Photoinduced amino-imino tautomerism: an infrared study of 2-amino-5-methylpyridine in a low-temperature argon matrix

The photoreaction of 2-amino-5-methylpyridine was investigated by matrix-isolation infrared spectroscopy and DFT calculation. Photoinduced reversible amino (N=C-NH(2))-imino (NH-C=NH) tautomerism was found between 2-amino-5-methylpyridine and 5-methyl-2(1H)-pyridinimine; the amino tautomer changes to the imino tautomer by UV irradiation (340>lambda>or= 300 nm) and the reverse change occurs by longer-wavelength light irradiation (420>lambda>or= 340 nm). The results of the CASSCF calculation revealed that the amino-imino tautomerism proceeds in vibrational relaxation process from electronic excited state to the ground state. The IR spectra of 2-amino-5-methylpyridine in the T(1) state and 5-methyl-2-pyridinamino radical were also obtained by UV irradiation (lambda>or= 300 nm).     

Low atmospheric temperature dependence on photodegradation of wood

This paper deals with the photodegradation of wood in low atmospheric temperature region (-40 to 50 degrees C), and discusses the changes in color and IR spectra. The color and IR spectra of wood hardly changed with photo-irradiation at -40 degrees C but did at -20 degrees C. Therefore, it is thought that the photo-energy causes the scission of chemical bonds or the production of radicals; however, heat energy is needed to yellow of wood (i.e., to produce quinone). In case of softwoods, the absorption of the carbonyl band consisted of two sub-bands, and the carbonyl band at around 1710 cm(-1) increased at lower temperature than that at around 1760 cm(-1) by light-irradiation. Such IR measurement due to lignin degradation means that the photodegradation of softwoods is faster than that of hardwoods. The color change by exposure to light in the low temperature was also caused by the degradation of lignin.     

Cobalt Ion-Doped TiO(2) Photocatalyst Response to Visible Light

Photocatalytic activity under visible light irradiation was generated by doping a small amount of Co(2+) ions into TiO(2) particles. Nanometer-sized particles with the composition xCoO-(100-x) TiO(2) (xCo/TiO(2); 0300 nm) light irradiation but also induced the visible light (lambda>400 nm) response. The highest photocatalytic activities were obtained at x=0.03 for both irradiations. Copyright 2000 Academic Press.     

Comparative study of photodegradation of wood by a UV laser and a xenon light source

A detailed study of photodegradation of wood surfaces by xenon light source and a UV laser has been carried out. Silver birch, rubberwood, Scots pine and chir pine wood veneers were irradiated with a xenon light source or a 244 nm argon ion laser. The changes in chemical structure of wood surfaces were monitored by UV resonance Raman (UVRR), photoacoustic Fourier transform infrared (FTIR-PAS) and UV-vis reflectance spectroscopies. The depth profile of xenon lamp irradiated wood surfaces was carried out by measuring FTIR-PAS spectra at different moving mirror velocities. The UVRR and FTIR-PAS spectra of irradiated wood surfaces showed degradation of aromatic structure in lignin combined with strong formation of carbonyl structures. The FTIR-PAS spectra measured from xenon irradiated wood surfaces indicate that hardwood lignin degrades at a faster rate than softwood lignin. The UVRR spectra of xenon irradiated wood show a significant decrease in the intensities of aromatic structures at 1602 cm⁻¹. This is accompanied by a significant band broadening and notable shift towards longer wavenumbers, which has been attributed to the formation of o- and p-quinone structures as degradation products. The formation of quinone structures was also supported by the generation of a broad absorption band between 350 and 600 nm in UV-vis reflectance spectra of irradiated wood surfaces. There was a significant broadening in the region of 1500-1000 cm⁻¹ in UVRR spectra due to the formation of unsaturated structures as a result of lignin degradation. The UVRR spectra of laser irradiated wood showed similar behaviour i.e., overall broadening and a rapid reduction in the intensity of lignin aromatic structure. The rate of degradation by laser was very high. However, the extent of band broadening was higher in xenon irradiated wood indicating the generation of several different types of structures as compared to laser irradiation, which produces only particular type of structures. UVRR spectra of laser irradiated Whatman paper showed significant photodegradation of cellulose by UV laser. The UV degradation rate of lignin was much higher than cellulose.     

Dynamics of intramolecular hydrogen-atom migrations in 2-hydroxy-3-nitropyridine studied by low-temperature matrix-isolation infrared spectroscopy and density functional theory calculation

Intramolecular hydrogen-atom migrations in 2-hydroxy-3-nitropyridine have been investigated by low-temperature matrix-isolation infrared (IR) spectroscopy with the aid of density functional theory (DFT) calculation. An IR spectrum measured after deposition was assigned to an enol isomer, the conformation of which is anti in relation to OH versus N in the pyridine ring. When the matrix sample was exposed to UV and visible light (lambda>350 nm), an IR spectrum consistent with a keto product was observed. During the irradiation, an IR spectrum of a transient species, a photoreaction intermediate between anti-enol and keto, was observed, which was assigned to syn-enol. The bands of syn-enol disappeared completely when the irradiation was stopped, while those of the original isomer, anti-enol, reappeared. No reverse isomerization was observable in the corresponding deuterated species. This led to the conclusion that the isomerization from syn to anti occurs through hydrogen-atom tunneling. On the other hand, an aci-nitro form was produced by UV irradiation (lambda=365+/-10 nm) without visible light. The conformation around the aci-nitro group was determined to be cis-cis by comparison with the spectral patterns obtained by the DFT/B3LYP/6-31++G** calculation. The dynamics of the hydrogen-atom migrations between anti- and syn-enols, syn-enol and keto, and anti-enol and aci-nitro are discussed in terms of the potential surfaces obtained by the DFT calculation.     

Thermal degradation of wood during photodegradation

In this study, wood samples were exposed to light irradiations (direct sunlight, xenon lamp, mercury vapour lamp) and thermal treatments were carried out in dry- and in humid conditions at 90°C. One part of the samples was covered by an aluminium plate during light irradiation. The samples under the aluminium plate also suffered considerable chemical changes, monitored by infrared technique and colour measurement. The sunlight produced greater colour change under the aluminium plate than the artificial light sources. During light irradiation, the carbonyl band having two maximum at 1700 and 1,746 cm(-1) increased and the peak of the aromatic skeletal vibration arising from lignin (1,510 cm(-1)) decreased together with the guaiacyl vibrations at 1,275 cm(-1). There was absorption decrease at 1,174 cm(-1) because of the ether band splitting. Under the covered surface only the ether band at 1,174 cm(-1) decreased and one carbonyl band increased with a maximum at 1,715 cm(-1). Degradation of lignin was negligible for the covered surface. Colour change generated by thermal degradation was much greater in humid condition than in dry condition.     

Photodegradation of 5-methyltetrahydrofolate: biophysical aspects

5-methyltetrahydrofolate (5MTHF) absorbs UV radiation and has an absorption coefficient of 24250+/-1170 M(-1) cm(-1) at 290 nm. It has a weak fluorescence emission in the wavelength region around 360 nm. Our data demonstrated induction of 5-methyldihydrofolate by exposure to UVB and, after continues irradiation, p-aminobenzoyl-L-glutamic acid was found. The photodegradation of 5MTHF follows a first order kinetic with a degradation rate constant of 9.2 x 10(-3) min(-1) under our conditions (fluence rate of 2.15 mW cm(-2), exposure wavelengths from 280 to 350 nm). Our results indicate that a direct degradation of 5MTHF by UV exposure in humans in vivo is rather unlikely. 5MTHF mainly absorbs, and is degraded by, UVB and UVC, radiation that does not penetrate the earth's atmosphere and the human skin well.     

Photoinduced amphiphilic property of InNbO4 thin film

An InNbO4 thin film was prepared by a sol-gel method. The photoinduced amphiphilic (hydrophilic and lipophilic) property on the film was determined from the changes in contact angles for water and diiodomethane (CH2I2) under light (lambda>300 nm) irradiation. Before light irradiation, the contact angles for water and diiodomethane were approximately 50 degrees and 40 degrees, respectively. Subsequently, light irradiation induced the contact angles to decrease. The critical contact angles for water and diiodomethane were approximately 0 degrees and 15 degrees, respectively, suggesting that this film possesses the photoinduced superhydrophilic and amphiphilic properties. This is the first report of a complex oxide with a photoinduced amphiphilicity. In the photoinduced amphiphilic conversion process, it was also found that both the total gas-solid surface free energy and the gas-solid surface free energy of the polar component increased.     

CH3--MoF], [CH2=MoHF], and [CH[triple bond]MoH2F] formed by reaction of laser-ablated molybdenum atoms with methyl fluoride: persistent photoreversible interconversion through alpha-hydrogen migration and agostic interaction

Simple molybdenum methyl, carbene, and carbyne complexes, [CH3--MoF], [CH2=MoHF], and [CH[triple chemical bond]MoH(2)F], were formed by the reaction of laser-ablated molybdenum atoms with methyl fluoride and isolated in an argon matrix. These molecules provide a persistent photoreversible system through alpha-hydrogen migration between the carbon and metal atoms: The methyl and carbene complexes are produced by applying UV irradiation (240-380 nm) while the carbyne complex is depleted, and the process reverses on irradiation with visible light (lambda>420 nm). An absorption at 589.3 cm(-1) is attributed to the Mo--F stretching mode of [CH3--MoF], which is in fact the most stable of the plausible products. Density functional theory calculations show that one of the alpha-hydrogen atoms of the carbene complex is considerably bent toward the metal atom (angle-spherical HCMo=84.5 degrees ), which provides evidence of a strong agostic interaction in the triplet ground state. The calculated C[triple chemical bond]Mo bond length in the carbyne is in the range of triple-bond values in methylidyne complexes.     

Photodegradation of substituted stilbene compounds: what colors aging paper yellow?

Photodegradation of lignin is one of the major postprocessing problems in paper production, as this renders yellowing of the paper and reduced paper quality. In this study, we have explored the photochemical properties of substituted stilbene derivatives believed to be key chromophores in the photodegradation of lignin derived from cinnamyl alcohol. In particular, the present work focuses on the computation of UV/vis electronic absorption spectra for different methoxylated stilbenes and their proposed photodegradation products. All calculations were performed using the time-dependent formalism of density functional theory (TD-DFT) and the B3LYP hybrid functional. It is concluded that the methodology employed is capable of reproducing not only the overall spectra, but also subtle features owing to the effects of different substitution patterns. For the strongly absorbing first excited singlet state (HOMO --> LUMO excitation) of the methoxylated stilbenes, the calculated transition energies are, albeit somewhat fortuitously, in excellent agreement with experimental data. The light-induced yellowing indirectly caused by the presence of stilbenes can be rationalized in terms of the absorption spectra of the resulting photodegraded o-quinones, for which distinct transitions in the 420-500 nm region of the visible spectrum lacking prior to degradation are observed.     

STUDIES ON THE PHOTO DEGRADATION OF TRYPTOPHAN

Abstract— The photodegradation of tryptophan in aqueous solution increases steadily with decrease in wavelength over the U.V. range 370–230 mμ, but the action spectrum does not parallel the absorption spectrum over these wavelengths. Thus appreciable decomposition occurs at wavelengths greater than 300 mp, even though at the concentrations tested the indole ring does not absorb energy in this region, and a photosensitive auto-oxidation mechanism is suggested to account for this.
The quantum efficiency for the destruction tryptophan at 265 mμ is 0 01 at pH 4 , 5 and 7 , but increases to 0.02 at pH 12. For the acetyl, ester and amide derivatives it is 0.03 in neutral solution.
The yellowing of tryptophan solutions during irradiation is accompanied by the appearance of an absorption maximum at about 305 mμ in the differential absorption spectrum. The increase in optical density at this wavelength bears a direct relationship to the decrease in optical density at 280 mμ: the degree of yellowing is related to the degree of destruction of tryptophan. It is less pronounced in neutral solution than at higher or lower pH values.
The yellowing of N-substituted tryptophan derivatives at 265 mμ is appreciably less than that of tryptophan, whereas derivatives with a free amino group are indistinguishable from the unsubstituted amino acid in this respect.     

Photosensitizing properties of a boronated phthalocyanine: studies at the molecular and cellular level.

A synthetic procedure has been developed for the preparation of a Zn-phthalocyanine peripherally substituted with a dodecaborane. The absorption spectrum of the derivative is typical of the phthalocyanine chromophore. Moreover, the boronated phthalocyanine exhibits a high photosensitizing efficiency against a model biological substrate, such as N-acetyl-L-tryptophanamide, and a singlet oxygen quantum yield of 0.53 in dimethylformamide. Even though the presence of the dodecaborane moiety appears to decrease the affinity of the phthalocyanine for HT-1080 transformed human fibroblasts, the boronated phthalocyanine causes an essentially complete loss of cell viability upon irradiation with 600-700 nm light under mild conditions (1 microM concentration, 5-min irradiation at 10 mW/cm(2)).     

GC-MS and HPLC analyses of lignin degradation products in biomass hydrolyzates

Summary The gas-chromatographic separation of lignin degradation products of hydrolytically degraded biomass (e. g. poplar wood) and their mass spectrometric identification are described. For routine analysis of lignin degradation products of biomass hydrolyzates, a time saving HPLC gradient method using a UV-detection system (280 nm), is described. In addition to reference samples, solutions of hydrothermally degraded biomass were analyzed.

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GC-MS- und HPLC-Analytik von Ligninabbauprodukten in Biomassehydrolysaten

Zusammenfassung Die gas-chromatographische Trennung von Ligninabbauprodukten nach hydrolytischer Spaltung von Biomasse (z.B. Pappelholz) sowie die massenspektrometrische Identifizierung werden beschrieben. Für eine routinemäßige Analytik der Ligninabbauprodukte in Biomassehydrolysaten wird weiterhin die Trennung mittels einer Gradienten-HPLC-Methode aufgezeigt. Die Detektion erfolgte bei 280 nm. Neben Referenzproben wurden hydrothermale Abbaulösungen untersucht.

     

Photoinduced magnetization in copper octacyanomolybdate

This article describes the studies of a photomagnetic cyanide-bridged Cu-Mo bimetallic assembly, Cu(II)(2)[Mo(IV)(CN)(8)].8H(2)O (Cu(II), S = (1)/(2); Mo(IV), S = 0) (1), which has an intervalence transfer (IT) band from Mo(IV)-CN-Cu(II) to Mo(V)-CN-Cu(I) around 480 nm. Wide-angle X-ray scattering and X-ray spectroscopic studies provide precise information about the 3D connectivity and the local environment of the transition metal ions. Irradiating with blue light causes solid 1 to exhibit a spontaneous magnetization (Curie temperature = 25 K). The thermal reversibility is carefully studied and shows the long-time stability of the photoinduced state up to 100 K. Photoreversibility is also observed; i.e., the magnetization is induced by irradiation with light below 520 nm, while the magnetization is reduced by irradiation with light above 520 nm. The UV-vis absorption spectrum after irradiation shows a decrease of the IT band and the appearance of the reverse-IT band in the region of 600-900 nm (lambda(max) = 710 nm). This UV-vis absorption spectrum is recovered to the original spectrum by irradiation with 658-, 785-, and 840-nm light. In this photomagnetic effect, the excitation of the IT band causes an electron transfer from Mo(IV) to Cu(II), producing a ferromagnetic mixed-valence isomer of Cu(I)Cu(II)[Mo(V)(CN)(8)].8H(2)O (Cu(I), S = 0; Cu(II), S = (1)/(2); Mo(V), S = (1)/(2)) (1'). 1' returns to 1 by irradiation of the reverse-IT band, which obeys the scheme for the potential energy surface in mixed-valence class II compounds.     

Formation of free radicals in photoirradiated cellulose. VI. Effect of lignin

The effect of lignin on free-radical formation in photoirradiated pulp was studied by means of electron spin resonance spectroscopy. Samples were irradiated with light of wavelength longer than 3400 Å as well as longer than 2537 Å. Radical formation in aspen lignin was observed before and after irradiation with light of wavelength longer than 3400 Å. Upon irradiation, free radicals were formed in a pulp sample with 0% lignin content only when oxygen was present. On irradiation with light longer than 2537 Å, in the presence of nitrogen, oxygen, and under vacuum, free-radical formation was found in all cases. The largest concentration of radicals was obtained under vacuum, the smallest in oxygen. It is evident that cellulose was protected from radiation by the presence of lignin. Increased aspen lignin content in pulp led to a decrease in the relative signal intensity of ESR spectrum, that is, a decrease of the yield of free radicals in the cellulose fraction in pulp. Irradiation with light in the presence of oxygen caused significant yellowing of the pulp sample, and these photooxidation and discoloration reactions occurred primarily on the surface of the sample irradiated.     

Comparison of the optical properties of radiata pine TMP and CTMP pulps and the starting wood

The reflectivity, specific light scattering coefficients and specific light absorption coefficients of earlywood and latewood in each ring from three radiata pine discs were determined at 457 nm. Significant ring-to-ring and within-ring variation was observed. A method was developed to determine the light absorption spectra of the wood. Comparison with that of TMP pulp showed that during refining a chromophore with a maximum absorption at 430 nm was produced. It is suggested that this is the same chromophore as that produced during light-induced yellowing of mechanical pulps, possibly ano-quinone. Chromophore formation was less when sulfite was present.     

Photooxidized products of recombinant alpha A-crystallin and W9F mutant

Human lenses contain many photosensitizers that absorb light at wavelengths above 300 nm, most notably UVA light (320-400 nm). Kynurenine (Kyn) and 3-hydroxykynurenine (HK), two of the best-known photosensitizers in the human lens, may play a significant role in photooxidation-related changes in lens proteins, such as conformational change and aggregation. In vitro irradiation experiments with proteins indicate that the Trp residue (with maximal absorption at 295 nm) is more susceptible to photooxidation by UVB light (280-320 nm) than by UVA light, but most UVB light below 300 nm is screened by the cornea and little reaches the lens, especially the nuclear region where nuclear color develops. Therefore, if photooxidation is an important contributor to nuclear color or nuclear cataract, it must arise from a photosensitized reaction. In the present study, we use recombinant alpha A- and its Trp-deficient mutant W9F as models to study the effects of UVA irradiation in the presence of HK or Kyn and of UVB (300 nm) irradiation on alpha-crystallins. alpha A-crystallin showed a large decrease in Trp fluorescence and a large increase in non-Trp (blue) fluorescence after the HK-sensitized or 300 nm photooxidation. For the W9F mutant, a smaller decrease in protein fluorescence (lambda ex at 280 nm) and a smaller increase in blue fluorescence than for the wild-type alpha A-crystallin were observed. A decrease in the near-UV CD was also observed for both photooxidized alpha A and the W9F mutant. The effect of Kyn sensitization is smaller than that of HK sensitization. A study of chaperone-like activity indicated that only 300 nm photooxidized alpha A and the W9F mutant increased the ability to protect insulin from dithiothreitol-induced aggregation. Thus, sensitized photooxidation can occur in amino acids other than Trp by UVA in the presence of HK or Kyn with effects similar to, albeit smaller than, those of direct UVB (300 nm) photooxidation.     

侧链型偶氮聚合物液晶在不同波长激发条件下的光致取向

合成了聚甲基丙烯酸 (6 [4 (4 氰基偶氮苯 )苯氧基 ]己酯 ) (Poly(6 [4 (4 cyanophenylazo)phenoxy]hexylmethacrylate) (PM6ABCN) ) ,采用溶液挥发法在玻璃载片上成膜 ,研究了薄膜样品在Tg 温度以下的光致取向 .用波长分别为 36 6、40 0和 436nm的偏振光照射 ,发现PM6ABCN薄膜的光致取向过程不仅依赖于光的强度 ,还依赖所使用的偏振光的波长 .在低于Tg 的温度下 ,用波长为 40 0nm ,功率为 2 0mW cm2 的光照射 10 0s可以使样品的取向达到饱和 ,而用 36 6nm的光导致的取向程度要小于 40 0nm和 436nm的光 ,因为 36 6nm的光会在稳态时产生更多数目的cis异构体     

Wavelength sensitivity of the photoinduced reaction in polycarbonate

Bisphenol A polycarbonate (PC) was irradiated with monochromatic light of wavelengths 260, 280, 300, 320, 340, 400, and 500 nm by use of the Okazaki large spectrograph (OLS). The quantum yield of main-chain scission (?_cs), efficiency of photo-Fries rearrangement (E_r), and effects of wavelength on ?_cs and E_r were investigated. It was found that photodegradation and photo-Fries rearrangement of PC took place by the irradiation of 260–300 nm light, but did not by the irradiation at λ ≧ 320 nm. The ?_cs has a maximum value in the case of the irradiation with 260 nm light, while E_r was found to have a maximum value by the irradiation of 280 nm light. © 1993 John Wiley & Sons, Inc.     

Spectroscopic studies and normal coordinate analysis of bilirubin

The infrared spectrum of bilirubin has been recorded in the spectral region 200-4000 cm(-1). The Raman spectrum has also been recorded using the second harmonic (530 nm) radiation of a 200 mW Nd-YAG laser. In order to confirm the vibrational assignment of the bands obtained from experimental observation, a normal coordinate analysis has been carried out using the semi-empirical AM1 method through MOPAC 5.1 computer program. Electronic absorption spectrum of bilirubin dissolved in CHCl3 has been recorded in the spectral region 300-600 nm. A broad spectrum is observed with peak maxima at 454.2 nm. The photoacoustic spectrum of this molecule (in the powder form) has also been recorded for the first time which shows certain discrete features.