous-ethanolic solutions is based on this mechanism.     

Photodegradation of polyimides. III. The effect of chemical composition,radiation source,atmosphere, and processing

Polymide films made from diarylanhydrides with oxygen, carbonyl, and hexafluoroisopropylidene 6F bridging groups are photolabile when irradiated with unfiltered light from a medium-pressure mercury lamp. The presence of oxygen is necessary to the photolytic degradation process, which results in the ultimate oxidative ablation of thin polyimide films. Films based on the 6F dianhydride containing a hexafluoroisopropylidene hinge group are the most unstable. However, photolysis of 6F dianhydride based polyimides in an inert atmosphere with either the unfiltered medium pressure mercury lamp or an electron beam results in no detectable changes even after long exposure times.     

双叠氮化合物光敏性能的研究

本文研究了五个结构不同的双叠氮化合物的光敏性能。用未经分光的高压汞灯辐照双叠氮化合物的甲苯溶液后发现,共轭结构的双叠氮化合物紫外吸收谱上存在着两个等吸收点;证明了在某一时间区间内,这些双叠氮化合物光解反应为一级反应;通过对光解产物的分离和鉴定,证实了双叠氮化合物光解反应的双光子机理,测定了它们的量子效率。     

Application of solid-phase extraction and micellar electrokinetic capillary chromatography to the study of hydrolytic and photolytic degradation of phenoxy acid and phenylurea herbicides

A degradation study of two phenoxy acid [(2,4-dichlorophenoxy) propanoic acid and (2,4,5-trichlorophenoxy) acetic acid] and two phenylurea (diuron and monolinuron) herbicides, spiked at 50 ppb in water, was performed. Some samples were subjected to neutral and basic hydrolysis; other samples were subjected to photolysis using either sunlight or a xenon arc lamp. After degradation, the water samples were preconcentrated using solid-phase extraction (SPE) with Carbopack B columns and analysed by a micellar electrokinetic capillary chromatography (MECC) system with UV detection at 210 nm. Phenoxyacetic acids were not degraded neither by hydrolysis nor by sunlight photolysis, but they were photodegraded when they were exposed to a xenon arc lamp, with half-lives around 300 min. Phenylurea herbicides were hydrolysed at the two-tested pH, with half-lives varying from 25 to 290 days. The main hydrolysis products were the corresponding chloroanilines. Diuron and monolinuron were also degraded when they were exposed to sunlight and xenon arc lamp. The main photodegradation pathway for diuron corresponded to dehalogenation, while for monolinuron dealkylation and hydroxylation were also postulated. The toxicity of the studied herbicides and their degradation products was evaluated by means of Microtox tests. The obtained results indicated that the toxicity of the degraded samples was higher than the toxicity of the herbicides.     

Ultraviolet Photolysis and Ortho-Phthalaldehyde-Mercaptoethanol Derivatization of Pharmaceuticals for Enhanced Fluorescence Detection in HPLC

Abstract

Several classes of nitrogenous pharmaceutical were examined for fluorescence after ultraviolet (UV) radiation induced photolysis followed by reaction with o-phthalaldehyde-2-mercaptoethanol (OPA-MERC), and after UV photolysis alone. Photolyses were examined in water, mixtures of methanol/water (1:1), and acetonitrile/water (1:1). Acetone was assessed as a photosensitizer to enhance photolysis and fluorescence response. Flow injection analysis and high-performance liquid chromatographic techniques were used for several pharmaceuticals. The analytes were subjected to UV photolysis and reaction with OPA-MERC reagent for generation of fluorophores that responded to fluorescence detection. During photolysis, solvent type as well as the presence of photosensitizers seem to play a significant role in the formation of primary amines and fluorophores. Photochemical transformation products of some of the pharmaceutical chemicals are proposed. Analytical figures of merit were determined for some analytes. This fluorescence detection approach is applicable for a number of pharmaceuticals at nanogram level.